2.2 Conditional Branches

Conditional branches let a program choose different code paths for different situations. Python uses if, elif, and else for branching.

if

An if statement checks a condition. If the condition is True, Python runs the indented block under it. If the condition is False, Python skips that block.

age = int(input("Enter your age: "))

if 0 < age < 18:
    print("Minor")

One possible run:

Enter your age: 17
Minor

In Python, indentation is not decoration. It is part of the syntax. Statements that belong to the same if block must use the same indentation level.

if-else

if-else is used for a two-way choice: run the if block when the condition is true; otherwise run the else block.

Here is a leap-year example. A year is a leap year if it satisfies one of these conditions:

• It is divisible by 4, and not divisible by 100.
• Or it is divisible by 400.

year = int(input("Enter a year: "))

is_leap_year = (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0)

if is_leap_year:
    print("Leap year")
else:
    print("Common year")

One possible run:

Enter a year: 2020
Leap year

Saving the complex condition into is_leap_year makes the if line easier to read. In real programs, this is often more maintainable than placing a long condition directly inside if.

if-elif-else

When there are more than two cases, use if-elif-else. Python checks conditions from top to bottom. It runs the first branch whose condition is True, then skips the remaining branches.

c = input("Enter a character: ")

if 'a' <= c <= 'z':
    print("Lowercase")
elif 'A' <= c <= 'Z':
    print("Uppercase")
elif '0' <= c <= '9':
    print("Digit")
else:
    print("Special character")

One possible run:

Enter a character: T
Uppercase

The order of elif branches matters. Put more specific or higher-priority conditions earlier, and use else as the fallback case.