Conditionals: if / elif / else

So far every snippet in this course has run top to bottom, every line, every time. Real programs need to make decisions - do this if a condition holds, do something else if it doesn't. That's what conditionals are for.

• -== equal to (note: two equals signs - one = is assignment, a very common typo bug)
• -!= not equal to
• -< / > less than / greater than
• -<= / >= less than or equal to / greater than or equal to

Python evaluates conditions top to bottom and runs the first branch that matches, then skips the rest entirely - even if a later condition would also be true. elif (short for "else if") lets you chain as many conditions as you need; else is optional and catches anything not matched above. Note that Python uses indentation, not braces, to mark what belongs inside each branch - four spaces is the near-universal convention, and getting the indentation wrong is a real syntax error, not just a style nitpick.

and requires both sides to be true, or requires at least one, and not flips a condition's truth value. Unlike some languages, Python spells these out as words rather than symbols like && or || - one of many small ways Python favors reading like plain English.

Nesting is what you reach for when a decision genuinely depends on the outcome of a previous decision. It's powerful, but it's also the fastest way to make code hard to read - three or four levels deep and a reader has to hold a lot of context in their head at once. Often, combining conditions with and (like age >= 18 and has_ticket) reads more clearly than nesting two separate if statements, when the logic allows it.

Unlike languages with a dedicated switch statement, Python has traditionally handled "check this value against several options" purely with if / elif / else chains - and that's exactly what you should reach for as a beginner. Modern Python (3.10+) does add a match/case statement for this exact pattern, worth knowing exists even though this course won't lean on it: