assignment of variables in python

Python Numerical Methods

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Variables and Assignment ¶

When programming, it is useful to be able to store information in variables. A variable is a string of characters and numbers associated with a piece of information. The assignment operator , denoted by the “=” symbol, is the operator that is used to assign values to variables in Python. The line x=1 takes the known value, 1, and assigns that value to the variable with name “x”. After executing this line, this number will be stored into this variable. Until the value is changed or the variable deleted, the character x behaves like the value 1.

TRY IT! Assign the value 2 to the variable y. Multiply y by 3 to show that it behaves like the value 2.

A variable is more like a container to store the data in the computer’s memory, the name of the variable tells the computer where to find this value in the memory. For now, it is sufficient to know that the notebook has its own memory space to store all the variables in the notebook. As a result of the previous example, you will see the variable “x” and “y” in the memory. You can view a list of all the variables in the notebook using the magic command %whos .

TRY IT! List all the variables in this notebook

Note that the equal sign in programming is not the same as a truth statement in mathematics. In math, the statement x = 2 declares the universal truth within the given framework, x is 2 . In programming, the statement x=2 means a known value is being associated with a variable name, store 2 in x. Although it is perfectly valid to say 1 = x in mathematics, assignments in Python always go left : meaning the value to the right of the equal sign is assigned to the variable on the left of the equal sign. Therefore, 1=x will generate an error in Python. The assignment operator is always last in the order of operations relative to mathematical, logical, and comparison operators.

TRY IT! The mathematical statement x=x+1 has no solution for any value of x . In programming, if we initialize the value of x to be 1, then the statement makes perfect sense. It means, “Add x and 1, which is 2, then assign that value to the variable x”. Note that this operation overwrites the previous value stored in x .

There are some restrictions on the names variables can take. Variables can only contain alphanumeric characters (letters and numbers) as well as underscores. However, the first character of a variable name must be a letter or underscores. Spaces within a variable name are not permitted, and the variable names are case-sensitive (e.g., x and X will be considered different variables).

TIP! Unlike in pure mathematics, variables in programming almost always represent something tangible. It may be the distance between two points in space or the number of rabbits in a population. Therefore, as your code becomes increasingly complicated, it is very important that your variables carry a name that can easily be associated with what they represent. For example, the distance between two points in space is better represented by the variable dist than x , and the number of rabbits in a population is better represented by nRabbits than y .

Note that when a variable is assigned, it has no memory of how it was assigned. That is, if the value of a variable, y , is constructed from other variables, like x , reassigning the value of x will not change the value of y .

EXAMPLE: What value will y have after the following lines of code are executed?

WARNING! You can overwrite variables or functions that have been stored in Python. For example, the command help = 2 will store the value 2 in the variable with name help . After this assignment help will behave like the value 2 instead of the function help . Therefore, you should always be careful not to give your variables the same name as built-in functions or values.

TIP! Now that you know how to assign variables, it is important that you learn to never leave unassigned commands. An unassigned command is an operation that has a result, but that result is not assigned to a variable. For example, you should never use 2+2 . You should instead assign it to some variable x=2+2 . This allows you to “hold on” to the results of previous commands and will make your interaction with Python must less confusing.

You can clear a variable from the notebook using the del function. Typing del x will clear the variable x from the workspace. If you want to remove all the variables in the notebook, you can use the magic command %reset .

In mathematics, variables are usually associated with unknown numbers; in programming, variables are associated with a value of a certain type. There are many data types that can be assigned to variables. A data type is a classification of the type of information that is being stored in a variable. The basic data types that you will utilize throughout this book are boolean, int, float, string, list, tuple, dictionary, set. A formal description of these data types is given in the following sections.

Python Variables

In Python, a variable is a container that stores a value. In other words, variable is the name given to a value, so that it becomes easy to refer a value later on.

Unlike C# or Java, it's not necessary to explicitly define a variable in Python before using it. Just assign a value to a variable using the = operator e.g. variable_name = value . That's it.

The following creates a variable with the integer value.

In the above example, we declared a variable named num and assigned an integer value 10 to it. Use the built-in print() function to display the value of a variable on the console or IDLE or REPL .

In the same way, the following declares variables with different types of values.

Multiple Variables Assignment

You can declare multiple variables and assign values to each variable in a single statement, as shown below.

In the above example, the first int value 10 will be assigned to the first variable x, the second value to the second variable y, and the third value to the third variable z. Assignment of values to variables must be in the same order in they declared.

You can also declare different types of values to variables in a single statement separated by a comma, as shown below.

Above, the variable x stores 10 , y stores a string 'Hello' , and z stores a boolean value True . The type of variables are based on the types of assigned value.

Assign a value to each individual variable separated by a comma will throw a syntax error, as shown below.

Variables in Python are objects. A variable is an object of a class based on the value it stores. Use the type() function to get the class name (type) of a variable.

In the above example, num is an object of the int class that contains integre value 10 . In the same way, amount is an object of the float class, greet is an object of the str class, isActive is an object of the bool class.

Unlike other programming languages like C# or Java, Python is a dynamically-typed language, which means you don't need to declare a type of a variable. The type will be assigned dynamically based on the assigned value.

The + operator sums up two int variables, whereas it concatenates two string type variables.

Object's Identity

Each object in Python has an id. It is the object's address in memory represented by an integer value. The id() function returns the id of the specified object where it is stored, as shown below.

Variables with the same value will have the same id.

Thus, Python optimize memory usage by not creating separate objects if they point to same value.

Naming Conventions

Any suitable identifier can be used as a name of a variable, based on the following rules:

• The name of the variable should start with either an alphabet letter (lower or upper case) or an underscore (_), but it cannot start with a digit.
• More than one alpha-numeric characters or underscores may follow.
• The variable name can consist of alphabet letter(s), number(s) and underscore(s) only. For example, myVar , MyVar , _myVar , MyVar123 are valid variable names, but m*var , my-var , 1myVar are invalid variable names.
• Variable names in Python are case sensitive. So, NAME , name , nAME , and nAmE are treated as different variable names.
• Variable names cannot be a reserved keywords in Python.
• Compare strings in Python
• Convert file data to list
• Convert User Input to a Number
• Convert String to Datetime in Python
• How to call external commands in Python?
• How to count the occurrences of a list item?
• How to flatten list in Python?
• How to merge dictionaries in Python?
• How to pass value by reference in Python?
• Remove duplicate items from list in Python
• More Python articles

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Variables & Assignment 

There are reading-comprehension exercises included throughout the text. These are meant to help you put your reading to practice. Solutions for the exercises are included at the bottom of this page.

Variables permit us to write code that is flexible and amendable to repurpose. Suppose we want to write code that logs a student’s grade on an exam. The logic behind this process should not depend on whether we are logging Brian’s score of 92% versus Ashley’s score of 94%. As such, we can utilize variables, say name and grade , to serve as placeholders for this information. In this subsection, we will demonstrate how to define variables in Python.

In Python, the = symbol represents the “assignment” operator. The variable goes to the left of = , and the object that is being assigned to the variable goes to the right:

Attempting to reverse the assignment order (e.g. 92 = name ) will result in a syntax error. When a variable is assigned an object (like a number or a string), it is common to say that the variable is a reference to that object. For example, the variable name references the string "Brian" . This means that, once a variable is assigned an object, it can be used elsewhere in your code as a reference to (or placeholder for) that object:

Valid Names for Variables 

A variable name may consist of alphanumeric characters ( a-z , A-Z , 0-9 ) and the underscore symbol ( _ ); a valid name cannot begin with a numerical value.

var : valid

_var2 : valid

ApplePie_Yum_Yum : valid

2cool : invalid (begins with a numerical character)

I.am.the.best : invalid (contains . )

They also cannot conflict with character sequences that are reserved by the Python language. As such, the following cannot be used as variable names:

for , while , break , pass , continue

in , is , not

if , else , elif

def , class , return , yield , raises

import , from , as , with

try , except , finally

There are other unicode characters that are permitted as valid characters in a Python variable name, but it is not worthwhile to delve into those details here.

Mutable and Immutable Objects 

The mutability of an object refers to its ability to have its state changed. A mutable object can have its state changed, whereas an immutable object cannot. For instance, a list is an example of a mutable object. Once formed, we are able to update the contents of a list - replacing, adding to, and removing its elements.

To spell out what is transpiring here, we:

Create (initialize) a list with the state [1, 2, 3] .

Assign this list to the variable x ; x is now a reference to that list.

Using our referencing variable, x , update element-0 of the list to store the integer -4 .

This does not create a new list object, rather it mutates our original list. This is why printing x in the console displays [-4, 2, 3] and not [1, 2, 3] .

A tuple is an example of an immutable object. Once formed, there is no mechanism by which one can change of the state of a tuple; and any code that appears to be updating a tuple is in fact creating an entirely new tuple.

Mutable & Immutable Types of Objects 

The following are some common immutable and mutable objects in Python. These will be important to have in mind as we start to work with dictionaries and sets.

Some immutable objects

numbers (integers, floating-point numbers, complex numbers)

“frozen”-sets

Some mutable objects

dictionaries

NumPy arrays

Referencing a Mutable Object with Multiple Variables 

It is possible to assign variables to other, existing variables. Doing so will cause the variables to reference the same object:

What this entails is that these common variables will reference the same instance of the list. Meaning that if the list changes, all of the variables referencing that list will reflect this change:

We can see that list2 is still assigned to reference the same, updated list as list1 :

In general, assigning a variable b to a variable a will cause the variables to reference the same object in the system’s memory, and assigning c to a or b will simply have a third variable reference this same object. Then any change (a.k.a mutation ) of the object will be reflected in all of the variables that reference it ( a , b , and c ).

