Python if..else Statement

Introduction

Before we begin talking about Python if..else Statement, let's briefly understand – What is Python?

One of the most important concepts in computer programming is decision-making. With slight adjustments, Python supports the common flow control statements seen in other languages. One of the simplest and most well-known statements for executing code depending on a specific condition is the if control statement.

In Python, if-else is a control structure that allows you to conditionally execute different blocks of code based on certain conditions. It is used to perform branching in a program, where different actions are taken depending on whether a condition is True or False.

In this tutorial, you will understand Python if..else Statement. We will also address a few FAQs on Python if..else Statement.

Python `if` Statement

The simplest if statement in Python looks like this:

if EXPRESSION:
  STATEMENT

The if statement begins with the if keyword and is followed by the conditional expression.

The colon (:) must come after the EXPRESSION. The STATEMENT is executed if the EXPRESSION evaluates to True. When EXPRESSION returns False, the STATEMENT is ignored and nothing happens. STATEMENT can refer to any statement, including numerous statements or nested if statements. Use the pass statement to carry out no statements.

The STATEMENT block begins with an indentation and concludes with the first unintended line. Most individuals either prefer to indent 4-space or 2-space. The official Python Code Style Guide suggests using 4-spaces per indentation level and avoiding mixing tabs and spaces for indentation.

Consider the following example script, which determines whether a given number is greater than 5.

number = int(input('Enter a number: '))

if number > 5:
    print(number, 'is greater than 5.')

Save the code in a file and execute it from the command line:

python test.py

You will be prompted to input a number by the script. For instance, if you enter 10, the conditional expression will evaluate to True (10 is greater than 5), and the print function will be called.

Output

10 is greater than 5.

Standard comparison operations are supported by Python:

a == b - True if variables a and b are equal.
a != b - True if variables a and b are not equal.
a > b - True if variable a is greater than variable b.
a >= b - True if variable a is equal to or greater than variable b.
a < b - True if variable a is less than variable b.
a <= b - True if variable a is equal to or less than variable b.

The in keyword can also be used to determine whether a value is contained in an iterable (such as a string, list, tuple, dictionary, etc.):

s = 'vegastack'
if 've' in s:
    print('True.')

Here's another example using a dictionary:

d = {'a': 2, 'b': 4}
if 'a' in d:
    print('True.')

When used on a dictionary, the in keyword determines whether the dictionary contains a specific key.

Use the logical not operator to negate the conditional expression:

number = int(input('Enter a number: '))

if not number < 5:
    print(number, 'is greater than 5.')

`if..else` Statement

A condition is tested in an if..else statement, and based on the outcome, one of the two sentences is executed.

The if..else statement in Python takes the following form:

if EXPRESSION:
    STATEMENT1
else:
    STATEMENT2

If EXPRESSION evaluates to True, STATEMENT1 is carried out. Otherwise, STATEMENT2 is carried out if EXPRESSION returns False. Only one else clause is acceptable in the statement.

The else keyword must have a (:) colon at the end and be indented at the same level as the corresponding if keyword.

Let us update the previous example script to include an else clause:

number = int(input('Enter a number: '))

if number > 5:
    print(number, 'is greater than 5.')
else:
    print(number, 'is equal or less than 5.')

The script will produce a different message if you execute the code and enter a number depending on whether the value is greater or less/equal to 5.

`if..elif..else` Statement

else if is shortened to form the elif keyword.

The if..elif..else statement in Python has the following form:

if EXPRESSION1:
    STATEMENT1
elif: EXPRESSION2:
    STATEMENT2
else:
    STATEMENT3

The STATEMENTS1 are carried out if the evaluation of EXPRESSION1 is True. The STATEMENTS2 are carried out if the EXPRESSION2 evaluates to True. The STATEMENTS3 is executed if none of the expressions return True.

The elif keyword must have a (:) colon at the end and be indented the same amount as the if keyword. The statement may contain one or more elif clauses. The else clause is not mandatory. None of the statements are executed if the else clause is absent and every expression evaluates to False.

The conditions are evaluated in order. The remaining conditions are not checked once a condition returns True, and the program control jumps to the end of the if statements.

Let us modify the previous script by adding an elif clause:

number = int(input('Enter a number: '))

if number > 5:
    print(number, 'is greater than 5.')
elif number < 5:
    print(number, 'is less than 5.')
else:
    print(number, 'is equal to 5.')

In contrast to the majority of programming languages, Python lacks switch or case statements. The switch or case statements can be replaced by a series of multiple elif statements.

Nested `if` Statements

You can nest if statements inside other if statements in Python. In general, you should aim to use elif rather than nesting if statements and always try to avoid excessive indentation.

The script that follows will ask you to enter three numbers and will print the largest one.

number1 = int(input('Enter the first number: '))
number2 = int(input('Enter the second number: '))
number3 = int(input('Enter the third number: '))

if number1 > number2:
    if number1 > number3:
        print(number1, 'is the largest number.')
    else:
        print(number3, 'is the largest number.')
else:
    if number2 > number3:
        print(number2, 'is the largest number.')
    else:
        print(number3, 'is the largest number.')

This is how the output will look:

Output

Enter the first number: 455 
Enter the second number: 567
Enter the third number: 354
567 is the largest number.

Multiple Conditions

You can combine numerous conditions in the if statements using the logical or, and, and operators.

Here is a different script that will print the largest of the three integers. In this version, the logical and operator and elif will be used in place of the nested if statements.

number1 = int(input('Enter the first number: '))
number2 = int(input('Enter the second number: '))
number3 = int(input('Enter the third number: '))

if number1 > number2 and number1 > number3:
    print(number1, 'is the largest number.')
elif number2 > number3 and number2 > number3:
    print(number2, 'is the largest number.')
else:
    print(number3, 'is the largest number.')

FAQs on Python if..else Statement

Can you have multiple `else` statements in a single `if` block?

No, in Python, you can only have one else statement associated with an if block. It is used as a final catch-all block when all preceding conditions are false.

What happens if the condition in the `if` statement is not true?

If the condition in the if statement is false, the code block within the else statement (if present) will be executed. Otherwise, it will be skipped.

Can you nest `if-else` statements within each other?

Yes, you can nest if-else statements within each other to handle more complex conditional logic.

Can you use logical operators (such as `and`, `or`, `not`) in the conditions of `if-else` statements?

Yes, you can use logical operators to combine multiple conditions within the if statement.

What happens if there is no `else` statement in an `if-else` block?

If there is no else statement, the code block associated with the if statement will be executed if the condition is true. If the condition is false, the program will simply move on to the next line of code.

Can the `if-else` statement be used inside a function?

Yes, the if-else statement can be used anywhere in Python code, including inside functions.

What is the difference between `if-elif-else` and multiple `if-else` statements?

The if-elif-else structure allows you to check multiple conditions in sequence until one of them is true. In contrast, using multiple if-else statements would check each condition independently, which could result in multiple blocks of code being executed.