Conditional Branching

Making decisions

Programs would be incredibly boring if they just did the exact same thing every single time they ran.

To write intelligent software, your code needs to make decisions based on changing conditions.

We do this using if, elif, and else statements.

The basic if statement

An if statement checks a Boolean condition. If it is True, the indented code underneath it runs. If it is False, Python skips over it entirely.

python
temperature = 35

if temperature > 30:
    print("It's a hot day!")
    print("Drink plenty of water.")

Because 35 is greater than 30, both print statements will execute. Notice that the code inside the block must be indented (this is usually done with 4 spaces).

Expanding with else

What if you want something to happen when the condition is False? Use else.

python
password = "123"

if password == "secret":
    print("Access granted.")
else:
    print("Access denied!")

The else block catches everything that fails the if test.

Chaining with elif

If you have more than two possibilities, you can use elif (short for "else if").

Python will check them back-to-back until it finds one that is True.

python
score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

Here, Python checks score >= 90 (False). Then it checks score >= 80 (True!). It prints "Grade: B" and immediately exits the entire chain. It never bothers checking the C or F conditions.

What this lesson should give you

After this lesson, you should understand how to:

• write an if statement to run code conditionally
• use else as a fallback option
• chain multiple conditions sequentially using elif
• properly indent code blocks to tell Python which lines belong to the statement