Of course, assigning two variables to identical but distinct lists means that a change to one list will not affect the other:

Reading Comprehension: Does slicing a list produce a reference to that list?

Suppose x is assigned a list, and that y is assigned a “slice” of x . Do x and y reference the same list? That is, if you update part of the subsequence common to x and y , does that change show up in both of them? Write some simple code to investigate this.

Reading Comprehension: Understanding References

Based on our discussion of mutable and immutable objects, predict what the value of y will be in the following circumstance:

Reading Comprehension Exercise Solutions: 

Does slicing a list produce a reference to that list?: Solution

Based on the following behavior, we can conclude that slicing a list does not produce a reference to the original list. Rather, slicing a list produces a copy of the appropriate subsequence of the list:

Understanding References: Solutions

Integers are immutable, thus x must reference an entirely new object ( 9 ), and y still references 3 .

Python Variables – The Complete Beginner's Guide

Variables are an essential part of Python. They allow us to easily store, manipulate, and reference data throughout our projects.

This article will give you all the understanding of Python variables you need to use them effectively in your projects.

If you want the most convenient way to review all the topics covered here, I've put together a helpful cheatsheet for you right here:

Download the Python variables cheatsheet (it takes 5 seconds).

What is a Variable in Python?

So what are variables and why do we need them?

Variables are essential for holding onto and referencing values throughout our application. By storing a value into a variable, you can reuse it as many times and in whatever way you like throughout your project.

You can think of variables as boxes with labels, where the label represents the variable name and the content of the box is the value that the variable holds.

In Python, variables are created the moment you give or assign a value to them.

How Do I Assign a Value to a Variable?

Assigning a value to a variable in Python is an easy process.

You simply use the equal sign = as an assignment operator, followed by the value you want to assign to the variable. Here's an example:

In this example, we've created two variables: country and year_founded. We've assigned the string value "United States" to the country variable and integer value 1776 to the year_founded variable.

There are two things to note in this example:

• Variables in Python are case-sensitive . In other words, watch your casing when creating variables, because Year_Founded will be a different variable than year_founded even though they include the same letters
• Variable names that use multiple words in Python should be separated with an underscore _ . For example, a variable named "site name" should be written as "site name" ._ This convention is called snake case (very fitting for the "Python" language).

How Should I Name My Python Variables?

There are some rules to follow when naming Python variables.

Some of these are hard rules that must be followed, otherwise your program will not work, while others are known as conventions . This means, they are more like suggestions.

Variable naming rules

• Variable names must start with a letter or an underscore _ character.
• Variable names can only contain letters, numbers, and underscores.
• Variable names cannot contain spaces or special characters.

Variable naming conventions

• Variable names should be descriptive and not too short or too long.
• Use lowercase letters and underscores to separate words in variable names (known as "snake_case").

What Data Types Can Python Variables Hold?

One of the best features of Python is its flexibility when it comes to handling various data types.

Python variables can hold various data types, including integers, floats, strings, booleans, tuples and lists:

Integers are whole numbers, both positive and negative.

Floats are real numbers or numbers with a decimal point.

Strings are sequences of characters, namely words or sentences.

Booleans are True or False values.

Lists are ordered, mutable collections of values.

Tuples are ordered, immutable collections of values.

There are more data types in Python, but these are the most common ones you will encounter while working with Python variables.

Python is Dynamically Typed

Python is what is known as a dynamically-typed language. This means that the type of a variable can change during the execution of a program.

Another feature of dynamic typing is that it is not necessary to manually declare the type of each variable, unlike other programming languages such as Java.

You can use the type() function to determine the type of a variable. For instance:

What Operations Can Be Performed?

Variables can be used in various operations, which allows us to transform them mathematically (if they are numbers), change their string values through operations like concatenation, and compare values using equality operators.

Mathematic Operations

It's possible to perform basic mathematic operations with variables, such as addition, subtraction, multiplication, and division:

It's also possible to find the remainder of a division operation by using the modulus % operator as well as create exponents using the ** syntax:

String operators

Strings can be added to one another or concatenated using the + operator.

Equality comparisons

Values can also be compared in Python using the < , > , == , and != operators.

These operators, respectively, compare whether values are less than, greater than, equal to, or not equal to each other.

Finally, note that when performing operations with variables, you need to ensure that the types of the variables are compatible with each other.

For example, you cannot directly add a string and an integer. You would need to convert one of the variables to a compatible type using a function like str() or [int()](https://www.freecodecamp.org/news/python-string-to-int-convert-a-string-example/) .

Variable Scope

The scope of a variable refers to the parts of a program where the variable can be accessed and modified. In Python, there are two main types of variable scope:

Global scope : Variables defined outside of any function or class have a global scope. They can be accessed and modified throughout the program, including within functions and classes.

Local scope : Variables defined within a function or class have a local scope. They can only be accessed and modified within that function or class.

In this example, attempting to access local_var outside of the function_with_local_var function results in a NameError , as the variable is not defined in the global scope.

Don't be afraid to experiment with different types of variables, operations, and scopes to truly grasp their importance and functionality. The more you work with Python variables, the more confident you'll become in applying these concepts.

Finally, if you want to fully learn all of these concepts, I've put together for you a super helpful cheatsheet that summarizes everything we've covered here.

Just click the link below to grab it for free. Enjoy!

Download the Python variables cheatsheet

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Python for absolute beginners, next episode, variables and assignment.

Overview Teaching: 15 min Exercises: 15 min Questions How can I store data in programs? Objectives Write scripts that assign values to variables and perform calculations with those values. Correctly trace value changes in scripts that use assignment.

Use variables to store values

Variables are one of the fundamental building blocks of Python. A variable is like a tiny container where you store values and data, such as filenames, words, numbers, collections of words and numbers, and more.

The variable name will point to a value that you “assign” it. You might think about variable assignment like putting a value “into” the variable, as if the variable is a little box 🎁

(In fact, a variable is not a container as such but more like an adress label that points to a container with a given value. This difference will become relevant once we start talking about lists and mutable data types.)

You assign variables with an equals sign ( = ). In Python, a single equals sign = is the “assignment operator.” (A double equals sign == is the “real” equals sign.)

• Variables are names for values.
• In Python the = symbol assigns the value on the right to the name on the left.
• The variable is created when a value is assigned to it.
• Here, Python assigns an age to a variable age and a name in quotation marks to a variable first_name :

Variable names

Variable names can be as long or as short as you want, but there are certain rules you must follow.

• Cannot start with a digit.
• Cannot contain spaces, quotation marks, or other punctuation.
• May contain an underscore (typically used to separate words in long variable names).
• Having an underscore at the beginning of a variable name like _alistairs_real_age has a special meaning. So we won’t do that until we understand the convention.
• The standard naming convention for variable names in Python is the so-called “snake case”, where each word is separated by an underscore. For example my_first_variable . You can read more about naming conventions in Python here .

Use meaningful variable names

Python doesn’t care what you call variables as long as they obey the rules (alphanumeric characters and the underscore). As you start to code, you will almost certainly be tempted to use extremely short variables names like f . Your fingers will get tired. Your coffee will wear off. You will see other people using variables like f . You’ll promise yourself that you’ll definitely remember what f means. But you probably won’t.

So, resist the temptation of bad variable names! Clear and precisely-named variables will:

• Make your code more readable (both to yourself and others).
• Reinforce your understanding of Python and what’s happening in the code.
• Clarify and strengthen your thinking.

Use meaningful variable names to help other people understand what the program does. The most important “other person” is your future self!

Python is case-sensitive

Python thinks that upper- and lower-case letters are different, so Name and name are different variables. There are conventions for using upper-case letters at the start of variable names so we will use lower-case letters for now.

Off-Limits Names

The only variable names that are off-limits are names that are reserved by, or built into, the Python programming language itself — such as print , True , and list . Some of these you can overwrite into variable names (not ideal!), but Jupyter Lab (and many other environments and editors) will catch this by colour coding your variable. If your would-be variable is colour-coded green, rethink your name choice. This is not something to worry too much about. You can get the object back by resetting your kernel.

Use print() to display values

We can check to see what’s “inside” variables by running a cell with the variable’s name. This is one of the handiest features of a Jupyter notebook. Outside the Jupyter environment, you would need to use the print() function to display the variable.

You can run the print() function inside the Jupyter environment, too. This is sometimes useful because Jupyter will only display the last variable in a cell, while print() can display multiple variables. Additionally, Jupyter will display text with \n characters (which means “new line”), while print() will display the text appropriately formatted with new lines.

• Python has a built-in function called print() that prints things as text.
• Provide values to the function (i.e., the things to print) in parentheses.
• To add a string to the printout, wrap the string in single or double quotations.
• The values passed to the function are called ‘arguments’ and are separated by commas.
• When using the print() function, we can also separate with a ‘+’ sign. However, when using ‘+’ we have to add spaces in between manually.
• print() automatically puts a single space between items to separate them.
• And wraps around to a new line at the end.

Variables must be created before they are used

If a variable doesn’t exist yet, or if the name has been misspelled, Python reports an error (unlike some languages, which “guess” a default value).

The last line of an error message is usually the most informative. This message lets us know that there is no variable called eye_color in the script.

