How Do if, elif and else Work in Python?

An if statement runs its body when the condition is truthy. When that happens, the whole chain is finished. Python does not carry on to the elif branches, and it does not run the else either.

score = 75

if score >= 90:
    grade = 'A'
elif score >= 75:
    grade = 'B'
elif score >= 60:
    grade = 'C'
else:
    grade = 'F'

print(grade)          # B

The chain is evaluated strictly from the top. Python tests score >= 90, which is false, and then it tests score >= 75, which is true. It binds grade to B and skips the remaining branches completely. Only one branch ever runs.

Two structural details are worth mentioning in an interview. The elif is only a short way of writing else: if, so you can have as many of them as you like, but a plain if can never come after an elif. The chain has to start with if. And the else branch is optional, so a chain may simply end with no else and do nothing when nothing matches.

The condition itself is tested for truthiness and not for equality with True. A non-empty string, a number that is not zero, and a collection that is not empty are all truthy. That makes if value: a test for emptiness rather than a test for presence, so if a value may legitimately be zero or an empty list, write if value is not None: instead.

One last practical point is that Python has no switch statement, so a chain of if and elif on the same variable is the usual way to express a choice between many values. Python 3.10 added the match statement as a more expressive alternative.

A single equals sign is an assignment, and a double equals sign is a comparison. Only the comparison makes sense inside a condition, and the parser catches the mistake for you instead of letting it slip through as a bug.

value = 5

if value == 5:
    print('value is five')

# One equals sign in a condition is a SyntaxError, and it is caught
# before anything runs at all.
try:
    compile('if value = 5:\n    pass', '', 'exec')
except SyntaxError as error:
    print('SyntaxError:', error.msg)

The first block prints "value is five". The second block reports the parser's own message, which is a SyntaxError about invalid syntax, and it even mentions that you probably meant a comparison.

This is worth contrasting with other languages, where the same mistake is legal but wrong. In C, if (x = 5) assigns 5 to x and then tests the result, so it compiles and quietly does the wrong thing. In JavaScript, the same mistake is valid, because an assignment gives back the value that was assigned, so the condition is always true. Python refuses to compile it at all, and that turns a silent bug into an immediate error.

Python also has the walrus operator, which is written :=, and that one does allow an assignment inside an expression. It is occasionally useful for assigning and testing in one step, but it is a different operator and it cannot be used as the condition of an if statement on its own. The correct spelling is if (x := f()):.

Historically Python had no switch because the design philosophy of the language treats a chain of if, elif and else as perfectly good enough for the cases that switch is used for. Adding a dedicated construct would have introduced a second way of saying the same thing. Guido van Rossum's own stated objection was that he did not want to add a construct that he did not need, and the community's experience with if and elif was that it worked. The other side of that coin is that long chains on the same variable get very verbose, and the pattern of comparing one object against many constants is awkward to read without a switch.

Python 3.10 added the match statement, and it is much more than a switch. It performs structural pattern matching. A case pattern can match on the type of a value, on the shape of a sequence, on the keys of a mapping, or on a plain literal. Most importantly, it can bind the parts that it matched to names, and a traditional switch cannot do that at all.

Two design points came straight from earlier proposals. match and case are soft keywords, so existing code that uses those words as identifiers keeps working. And the statement is named match rather than switch to avoid confusion with the C preprocessor keyword, and that explains a naming choice that still confuses people today.

The trade-off to explain in an interview answer is that match is more expressive but easier to misuse, and that a chain of simple equality tests is still clearer than a match statement with a single capture pattern. Reach for match when you are dispatching on structure, such as a type, a tuple shape, or a set of keys, and keep if and elif for plain value comparisons.