Control code flow with conditional logic (if, elif, else)

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Now that your program can evaluate conditions, you can use them to guide the paths your code can take. Python uses if, elif, and else statements to guide the flow of your program based on True or False results. This approach is called conditional logic.

The if statement

An if statement runs a block of code only when a condition is True. The code inside must be indented—Python uses indentation to know which lines belong to the block:

score = 85

if score >= 90:
    print("Congratulations!")
    print("Grade: A")

print("Program complete.")

If score is less than 90, the indented code inside the if block is skipped, and the program continues with the next line after the block.

Indentation and colon rules

Python uses strict rules to group code blocks together:

1. Colon (:): You must place a colon at the end of a conditional statement. That tells Python that the next indented lines belong to that conditional block.

2. Indentation: Every line in the same block must use the same indentation. Use four spaces (or a Tab) consistently.

To complete the code block, you must unindent the next line after the block. This unindented line signals to Python that the block has ended. In the example above, the line print("Program complete.") is unindented, so it runs regardless of the if condition.

Adding alternative paths with else

Use else to run a different block of code when the condition is False:

score = 75

if score >= 90:
    print("Grade: A")
else:
    print("Grade: B or lower")

Chaining conditions with elif

When you have more than two possible paths, you can use elif (short for "else if") to check extra conditions.

First match wins

Python checks each condition in order. When it finds the first True condition, it runs that block and skips the rest. If none of the conditions are True, it runs the else block (if present).

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:     # Python stops here! 85 >= 80 is True.
    print("Grade: B")
elif score >= 70:     # Skipped entirely, even though 85 >= 70 is also true.
    print("Grade: C")
else:                 # Skipped entirely.
    print("Grade: D or lower")

Output:

Grade: B

Because Python exits the chain after the first True condition, be sure to order your conditions from most specific to least specific. Otherwise, you may never reach the conditions you expect.