Variables Persist Between Cells Variables defined in one cell exist in all other cells once executed, so the relative location of cells in the notebook do not matter (i.e., cells lower down can still affect those above). Notice the number in the square brackets [ ] to the left of the cell. These numbers indicate the order, in which the cells have been executed. Cells with lower numbers will affect cells with higher numbers as Python runs the cells chronologically. As a best practice, we recommend you keep your notebook in chronological order so that it is easier for the human eye to read and make sense of, as well as to avoid any errors if you close and reopen your project, and then rerun what you have done. Remember: Notebook cells are just a way to organize a program! As far as Python is concerned, all of the source code is one long set of instructions.

Variables can be used in calculations

• We can use variables in calculations just as if they were values. Remember, we assigned 42 to age a few lines ago.

This code works in the following way. We are reassigning the value of the variable age by taking its previous value (42) and adding 3, thus getting our new value of 45.

Use an index to get a single character from a string

• The characters (individual letters, numbers, and so on) in a string are ordered. For example, the string ‘AB’ is not the same as ‘BA’. Because of this ordering, we can treat the string as a list of characters.
• Each position in the string (first, second, etc.) is given a number. This number is called an index or sometimes a subscript.
• Indices are numbered from 0 rather than 1.
• Use the position’s index in square brackets to get the character at that position.

Use a slice to get a substring

A part of a string is called a substring. A substring can be as short as a single character. A slice is a part of a string (or, more generally, any list-like thing). We take a slice by using [start:stop] , where start is replaced with the index of the first element we want and stop is replaced with the index of the element just after the last element we want. Mathematically, you might say that a slice selects [start:stop] . The difference between stop and start is the slice’s length. Taking a slice does not change the contents of the original string. Instead, the slice is a copy of part of the original string.

Use the built-in function len() to find the length of a string

The built-in function len() is used to find the length of a string (and later, of other data types, too).

Note that the result is 6 and not 7. This is because it is the length of the value of the variable (i.e. 'helium' ) that is being counted and not the name of the variable (i.e. element )

Also note that nested functions are evaluated from the inside out, just like in mathematics. Thus, Python first reads the len() function, then the print() function.

Choosing a Name Which is a better variable name, m , min , or minutes ? Why? Hint: think about which code you would rather inherit from someone who is leaving the library: ts = m * 60 + s tot_sec = min * 60 + sec total_seconds = minutes * 60 + seconds Solution minutes is better because min might mean something like “minimum” (and actually does in Python, but we haven’t seen that yet).

Swapping Values Draw a table showing the values of the variables in this program after each statement is executed. In simple terms, what do the last three lines of this program do? x = 1.0 y = 3.0 swap = x x = y y = swap Solution swap = x # x->1.0 y->3.0 swap->1.0 x = y # x->3.0 y->3.0 swap->1.0 y = swap # x->3.0 y->1.0 swap->1.0 These three lines exchange the values in x and y using the swap variable for temporary storage. This is a fairly common programming idiom.

Predicting Values What is the final value of position in the program below? (Try to predict the value without running the program, then check your prediction.) initial = "left" position = initial initial = "right" Solution initial = "left" # Initial is assigned the string "left" position = initial # Position is assigned the variable initial, currently "left" initial = "right" # Initial is assigned the string "right" print(position) left The last assignment to position was “left”

Can you slice integers? If you assign a = 123 , what happens if you try to get the second digit of a ? Solution Numbers are not stored in the written representation, so they can’t be treated like strings. a = 123 print(a[1]) TypeError: 'int' object is not subscriptable

Slicing What does the following program print? library_name = 'social sciences' print('library_name[1:3] is:', library_name[1:3]) If thing is a variable name, low is a low number, and high is a high number: What does thing[low:high] do? What does thing[low:] (without a value after the colon) do? What does thing[:high] (without a value before the colon) do? What does thing[:] (just a colon) do? What does thing[number:negative-number] do? Solution library_name[1:3] is: oc It will slice the string, starting at the low index and ending an element before the high index It will slice the string, starting at the low index and stopping at the end of the string It will slice the string, starting at the beginning on the string, and ending an element before the high index It will print the entire string It will slice the string, starting the number index, and ending a distance of the absolute value of negative-number elements from the end of the string

Key Points Use variables to store values. Use meaningful variable names. Python is case-sensitive. Use print() to display values. Variables must be created before they are used. Variables persist between cells. Variables can be used in calculations. Use an index to get a single character from a string. Use a slice to get a substring. Use the built-in function len to find the length of a string.

Variable Assignment

• Discussion (8)

Think of a variable as a name attached to a particular object . In Python, variables need not be declared or defined in advance, as is the case in many other programming languages. To create a variable, you just assign it a value and then start using it. Assignment is done with a single equals sign ( = ).

00:00 Welcome to this first section, where we’ll talk about variable assignments in Python. First of all, I just want to mention that I’m going to be using the IPython shell.

00:09 The reason for that is just that it adds a bit of colors to the prompt and makes it a bit easier to see which types we’re working with, but you don’t need to do this install that I’m going to show you in just a second.

00:19 You can just use your normal Python interpreter and it’s going to work all the same. If you want to install IPython, all you need to do is go to your terminal and type pip3 install ipython , press Enter, and then wait until it installs.

00:36 I already got it installed. And then instead of typing python to get into the interpreter, you’re going to type ipython .

00:46 It gives us the same functionality, only you see there’s some colors involved and it looks a bit nicer. I can do clear and clear my screen. So it’s going to make it a bit easier for you to follow, but that’s all.

00:58 So, first stop: a standard variable assignment in Python. Unlike other languages, in Python this is very simple. We don’t need to declare a variable, all we need to do is give it a name, put the equal sign ( = ) and then the value that we want to assign. That’s it.

01:15 That’s a variable assignment in Python. I just assigned the value 300 to the variable n . So now I can print(n) and get the result.

01:26 Or, since I’m in an interpreter session, I can just put in n and it shows me that the output is going to be 300 . So, that’s the basic, standard variable assignment that you’re going to do many times in Python.

01:38 And it’s nice that you don’t need to declare the variable before. You simply can type it in like this. Now the variable n is referring to the value 300 .

01:48 What happens if I change it? So, I don’t need to stick with 300 through the lifetime of this variable. I can just change it to something else. I can say “Now this is this going to be 400 .”

02:00 Or, in Python, not even the type is fixed, so I can say n = "hello" and change it to a string.

02:10 And this is still all working fine. So you see, it feels very fluid, and this is because Python is a dynamically-typed language, so we don’t need to define types and define variables beforehand and then they’re unchangeable for the rest of the program—but it’s fluid, and we can just start off with n being an integer of the value of 300 and through the lifetime of the program, it can take on a couple of different identities.

02:36 So, apart from the standard variable assignment that we just saw before, n = 300 , we can also use a chained assignment in Python, which makes it quick to assign a couple of variables to the same value.

02:49 And that looks like this.

02:52 I can say n = m = x = y et cetera, and then give it a value. And now all of those variable names point to 400 , so I can say m is 400 , x is 400 , y is 400 , et cetera. That’s what is called a chained assignment in Python.

03:15 Another way is the multiple assignment statement, or multiple assignments, which works a little bit different and there’s something you need to take care of, but I still want to introduce you to it. If you go ahead here, I can assign two values at the same time in one line.

03:32 So I can say a, b = 300, 400 . The comma ( , ) is important, and it’s important that the amount of variables that you’re declaring here on the left side is the same amount of values that you have on the right side.

03:48 I can do this, and now b points to 400 , a points to 300 .

03:54 It doesn’t have to be two, there can be more, but just make sure that every time if you use this multiple assignment statement, that the amount of variables you use left is the same as the amount of values on the right. And as a last point in this section, I want to talk a little bit about variable types.

04:14 I already mentioned that variable types don’t have to be fixed in Python. I can start off with

04:21 n pointing to 300 , which as we know is an integer. Remember, you can always check what the type of a variable is by just saying type() and passing in the variable in there.

04:33 So it gives me as an output that this is an int (integer).

04:37 This is just the same as saying “What’s the type() of 300 or 200 ?” directly— it’s an integer—because all that I’m passing in here is a reference to this object. We’ll talk about this more in the next section.

04:52 But now I can easily change the type of this variable, because all I’m doing is pointing it to a different object. So now n is pointing to a string.

05:01 If I say type(n) now, it will tell me it’s a str (string).

05:08 And the reason for this is that variables in Python are simply references to objects. In the next section, we’ll talk much more what’s important about that and how in Python everything is an object.

05:19 And that it for this section! Let’s do a quick recap. Variable assignments in Python are pretty straightforward. We have the standard variable assignment that just goes <variable name> = <your value> .

05:32 We have another way of doing it that’s called chained assignments, where we can assign a couple of variable names to the same value by just using multiple equal signs.

05:43 Then there’s the multiple assignment statement, which works a little differently, and you have to take care to use commas and the same amount of variable names on the left side as values on the right side.

05:53 It’s going to assign, as expected, n to 300 , m to 400 . And then finally, we talked about variable types, that they are fluid in Python and that you can check what the variable type is by using the type() function.

06:07 And here’s a nice thing to see also, that n is just a pointer to the 300 integer, because we’re going to get the same result if we say type(n) or type(300) .

06:18 They’re both int (integer) objects. And this is a concept that we’re going to talk about more in the upcoming section when we talk about object references. See you over there.

iamscottdavis on Dec. 10, 2019

I installed ipython on my chromebook but it won’t run.

