Write 'if' statements
To express conditional logic in Python, you use if statements. When you're writing an if statement, you're relying on another concept we cover in this module, mathematical operators. Python supports the common logic operators from math: equals, not equals, less than, less than or equal to, greater than, and greater than or equal to. You're probably used to seeing these operators displayed using symbols, which is how they’re represented in Python, too.
- Equals:
a == b - Not Equals:
a != b - Less than:
a < b - Less than or equal to:
a <= b - Greater than:
a > b - Greater than or equal to:
a >= b
Test expressions
You need to use an if statement to run code only if a certain condition is satisfied. The first thing you do when you write an if statement is to check the condition by using a test expression. You then determine whether the statement evaluates to True or False. If it's True, the next indented code block is run:
a = 97
b = 55
# test expression
if a < b:
# statement to be run
print(b)
In this example, a < b is the test expression. The program evaluates the test expression and then runs the code within the if statement only if the test expression is True. If you evaluate the expression, you know that it's False, so any code you write in the if statement won't be run.
Note
In Python, None and 0 are also interpreted as False.
Write if statements
You use an if statement if you want to run code only if a certain condition is satisfied. The syntax of an if statement is always:
if test_expression:
# statement(s) to be run
For example:
a = 93
b = 27
if a >= b:
print(a)
Output: 93
In Python, the body of an if statement must be indented. Any code following a test expression that isn't indented will always be run:
a = 24
b = 44
if a <= 0:
print(a)
print(b)
Output:
44
In this example, the output is 44 because the test expression is False and the print(b) statement isn't indented at the same level as the if statement.