Making Decisions

So far your programs ran every line, top to bottom. Real programs make decisions: show a discount if the cart is big enough, or an error if a password is too short. That's what if is for.

if, elif, else

Change score and run it again. Three rules to remember:

1. Each condition line ends with a colon :.
2. The code that belongs to a branch is indented, by 4 spaces by convention.
3. Python checks branches top to bottom and runs only the first one that's True. else runs when none of them are.

Indentation is the syntax

Python has no curly braces around blocks; indentation is the structure. Mixing indentation levels causes an IndentationError:

Combining conditions

Use and, or and not from Day 4 inside a condition:

Truthiness: what counts as True?

An if doesn't need a comparison. Python treats every value as truthy or falsy:

Nested decisions

An if can sit inside another one. Keep nesting shallow, because two levels is already plenty:

match: choosing between many options

When you compare one value against several fixed options, match (Python 3.10+) is often clearer than a long elif chain:

| means "or", and case _ matches anything else, like else.

One-line conditional expression

For a simple either/or value, use the conditional expression A if condition else B:

Recap

• if / elif / else branch on conditions; only the first true branch runs.
• Lines ending with : start a block; the block is defined by indentation.
• Empty values (0, "", [], None) are falsy; almost everything else is truthy.
• match picks between fixed options; x if cond else y chooses a value in one line.