Martin Breuss RP Team on Dec. 10, 2019

You might have to close and re-open your terminal @iamscottdavis

Geir Arne Hjelle RP Team on Dec. 10, 2019

I’ve recently had some weird issues with prompt_toolkit , one of the dependencies of IPython. Maybe that’s what you’re running into?

I got a cryptic error message like TypeError: __init__() got an unexpected keyword argument 'inputhook' . If this is your problem as well, the best solution should be to update to IPython >= 7.10 which should have fixed this. Another workaround is to downgrade prompt_toolkit to version 2.

See some discussion on github.com/ipython/ipython/issues/11975

If you are having other problems, feel free to post your error messages :)

kiran on July 18, 2020

if i declare the variable in any one loop in python.

now my question is a is local/global variable? in C it is local variable but what about python? in Python even i declare a variable with in the loop it become a global variable?

Martin Breuss RP Team on July 18, 2020

In Python, it will keep the last value it got assigned within the loop also outside in the global scope. That is why a is still accessible and has a value in your example, also outside of the loop’s scope.

DoubleA on Jan. 20, 2021

Hey Martin,

Thanks for pulling this stuff together and explaining it so clearly. I came accross some sort of a variation of the multiple assignment you discussed. Basically, it seems that the number of variable names and variables can be diffrent. What I mean is this:

then print(a,b,c) gives me the following output:

Am I right saying that, basically, what happens above is that the variable c having an asterisk before it will get assigned a list of the two values, incl. the excessive (‘string’) one?

Referring back to Kiran’s question:

When I run this code:

I see that the globals() function returns, amongst other things, the value of the variables a , b and the last value of the iterable elem . Both variables a and b appear to be visible in the global scope as key-value pairs of the following dict:

Bartosz Zaczyński RP Team on Jan. 21, 2021

@DoubleA The “starred” expression syntax you were referring to before can be used for extended iterable unpacking .

Become a Member to join the conversation.

Leon Lovett

Python Variables – A Guide to Variable Assignment and Naming

In Python, variables are essential elements that allow developers to store and manipulate data. When writing Python code, understanding variable assignment and naming conventions is crucial for effective programming.

Python variables provide a way to assign a name to a value and use that name to reference the value later in the code. Variables can be used to store various types of data, including numbers, strings, and lists.

In this article, we will explore the basics of Python variables, including variable assignment and naming conventions. We will also dive into the different variable types available in Python and how to use them effectively.

Key Takeaways

• Python variables are used to store and manipulate data in code.
• Variable assignment allows developers to assign a name to a value and reference it later.
• Proper variable naming conventions are essential for effective programming.
• Python supports different variable types, including integers, floats, strings, and lists.

Variable Assignment in Python

Python variables are created when a value is assigned to them using the equals sign (=) operator. For example, the following code snippet assigns the integer value 5 to the variable x :

From this point forward, whenever x is referenced in the code, it will have the value 5.

Variables can also be assigned using other variables or expressions. For example, the following code snippet assigns the value of x plus 2 to the variable y :

It is important to note that variables in Python are dynamically typed, meaning that their type can change as the program runs. For example, the following code snippet assigns a string value to the variable x , then later reassigns it to an integer value:

x = "hello" x = 7

Common Mistakes in Variable Assignment

One common mistake is trying to reference a variable before it has been assigned a value. This will result in a NameError being raised. For example:

print(variable_name) NameError: name ‘variable_name’ is not defined

Another common mistake is assigning a value to the wrong variable name. For example, the following code snippet assigns the value 5 to the variable y instead of x :

y = 5 print(x) NameError: name ‘x’ is not defined

To avoid these mistakes, it is important to carefully review code and double-check variable names and values.

Using Variables in Python

Variables are used extensively in Python code for a variety of purposes, from storing user input to performing complex calculations. The following code snippet demonstrates the basic usage of variables in a simple addition program:

number1 = input("Enter the first number: ") number2 = input("Enter the second number: ") sum = float(number1) + float(number2) print("The sum of", number1, "and", number2, "is", sum)

This program prompts the user to enter two numbers, converts them to floats using the float() function, adds them together, and prints the result using the print() function.

Variables can also be used in more complex operations, such as string concatenation and list manipulation. The following code snippet demonstrates how variables can be used to combine two strings:

greeting = "Hello" name = "Alice" message = greeting + ", " + name + "!" print(message)

This program defines two variables containing a greeting and a name, concatenates them using the plus (+) operator, and prints the result.

Variable Naming Conventions in Python

In Python, proper variable naming conventions are crucial for writing clear and maintainable code. Consistently following naming conventions makes code more readable and easier to understand, especially when working on large projects with many collaborators. Here are some commonly accepted conventions:

It’s recommended to use lowercase or snake case for variable names as they are easier to read and more commonly used in Python. Camel case is common in other programming languages, but can make Python code harder to read.

Variable names should be descriptive and meaningful. Avoid using abbreviations or single letters, unless they are commonly understood, like “i” for an iterative variable in a loop. Using descriptive names will make your code easier to understand and maintain by you and others.

Lastly, it’s a good practice to avoid naming variables with reserved words in Python such as “and”, “or”, and “not”. Using reserved words can cause errors in your code, making it hard to debug.

Scope of Variables in Python

Variables in Python have a scope, which dictates where they can be accessed and used within a code block. Understanding variable scope is important for writing efficient and effective code.

Local Variables in Python

A local variable is created within a particular code block, such as a function. It can only be accessed within that block and is destroyed when the block is exited. Local variables can be defined using the same Python variable assignment syntax as any other variable.

Example: def my_function():     x = 10     print(“Value inside function:”, x) my_function() print(“Value outside function:”, x) Output: Value inside function: 10 NameError: name ‘x’ is not defined

In the above example, the variable ‘x’ is a local variable that is defined within the function ‘my_function()’. It cannot be accessed outside of that function, which is why the second print statement results in an error.

Global Variables in Python

A global variable is a variable that can be accessed from anywhere within a program. These variables are typically defined outside of any code block, at the top level of the program. They can be accessed and modified from any code block within the program.

Example: x = 10 def my_function():     print(“Value inside function:”, x) my_function() print(“Value outside function:”, x) Output: Value inside function: 10 Value outside function: 10

In the above example, the variable ‘x’ is a global variable that is defined outside of any function. It can be accessed from within the ‘my_function()’ as well as from outside it.

When defining a function, it is possible to access and modify a global variable from within the function using the ‘global’ keyword.

Example: x = 10 def my_function():     global x     x = 20 my_function() print(x) Output: 20

In the above example, the ‘global’ keyword is used to indicate that the variable ‘x’ inside the function is the same as the global variable ‘x’. The function modifies the global variable, causing the final print statement to output ’20’ instead of ’10’.

One of the most fundamental concepts in programming is the use of variables. In Python, variables allow us to store and manipulate data efficiently. Here are some practical examples of how to use variables in Python:

Mathematical Calculations

Variables are often used to perform mathematical calculations in Python. For instance, we can assign numbers to variables and then perform operations on those variables. Here’s an example:

x = 5 y = 10 z = x + y print(z) # Output: 15

In this code, we have assigned the value 5 to the variable x and the value 10 to the variable y. We then create a new variable z by adding x and y together. Finally, we print the value of z, which is 15.

String Manipulation

Variables can also be used to manipulate strings in Python. Here is an example:

first_name = “John” last_name = “Doe” full_name = first_name + ” ” + last_name print(full_name) # Output: John Doe

In this code, we have assigned the strings “John” and “Doe” to the variables first_name and last_name respectively. We then create a new variable full_name by combining the values of first_name and last_name with a space in between. Finally, we print the value of full_name, which is “John Doe”.

Working with Data Structures

Variables are also essential when working with data structures such as lists and dictionaries in Python. Here’s an example:

numbers = [1, 2, 3, 4, 5] sum = 0 for num in numbers:     sum += num print(sum) # Output: 15

In this code, we have assigned a list of numbers to the variable numbers. We then create a new variable sum and initialize it to 0. We use a for loop to iterate over each number in the list, adding it to the sum variable. Finally, we print the value of sum, which is 15.

As you can see, variables are an essential tool in Python programming. By using them effectively, you can manipulate data and perform complex operations with ease.

Variable Types in Python

Python is a dynamically typed language, which means that variables can be assigned values of different types without explicit type declaration. Python supports a wide range of variable types, each with its own unique characteristics and uses.

Numeric Types:

Python supports several numeric types, including integers, floats, and complex numbers. Integers are whole numbers without decimal points, while floats are numbers with decimal points. Complex numbers consist of a real and imaginary part, expressed as a+bi.

Sequence Types:

Python supports several sequence types, including strings, lists, tuples, and range objects. Strings are sequences of characters, while lists and tuples are sequences of values of any type. Range objects are used to represent sequences of numbers.

Mapping Types:

Python supports mapping types, which are used to store key-value pairs. The most commonly used mapping type is the dictionary, which supports efficient lookup of values based on their associated keys.

Boolean Type:

Python supports a Boolean type, which is used to represent truth values. The Boolean type has two possible values: True and False.

Python has a special value called None, which represents the absence of a value. This type is often used to indicate the result of functions that do not return a value.

Understanding the different variable types available in Python is essential for effective coding. Each type has its own unique properties and uses, and choosing the right type for a given task can help improve code clarity, efficiency, and maintainability.

Python variables are a fundamental concept that every aspiring Python programmer must understand. In this article, we have covered the basics of variable assignment and naming conventions in Python. We have also explored the scope of variables and their different types.

