Chapter 2.1: Conditionals – If, Else, Elif

In this chapter, we will discuss how to control the flow of our Python programs using conditional statements: if, else, and elif.

The if Statement

The if statement is the most basic type of conditional statement in Python. It checks a condition and executes a block of code only if the condition is true.

x = 5

if x > 0:
    print("x is positive")

In this example, x > 0 is the condition. Since x is 5, and 5 is indeed greater than 0, the print statement is executed.

Note the use of indentation to define the block of code to be executed if the condition is true. Python uses indentation (typically four spaces or one tab) to define blocks of code.

The else Statement

The else statement follows an if statement and defines a block of code to be executed if the condition in the if statement is false.

x = -5

if x > 0:
    print("x is positive")
else:
    print("x is not positive")

In this case, x is -5, so x > 0 is False. The if block is skipped and the else block is executed instead.

The elif Statement

elif is a contraction of “else if”. It allows us to check multiple conditions and execute a specific block of code as soon as one of the conditions is true.

x = 0

if x > 0:
    print("x is positive")
elif x < 0:
    print("x is negative")
else:
    print("x is zero")

In this example, x is 0, so x > 0 is False and x < 0 is also False. Both the if and elif blocks are skipped, and the else block is executed.

Remember, the elif and else statements are optional. An if statement can exist on its own or be followed by any number of elif statements. The else statement, if used, must come last.

With if, else, and elif, you can make your Python programs make decisions and behave differently based on various conditions. This is a foundational concept in any programming language and critical in mastering Python.