It is important to remember that variables play a crucial role in programming, and their effective use can make your code more efficient and easier to read. Proper naming conventions and good coding practices can also help prevent errors and improve maintainability.

As you continue to explore the vast possibilities of Python programming, we encourage you to practice using variables in your code. With a solid understanding of Python variables, you will be well on your way to becoming a proficient Python programmer.

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1.6. Variables and Assignment ¶

Each set-off line in this section should be tried in the Shell.

Nothing is displayed by the interpreter after this entry, so it is not clear anything happened. Something has happened. This is an assignment statement , with a variable , width , on the left. A variable is a name for a value. An assignment statement associates a variable name on the left of the equal sign with the value of an expression calculated from the right of the equal sign. Enter

Once a variable is assigned a value, the variable can be used in place of that value. The response to the expression width is the same as if its value had been entered.

The interpreter does not print a value after an assignment statement because the value of the expression on the right is not lost. It can be recovered if you like, by entering the variable name and we did above.

Try each of the following lines:

The equal sign is an unfortunate choice of symbol for assignment, since Python’s usage is not the mathematical usage of the equal sign. If the symbol ↤ had appeared on keyboards in the early 1990’s, it would probably have been used for assignment instead of =, emphasizing the asymmetry of assignment. In mathematics an equation is an assertion that both sides of the equal sign are already, in fact, equal . A Python assignment statement forces the variable on the left hand side to become associated with the value of the expression on the right side. The difference from the mathematical usage can be illustrated. Try:

so this is not equivalent in Python to width = 10 . The left hand side must be a variable, to which the assignment is made. Reversed, we get a syntax error . Try

This is, of course, nonsensical as mathematics, but it makes perfectly good sense as an assignment, with the right-hand side calculated first. Can you figure out the value that is now associated with width? Check by entering

In the assignment statement, the expression on the right is evaluated first . At that point width was associated with its original value 10, so width + 5 had the value of 10 + 5 which is 15. That value was then assigned to the variable on the left ( width again) to give it a new value. We will modify the value of variables in a similar way routinely.

Assignment and variables work equally well with strings. Try:

Try entering:

Note the different form of the error message. The earlier errors in these tutorials were syntax errors: errors in translation of the instruction. In this last case the syntax was legal, so the interpreter went on to execute the instruction. Only then did it find the error described. There are no quotes around fred , so the interpreter assumed fred was an identifier, but the name fred was not defined at the time the line was executed.

It is both easy to forget quotes where you need them for a literal string and to mistakenly put them around a variable name that should not have them!

Try in the Shell :

There fred , without the quotes, makes sense.

There are more subtleties to assignment and the idea of a variable being a “name for” a value, but we will worry about them later, in Issues with Mutable Objects . They do not come up if our variables are just numbers and strings.

Autocompletion: A handy short cut. Idle remembers all the variables you have defined at any moment. This is handy when editing. Without pressing Enter, type into the Shell just

Assuming you are following on the earlier variable entries to the Shell, you should see f autocompleted to be

This is particularly useful if you have long identifiers! You can press Alt-/ several times if more than one identifier starts with the initial sequence of characters you typed. If you press Alt-/ again you should see fred . Backspace and edit so you have fi , and then and press Alt-/ again. You should not see fred this time, since it does not start with fi .

1.6.1. Literals and Identifiers ¶

Expressions like 27 or 'hello' are called literals , coming from the fact that they literally mean exactly what they say. They are distinguished from variables, whose value is not directly determined by their name.

The sequence of characters used to form a variable name (and names for other Python entities later) is called an identifier . It identifies a Python variable or other entity.

There are some restrictions on the character sequence that make up an identifier:

The characters must all be letters, digits, or underscores _ , and must start with a letter. In particular, punctuation and blanks are not allowed.

There are some words that are reserved for special use in Python. You may not use these words as your own identifiers. They are easy to recognize in Idle, because they are automatically colored orange. For the curious, you may read the full list:

There are also identifiers that are automatically defined in Python, and that you could redefine, but you probably should not unless you really know what you are doing! When you start the editor, we will see how Idle uses color to help you know what identifies are predefined.

Python is case sensitive: The identifiers last , LAST , and LaSt are all different. Be sure to be consistent. Using the Alt-/ auto-completion shortcut in Idle helps ensure you are consistent.

What is legal is distinct from what is conventional or good practice or recommended. Meaningful names for variables are important for the humans who are looking at programs, understanding them, and revising them. That sometimes means you would like to use a name that is more than one word long, like price at opening , but blanks are illegal! One poor option is just leaving out the blanks, like priceatopening . Then it may be hard to figure out where words split. Two practical options are

• underscore separated: putting underscores (which are legal) in place of the blanks, like price_at_opening .
• using camel-case : omitting spaces and using all lowercase, except capitalizing all words after the first, like priceAtOpening

Use the choice that fits your taste (or the taste or convention of the people you are working with).

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Python Programming

Python Variables

Updated on:  August 31, 2021 | 19 Comments

A variable is a reserved memory area (memory address) to store value . For example, we want to store an employee’s salary. In such a case, we can create a variable and store salary using it. Using that variable name, you can read or modify the salary amount.

In other words, a variable is a value that varies according to the condition or input pass to the program. Everything in Python is treated as an object so every variable is nothing but an object in Python.

A variable can be either mutable or immutable . If the variable’s value can change, the object is called mutable, while if the value cannot change, the object is called immutable. We will learn the difference between mutable and immutable types in the later section of this article.

Also, Solve :

• Python variables and data type Quiz
• Basic Python exercise for beginners

Table of contents

Creating a variable, changing the value of a variable, integer variable, float variable, complex type, string variable, list type variable, get the data type of variable, delete a variable, variable’s case-sensitive, assigning a value to a constant in python, rules and naming convention for variables and constants, assigning a single value to multiple variables, assigning multiple values to multiple variables, local variable, global variable, object reference, unpack a collection into a variable.

Python programming language is dynamically typed, so there is no need to declare a variable before using it or declare the data type of variable like in other programming languages. The declaration happens automatically when we assign a value to the variable.

Creating a variable and assigning a value

We can assign a value to the variable at that time variable is created. We can use the assignment operator = to assign a value to a variable.

The operand, which is on the left side of the assignment operator, is a variable name. And the operand, which is the right side of the assignment operator, is the variable’s value.

In the above example, “John”, 25, 25800.60 are values that are assigned to name , age , and salary respectively.

Many programming languages are statically typed languages where the variable is initially declared with a specific type, and during its lifetime, it must always have that type.

But in Python, variables are dynamically typed  and not subject to the data type restriction. A variable may be assigned to a value of  one type , and then later, we can also re-assigned a value of a different type . Let’s see the example.

Create Number, String, List variables

We can create different types of variables as per our requirements. Let’s see each one by one.

A number is a data type to store numeric values. The object for the number will be created when we assign a value to the variable. In Python3, we can use the following three data types to store numeric values.

The  int is a data type that returns integer type values (signed integers); they are also called  ints or integers . The integer value can be positive or negative without a decimal point.

Note : We used the built-in Python method type() to check the variable type.

Floats are the values with the decimal point dividing the integer and the fractional parts of the number.  Use float data type to store decimal values.

In the above example, the variable salary assigned to value 10800.55, which is a float value.

The complex is the numbers that come with the real and imaginary part. A complex number is in the form of a+bj, where a and b contain integers or floating-point values.

In Python, a string is a set of characters  represented in quotation marks. Python allows us to define a string in either pair of  single  or  double quotation marks. For example, to store a person’s name we can use a string type.

To retrieve a piece of string from a given string, we can use to slice operator [] or [:] . Slicing provides us the subset of a string with an index starting from index 0 to index end-1.

To concatenate the string, we can use  the addition (+) operator.

If we want to represent  a group of elements (or value) as a single entity, we should go for the list variable type. For example, we can use them to store student names. In the list, the insertion order of elements is preserved. That means, in which order elements are inserted in the list, the order will be intact.

Read : Complete Guide on Python lists

The list can be accessed in two ways, either positive or negative index.  The list has the following characteristics:

• In the list insertion order of elements is preserved.
• Heterogeneous (all types of data types int , float , string ) elements are allowed.
• Duplicates elements are permitted.
• The list is mutable(can change).
• Growable in nature means based on our requirement, we can increase or decrease the list’s size.
• List elements should be enclosed within square brackets [] .

No matter what is stored in a variable (object), a variable can be any type like int , float , str , list , tuple , dict , etc. There is a built-in function called type() to get the data type of any variable.

The type() function has a simple and straightforward syntax.

Syntax of type() :

If we want to get the name of the datatype only as output, then we can use the __name__ attribute along with the type() function. See the following example where __name__ attribute is used.

Use the del keyword to delete the variable. Once we delete the variable, it will not be longer accessible and eligible for the garbage collector.

Now, let’s delete var1 and try to access it again.

Python is a case-sensitive language. If we define a variable with names a = 100 and A =200 then, Python differentiates between a and A . These variables are treated as two different variables (or objects).

Constant is a variable or value that does not change, which means it remains the same and cannot be modified. But in the case of Python, the constant concept is  not applicable . By convention, we can use only uppercase characters to define the constant variable if we don’t want to change it.

 Example

It is just convention, but we can change the value of MAX_VALUE variable.

As we see earlier, in the case of Python, the constant concept is not applicable. But if we still want to implement it, we can do it using the following way.

The declaration and assignment of constant in Python done with the module. Module means Python file ( .py ) which contains variables, functions, and packages.

So let’s create two modules, constant.py  and main.py , respectively.

• In the constant.py file, we will declare two constant variables,  PI and TOTAL_AREA .
• import constant module In main.py file.

To create a constant module write the below code in the constant.py file.

Constants are declared with uppercase later variables and separating the words with an underscore.

Create a  main.py  and write the below code in it.

Note : Constant concept is not available in Python. By convention, we define constants in an uppercase letter to differentiate from variables. But it does not prevent reassignment, which means we can change the value of a constant variable.

A name in a Python program is called an identifier. An identifier can be a variable name, class name, function name, and module name.

There are some rules to define variables in Python.

In Python, there are some conventions and rules to define variables and constants that should follow.

Rule 1 : The name of the variable and constant should have a combination of letters, digits, and underscore symbols.

• Alphabet/letters i.e., lowercase (a to z) or uppercase (A to Z)
• Digits(0 to 9)
• Underscore symbol (_)

Example 

Rule 2 : The variable name and constant name should make sense.

Note: we should always create a meaningful variable name so it will be easy to understand. That is, it should be meaningful.

It above example variable x does not make more sense, but student_name  is a meaningful variable.

Rule 3: Don’t’ use special symbols in a variable name

For declaring variable and constant, we cannot use special symbols like $, #, @, %, !~, etc. If we try to declare names with a special symbol, Python generates an error

Rule 4:  Variable and constant should not start with digit letters.

You will receive an error if you start a variable name with a digit. Let’s verify this using a simple example.

Here Python will generate a syntax error at 1studnet . instead of this, you can declare a variable like studnet_1 = "Jessa"

Rule 5:  Identifiers are case sensitive.

Here, Python makes a difference between these variables that is uppercase and lowercase, so that it will create three different variables total , Total , TOTAL .

Rule 6:  To declare constant should use capital letters.

Rule 6: Use an underscore symbol for separating the words in a variable name

If we want to declare variable and constant names having two words, use an underscore symbol for separating the words.

Multiple assignments

In Python, there is no restriction to declare a variable before using it in the program. Python allows us to create a variable as and when required.

We can do multiple assignments in two ways, either by assigning a single value to multiple variables  or assigning  multiple values to multiple variables .

we can assign a single value to multiple variables simultaneously using the assignment operator = .

Now, let’s create an example to assign the single value 10 to all three variables a , b , and c .

In the above example, two integer values 10 and 70 are assigned to variables roll_no and marks , respectively, and string literal, “Jessa,” is assigned to the variable name .

Variable scope

Scope : The scope of a variable refers to the places where we can access a variable.

Depending on the scope, the variable can categorize into two types  local variable and the global variable.

A local variable is a variable that is accessible inside a block of code only where it is declared. That means, If we declare a variable inside a method, the scope of the local variable is limited to the method only. So it is not accessible from outside of the method. If we try to access it, we will get an error.

In the above example, we created a function with the name test1 . Inside it, we created a local variable price. Similarly, we created another function with the name test2 and tried to access price, but we got an error "price is not defined" because its scope is limited to function test1() . This error occurs because we cannot access the local variable from outside the code block.

A Global variable is a variable that is defined outside of the method (block of code). That is accessible anywhere in the code file.

In the above example, we created a global variable price and tried to access it in test1 and test2 . In return, we got the same value because the global variable is accessible in the entire file.

Note : You must declare the global variable outside function.

Object/Variable identity and references

In Python, whenever we create an object, a number is given to it and uniquely identifies it. This number is nothing but a memory address of a variable’s value. Once the object is created, the identity of that object never changes.

No two objects will have the same identifier. The Object is for eligible garbage collection when deleted. Python has a built-in function id() to get the memory address of a variable.

For example, consider a library with many books (memory addresses) and many students (objects). At the beginning(when we start The Python program), all books are available. When a new student comes to the library (a new object created), the librarian gives him a book. Every book has a unique number (identity), and that id number tells us which book is delivered to the student (object)

It returns the same address location because both variables share the same value. But if we assign m to some different value, it points to a different object with a different identity.

See the following example

For m = 500 , Python created an integer object with the value 500 and set m as a reference to it. Similarly, n is assigned to an integer object with the value 400 and sets n as a reference to it. Both variables have different identities.

In Python, when we assign a value to a variable, we create an object and reference it.

For example, a=10 , here, an object with the value  10 is created in memory, and reference a now points to the memory address where the object is stored.

Suppose we created a=10 , b=10 , and  c=10 , the value of the three variables is the same. Instead of creating three objects, Python creates only one object as  10  with references such as  a , b , c .

We can access the memory addresses of these variables using the id() method. a , b refers to the same address  in memory, and c , d , e refers to the same address. See the following example for more details.

Here, an object in memory initialized with the value 10 and reference added to it, the reference count increments by ‘1’.

When Python executes the next statement that is b=10 , since it is the same value 10, a new object will not be created because the same object in memory with value 10 available, so it has created another reference, b . Now, the reference count for value 10 is ‘2’.

Again for  c=20 , a new object is created with reference ‘c’ since it has a unique value (20). Similarly, for d , and e  new objects will not be created because the object ’20’ already available. Now, only the reference count will be incremented.

We can check the reference counts of every object by using the getrefcount function of a  sys module. This function takes the object as an input and returns the number of references.

We can pass variable, name, value, function, class as an input to getrefcount() and in return, it will give a reference count for a given object.

See the following image for more details.

In the above picture, a , b pointing to the same memory address location (i.e., 140722211837248), and c , d , e pointing to the same memory address (i.e., 140722211837568 ). So reference count will be 2 and 3 respectively.

• In Python, we can create a tuple (or list) by packing a group of variables.
• Packing can be used when we want to collect multiple values in a single variable. Generally, this operation is referred to as tuple packing.

Here a , b , c , d  are packed in the tuple tuple1 .

Tuple unpacking  is the reverse operation of tuple packing . We can unpack tuple and assign tuple values to different variables.

Note: When we are performing unpacking, the number of variables and the number of values should be the same. That is, the number of variables on the left side of the tuple must exactly match a number of values on the right side of the tuple. Otherwise, we will get a ValueError .

Also, See :

• Class Variables
• Instance variables

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I’m  Vishal Hule , the Founder of PYnative.com. As a Python developer, I enjoy assisting students, developers, and learners. Follow me on  Twitter .

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Variables, Expressions, and Assignments

Variables, expressions, and assignments 1 #, introduction #.

In this chapter, we introduce some of the main building blocks needed to create programs–that is, variables, expressions, and assignments. Programming related variables can be intepret in the same way that we interpret mathematical variables, as elements that store values that can later be changed. Usually, variables and values are used within the so-called expressions. Once again, just as in mathematics, an expression is a construct of values and variables connected with operators that result in a new value. Lastly, an assignment is a language construct know as an statement that assign a value (either as a constant or expression) to a variable. The rest of this notebook will dive into the main concepts that we need to fully understand these three language constructs.

Values and Types #

A value is the basic unit used in a program. It may be, for instance, a number respresenting temperature. It may be a string representing a word. Some values are 42, 42.0, and ‘Hello, Data Scientists!’.

Each value has its own type : 42 is an integer ( int in Python), 42.0 is a floating-point number ( float in Python), and ‘Hello, Data Scientists!’ is a string ( str in Python).

The Python interpreter can tell you the type of a value: the function type takes a value as argument and returns its corresponding type.

Observe the difference between type(42) and type('42') !

Expressions and Statements #

On the one hand, an expression is a combination of values, variables, and operators.

A value all by itself is considered an expression, and so is a variable.

When you type an expression at the prompt, the interpreter evaluates it, which means that it calculates the value of the expression and displays it.

In boxes above, m has the value 27 and m + 25 has the value 52 . m + 25 is said to be an expression.

On the other hand, a statement is an instruction that has an effect, like creating a variable or displaying a value.

The first statement initializes the variable n with the value 17 , this is a so-called assignment statement .

The second statement is a print statement that prints the value of the variable n .

The effect is not always visible. Assigning a value to a variable is not visible, but printing the value of a variable is.

Assignment Statements #

We have already seen that Python allows you to evaluate expressions, for instance 40 + 2 . It is very convenient if we are able to store the calculated value in some variable for future use. The latter can be done via an assignment statement. An assignment statement creates a new variable with a given name and assigns it a value.

The example in the previous code contains three assignments. The first one assigns the value of the expression 40 + 2 to a new variable called magicnumber ; the second one assigns the value of π to the variable pi , and; the last assignment assigns the string value 'Data is eatig the world' to the variable message .

Programmers generally choose names for their variables that are meaningful. In this way, they document what the variable is used for.

Do It Yourself!

Let’s compute the volume of a cube with side \(s = 5\) . Remember that the volume of a cube is defined as \(v = s^3\) . Assign the value to a variable called volume .

Well done! Now, why don’t you print the result in a message? It can say something like “The volume of the cube with side 5 is \(volume\) ”.

Beware that there is no checking of types ( type checking ) in Python, so a variable to which you have assigned an integer may be re-used as a float, even if we provide type-hints .

Names and Keywords #

Names of variable and other language constructs such as functions (we will cover this topic later), should be meaningful and reflect the purpose of the construct.

In general, Python names should adhere to the following rules:

It should start with a letter or underscore.

It cannot start with a number.

It must only contain alpha-numeric (i.e., letters a-z A-Z and digits 0-9) characters and underscores.

They cannot share the name of a Python keyword.

If you use illegal variable names you will get a syntax error.

By choosing the right variables names you make the code self-documenting, what is better the variable v or velocity ?

The following are examples of invalid variable names.

These basic development principles are sometimes called architectural rules . By defining and agreeing upon architectural rules you make it easier for you and your fellow developers to understand and modify your code.

If you want to read more on this, please have a look at Code complete a book by Steven McConnell [ McC04 ] .

Every programming language has a collection of reserved keywords . They are used in predefined language constructs, such as loops and conditionals . These language concepts and their usage will be explained later.

The interpreter uses keywords to recognize these language constructs in a program. Python 3 has the following keywords:

False class finally is return

None continue for lambda try

True def from nonlocal while

and del global not with

as elif if or yield

assert else import pass break

except in raise

Reassignments #

It is allowed to assign a new value to an existing variable. This process is called reassignment . As soon as you assign a value to a variable, the old value is lost.

The assignment of a variable to another variable, for instance b = a does not imply that if a is reassigned then b changes as well.

You have a variable salary that shows the weekly salary of an employee. However, you want to compute the monthly salary. Can you reassign the value to the salary variable according to the instruction?

Updating Variables #

A frequently used reassignment is for updating puposes: the value of a variable depends on the previous value of the variable.

This statement expresses “get the current value of x , add one, and then update x with the new value.”

Beware, that the variable should be initialized first, usually with a simple assignment.

Do you remember the salary excercise of the previous section (cf. 13. Reassignments)? Well, if you have not done it yet, update the salary variable by using its previous value.

Updating a variable by adding 1 is called an increment ; subtracting 1 is called a decrement . A shorthand way of doing is using += and -= , which stands for x = x + ... and x = x - ... respectively.

Order of Operations #

Expressions may contain multiple operators. The order of evaluation depends on the priorities of the operators also known as rules of precedence .

For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3 - 1) is 4 , and (1 + 1)**(5 - 2) is 8 . You can also use parentheses to make an expression easier to read, even if it does not change the result.

Exponentiation has the next highest precedence, so 1 + 2**3 is 9 , not 27 , and 2 * 3**2 is 18 , not 36 .

Multiplication and division have higher precedence than addition and subtraction . So 2 * 3 - 1 is 5 , not 4 , and 6 + 4 / 2 is 8 , not 5 .

Operators with the same precedence are evaluated from left to right (except exponentiation). So in the expression degrees / 2 * pi , the division happens first and the result is multiplied by pi . To divide by 2π, you can use parentheses or write: degrees / 2 / pi .

In case of doubt, use parentheses!

Let’s see what happens when we evaluate the following expressions. Just run the cell to check the resulting value.

Floor Division and Modulus Operators #

The floor division operator // divides two numbers and rounds down to an integer.

For example, suppose that driving to the south of France takes 555 minutes. You might want to know how long that is in hours.

Conventional division returns a floating-point number.

Hours are normally not represented with decimal points. Floor division returns the integer number of hours, dropping the fraction part.

You spend around 225 minutes every week on programming activities. You want to know around how many hours you invest to this activity during a month. Use the \(//\) operator to give the answer.

The modulus operator % works on integer values. It computes the remainder when dividing the first integer by the second one.

The modulus operator is more useful than it seems.

For example, you can check whether one number is divisible by another—if x % y is zero, then x is divisible by y .

String Operations #

In general, you cannot perform mathematical operations on strings, even if the strings look like numbers, so the following operations are illegal: '2'-'1' 'eggs'/'easy' 'third'*'a charm'

But there are two exceptions, + and * .

The + operator performs string concatenation, which means it joins the strings by linking them end-to-end.

The * operator also works on strings; it performs repetition.

Speedy Gonzales is a cartoon known to be the fastest mouse in all Mexico . He is also famous for saying “Arriba Arriba Andale Arriba Arriba Yepa”. Can you use the following variables, namely arriba , andale and yepa to print the mentioned expression? Don’t forget to use the string operators.

Asking the User for Input #

The programs we have written so far accept no input from the user.

To get data from the user through the Python prompt, we can use the built-in function input .

When input is called your whole program stops and waits for the user to enter the required data. Once the user types the value and presses Return or Enter , the function returns the input value as a string and the program continues with its execution.

Try it out!

You can also print a message to clarify the purpose of the required input as follows.

The resulting string can later be translated to a different type, like an integer or a float. To do so, you use the functions int and float , respectively. But be careful, the user might introduce a value that cannot be converted to the type you required.

We want to know the name of a user so we can display a welcome message in our program. The message should say something like “Hello \(name\) , welcome to our hello world program!”.

Script Mode #

So far we have run Python in interactive mode in these Jupyter notebooks, which means that you interact directly with the interpreter in the code cells . The interactive mode is a good way to get started, but if you are working with more than a few lines of code, it can be clumsy. The alternative is to save code in a file called a script and then run the interpreter in script mode to execute the script. By convention, Python scripts have names that end with .py .

Use the PyCharm icon in Anaconda Navigator to create and execute stand-alone Python scripts. Later in the course, you will have to work with Python projects for the assignments, in order to get acquainted with another way of interacing with Python code.

This Jupyter Notebook is based on Chapter 2 of the books Python for Everybody [ Sev16 ] and Think Python (Sections 5.1, 7.1, 7.2, and 5.12) [ Dow15 ] .

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File handling, python modules, python numpy, python pandas, python matplotlib, python scipy, machine learning, python mysql, python mongodb, python reference, module reference, python how to, python examples, python variables - assign multiple values, many values to multiple variables.

Python allows you to assign values to multiple variables in one line:

Note: Make sure the number of variables matches the number of values, or else you will get an error.

One Value to Multiple Variables

And you can assign the same value to multiple variables in one line:

Unpack a Collection

If you have a collection of values in a list, tuple etc. Python allows you to extract the values into variables. This is called unpacking .

Unpack a list:

Learn more about unpacking in our Unpack Tuples Chapter.

Video: Python Variable Names

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Python Variables and Assignment

Python variables, variable assignment rules, every value has a type, memory and the garbage collector, variable swap, variable names are superficial labels, assignment = is shallow, decomp by var.

Introduction to Programming

Variables in python.

• The purpose of a variable is to store information within a program while it is running.
• A variable is a named storage location in computer memory. Use the name to access the value.
• To store a value in a variable, use the = operator (called the assignment operator).
• An = sign in Python is nothing like an equal sign in mathematics. Think of it more like an arrow going from right to left. The expression on the right is evaluated and then stored in the variable named on the left.
• For example, the line of code hourly_wage = 16.75 stores the value 16.75 in the variable called hourly_wage
• You can change the value of a variable with another assignment statement, such as hourly_wage = 17.25
• Every value has a type ( int for integers, float for decimals, str for text). In python, when you store a value in a variable (with = ), that variable then automatically has a type. For example, after the above assignment, hourly_wage is of type float .

Rules and conventions for naming variables in python

• The first character must be a letter or an underscore. For now, stick to letters for the first character.
• The remaining characters must be letters, numbers or underscores.
• No spaces are allowed in variable names.
• Legal examples: _pressure , pull , x_y , r2d2
• Invalid examples, these are NOT legal variable names: 4th_dimension , %profit , x*y , four(4) , repo man
• In python, it's a conventiion to use snake case to name variables. This means that we use all lower-case letters and we separate words in the variable name with underscores. Examples include age , x_coordinate , hourly_wage , user_password
• If the value stored in a variable is a true constant (in other words, its value will never change throughout the program), then we use all capital letters: COURSE_ENROLLMENT_LIMIT , MAX_PASSWORD_ATTEMPTS .
• For high quality code, it is crucial that you give descriptive names for variables. The variable names must help the reader of your program understand your intention.

Typical way we visualize variables

We usually draw variables by putting the value in a box, and labelling the box with the name of the variable:

Types of variables

Each variable has a name, a value, and a type. Types are necessary because different kinds of data are stored differently within the computer's memory. For now, we will learn three different types, for storing signed (positive or negative) whole numbers, signed decimals, and text.

Creating a variable with an assignment operator

A variable is created or declared when we assign a value to it using the assignment operator = . In python, the code looks like this: variable_name = <value> .

Notice that the left hand side of an assignment must be a variable name. Non-example:

After creating a variable, you can change the value stored in a variable with another assignment operator at any time. This is called reassignment .

Finding out the type of a variable or value

The type() function in python will return the type of either a variable or a value. Here are examples that show how to use it:

The output of the above code will be:

Casting (changing the type) of a variable or value

You can change the type of a value (called “casting”) using the int() , float() and str() functions. For example:

• int(23.7) (truncates the float value 23.7 to the int value 23. This is different from rounding - the decimal part is discarded, regardless of whether it is larger or smaller than 0.5.
• float(23) (outputting the result will give 23.0 rather than 23)
• str(23) (converts the integer 23 to the text "23" )
• int("23") (converts the string "23" into a numerical integer value 23 )
• float("23") (converts the string "23" into a numerical decimal value 23.0 )
• int("23.5") results in an error
• float("hello") results in an error

Doing arithmetic in python

Here are the basic arithmetic operators in python. In these examples, assume

An example of a use of the modulus operator is to determine if an integer is even or odd. Note that if x is an integer, then x%2 takes the value 0 or 1 . So x % 2 == 0 is True when x is even and False when x is odd.

Another example of integer division and modulus: When we divide 3 by 4, we get a quotient of 0 and a remainder of 3. So 3//4 results in 0 and 3%4 results in 3.

Warning note: In python, ^ is not an exponent!

Order of operations

The order of operations in python is similar to the order you are familiar with in math: parentheses, then exponentiation, then multiplication/division/modulus in order from left to right, then addition/subtraction in order from left to right.

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Different Forms of Assignment Statements in Python

We use Python assignment statements to assign objects to names. The target of an assignment statement is written on the left side of the equal sign (=), and the object on the right can be an arbitrary expression that computes an object.

There are some important properties of assignment in Python :-

• Assignment creates object references instead of copying the objects.
• Python creates a variable name the first time when they are assigned a value.
• Names must be assigned before being referenced.
• There are some operations that perform assignments implicitly.

Assignment statement forms :-

1. Basic form:

This form is the most common form.

2. Tuple assignment:

When we code a tuple on the left side of the =, Python pairs objects on the right side with targets on the left by position and assigns them from left to right. Therefore, the values of x and y are 50 and 100 respectively.

3. List assignment:

This works in the same way as the tuple assignment.

4. Sequence assignment:

In recent version of Python, tuple and list assignment have been generalized into instances of what we now call sequence assignment – any sequence of names can be assigned to any sequence of values, and Python assigns the items one at a time by position.

5. Extended Sequence unpacking:

It allows us to be more flexible in how we select portions of a sequence to assign.

Here, p is matched with the first character in the string on the right and q with the rest. The starred name (*q) is assigned a list, which collects all items in the sequence not assigned to other names.

This is especially handy for a common coding pattern such as splitting a sequence and accessing its front and rest part.

6. Multiple- target assignment:

In this form, Python assigns a reference to the same object (the object which is rightmost) to all the target on the left.

7. Augmented assignment :

The augmented assignment is a shorthand assignment that combines an expression and an assignment.

There are several other augmented assignment forms:

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What Is a Variable in Python?

It might seem like a straightforward concept, but variables are more interesting than you think. In an effort to expand our concept map, we’re here to cover one of the most basic programming concepts: variables in Python.

Table of Contents

Concept overview, where can variables be used, what if i don’t need a variable, is the equal sign the only way to create a variable, changing over time.

Most likely at some point in your life, you’ve heard the term “variable” used. Unfortunately, it has a lot of different definitions depending on the context. For example, in algebra, you might be familiar with variables like x and y , which are meant to represent sets of possible numbers. Meanwhile, in science, you might be familiar with concepts like dependent and independent variables (i.e., aspects of an experiment which can be manipulated to observe some outcome).

In the world of programming, variables are probably most closely associated with the kinds of variables you’ve seen in algebra. However, programming variables tend to have some unique features. For example, programming variables are not restricted to just numbers. We can use variables to “store” any type of data we want, from simpler data types like numbers and characters to more complex data types like entire data tables or objects.

In addition, programming variables are more “real” than mathematical variables in the sense that the data has to be stored somewhere. As a result, programming variables often don’t store data directly but rather store the address to where that data is stored. Of course, this varies from language to language.

That said, it’s very easy to assume that programming variables are similar to mathematical variables because the syntax often looks so familiar. For example, here’s how you might define your own variable in Python:

Given the use of the equal sign, it’s tempting to assume that all the rules from algebra apply. For instance, you might imagine that this statement can be rearranged as follows:

However, this is not legal code because the equal sign is not defining a relationship between x and 37. Instead, the equal sign is often called the assignment operator . In other words, x can be assigned the value 37, but 37 cannot be assigned the value x . Because of this confusion, some programming languages choose different symbols for their assignment operator, such as the walrus operator (i.e., := ) or an arrow (i.e., <- ).

Of course, as I said previously, we’re not restricted to storing numbers in our variables in Python. We can store whatever we like, including data structures and functions. In fact, as long as there is an expression on the right side of the equal sign, Python will be happy. As a result, all of the following lines of code are valid variable definitions in Python:

How cool is that? Now, in the remainder of this article, we’ll answer some of your questions about variables?

Variables are probably one of the most fundamental features of Python, so you will see them everywhere. For example, in a normal Python program, you might see a variable defined based on user input:

Likewise, you will also see variables used as function parameters. For example, the input function above accepts a prompt as an argument. That argument, is then saved in a variable for the function to use. Here’s what that might look like:

In fact, variables are so ubiquitous in Python, that most of the features wouldn’t work without them. For example, you can’t really use a while loop without a variable in the condition :

It’s possible to make the code above work using a function, but the function argument is just another variable. A similar argument can be made for if statements and other more complex structures like list comprehensions and pattern matching.

Because variables are so ubiquitous, there are times where you will receive one that you don’t need, such as part of a return value from a function. In Python, we have a fun convention for dealing with these situations—the underscore. For example, you might have a list of values from which you only want the first and last. In Python, we can use iterable unpacking :

In this example, the underscore is basically a dummy variable, which we dump blue and green into. It acts like any other variable, but it signals to the reader that we don’t care about its value.

Typically, we create variables using the assignment operator. However, it’s possible to create variables in more recent versions of Python using the walrus operator. It functions almost identically to the regular assignment operator, but it can only be used in certain scenarios. For example, you might be familiar with the traditional while loop for reading lines from a file:

Code like this is straightforward but some folks really don’t like duplicate code, so they opt for something like this:

This code only works when using the walrus operator. You cannot swap the walrus operator out for the usual assignment operator. Likewise, the following is not legal code:

The short explanation is that if you want to save and use a value at the same time, then you can use the walrus operator (e.g., in the context of a loop condition). Otherwise, you must use the regular assignment operator.

Like variables, this series is changing. With the addition of this article on variables, I think next time we’ll look at expressions. Let’s keep the concept map growing!

With that said, it’s time to call it today. Below you’ll find some related articles:

• The Haters Guide to Python
• Abusing Python’s Operator Overloading Feature
• 5 Things You Should Know Before You Pick Up Python

Likewise, here are some resources to help you learn more about Python (#ad):

Finally, feel free to check out my list of ways to grow the site . Otherwise, take care!

An activity I regularly do with my students is a concept map. Typically, we do it at the start and end of each semester to get an idea of how well our understanding of the material as matured over time. Naturally, I had the idea to extend this concept into its own series, just to see how deeply I can explore my own knowledge of Python. It should be a lot of fun, and I hope you enjoy it!

Jeremy Grifski

Jeremy grew up in a small town where he enjoyed playing soccer and video games, practicing taekwondo, and trading Pokémon cards. Once out of the nest, he pursued a Bachelors in Computer Engineering with a minor in Game Design. After college, he spent about two years writing software for a major engineering company. Then, he earned a master's in Computer Science and Engineering. Today, he pursues a PhD in Engineering Education in order to ultimately land a teaching gig. In his spare time, Jeremy enjoys spending time with his wife and kid, playing Overwatch 2, Lethal Company, and Baldur's Gate 3, reading manga, watching Penguins hockey, and traveling the world.

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Is there a way to set an instance variable without calling a descriptors __set__ method?

I have made a custom descriptor which I've assigned to a value in my class. However I would like to set the instance variable initially without using the descriptors __set__ method.

Is there a way to work around this and set self.bar directly? I have this question as originally I was using the @property descriptor, which when used allowed both the descriptor method and the private variable to be called, which was exactly what I needed. Since @property is a descriptor there must be a sensible way to do this (Even though the reason this works with @property is that the private variable is defined within the scope of my class, rather than the descriptor class as done with custom descriptors which allows for both to be modified within the class, but only the public version outside the scope of the class (I know private variables are not truly private but that's beside the point).) For example this same code using @property:

However in this case, the getters and setters take up way more room when more instance variables are introduced, and the toggle method from the original decorator has to be rewritten for each new setter added (which is what caused me to rewrite the @properties as a custom descriptor class as it seemed like a place to use oop to not have to repeat myself).

• python-descriptors

None of that scope stuff you were thinking about actually matters. Your property sets self._bar , and you bypass it by setting self._bar directly. Your custom descriptor does the exact same thing, just through setattr , so you still bypass it by setting the instance's _bar attribute directly:

• omg thank you for this answer it seems this entire problem was caused by pycharm being stupid and complaining that 'Unresolved attribute reference '_bar' for class 'Foo'', and I didn't bother to run and check if that was actually true I'm actually getting sick of pycharm... :/ Was thinking of deleting this question but someone might end up making the same assumptions I did and start searching for a solution to a problem that doesn't exist... –  r5ne Commented Aug 28 at 17:30
• Updating to the latest version of pycharm fixed this btw that's my bad. –  r5ne Commented Aug 28 at 18:09

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How to find first non-blank string variable during string assignment?

I have Python 3.12 on Windows 10.

I work with spreadsheets. I read each row one at a time, and grab one piece of data from that row, let’s say a job number, which could be in column A, B or C. Yes, the spreadsheet is a mess, I can’t change that. I have to work with what I’m given.

But the preferred order I look for the job number in the columns is B, A, C. I need to look for a job in column B. If that’s blank, go to column A. If that’s blank use column C. Currently this requires a bunch of lines to do.

Is there a quicker way to do this?

In perl it was something like (I can’t remember the exact syntax).

In Python I’m currently doing:

This will raise StopIteration if all of cola, colb, colc are empty.

What you did in Perl also works in Python, with slightly different syntax:

This will set job to '' if all of cola, colb, and colc are empty.

Perfect! Thank you! I made a note of this.