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if, elif, else: conditions in Python

How Python decides which code to run: if, else and elif, the comparison operators, truthy and falsy values, the conditional expression (x if C else y), and a first look at match.

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So far every line in our programs ran, top to bottom. Real programs make decisions: show a different message to a minor, refuse an empty password, grade a score. In Python that’s the if statement.

Every example here was run with Python 3.13. The behavior was checked against the tutorial section on if statements, the comparisons reference and the conditional expressions reference.

The if statement

An if has a condition, a colon, and an indented block. The block runs only when the condition is true:

temperature = 31
if temperature > 30:
    print("Hot")
    print("Drink water")
print("Done")

Hot
Drink water
Done

Both indented lines belong to the if. print("Done") is back at the left edge, so it always runs. Indentation is how Python knows where the block ends (indentation post).

else: the other branch

else runs when the condition is false. It has no condition of its own:

age = 15
if age >= 18:
    print("You can vote")
else:
    print("Too young to vote")

Too young to vote

Exactly one of the two blocks runs, never both and never neither.

elif: more than two options

elif is short for “else if”. You can have as many as you need, and else at the end is optional:

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(grade)

B

Python checks the conditions from the top and runs the first block whose condition is true; the rest are skipped, even if true. That’s why order matters. Here the checks are in the wrong order:

score = 95
if score >= 80:
    print("B")
elif score >= 90:
    print("A")

B

score >= 90 is never checked. Put the narrowest condition first.

The docs point out that an if/elif/elif chain is Python’s substitute for the switch or case statements of other languages.

Comparison operators

A condition is usually a comparison. Each one returns True or False:

Operator Meaning
== equal
!= not equal
< less than
<= less than or equal
> greater than
>= greater than or equal 
print(5 == 5, 5 != 3, 3 < 5, 5 <= 5, 7 > 9, 7 >= 7)
print("a" == "A", "apple" < "banana", 1 == 1.0)

True True True True False True
False True True

Strings compare character by character, and uppercase and lowercase are different characters. 1 == 1.0 is True, because numbers compare by value even across int and float.

Comparisons can be chained. The docs say x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once:

age = 25
print(18 <= age < 65)

True

Two more operators show up in conditions all the time. in checks membership, as in the lists post, and on a string it finds a substring. is checks identity; use it only with None:

print("z" in "pizza", "x" not in "pizza")

value = None
if value is None:
    print("no value")

True True
no value

Truthy and falsy values

A condition doesn’t have to be True or False. Python accepts any value and tests its truthiness, the same test bool() runs in the type conversion post:

name = ""
if name:
    print("Hello", name)
else:
    print("No name given")

items = [1, 2]
if items:
    print(len(items), "items")

No name given
2 items

if name: reads as “if there is a name”. It’s shorter than if name != "": and is the usual Python style.

Nested conditions

A block can contain another if:

age = 20
has_ticket = True
if age >= 18:
    if has_ticket:
        print("Come in")
    else:
        print("Buy a ticket")
else:
    print("Adults only")

Come in

Nesting works, but every level pushes the code further right. Often and gives a flatter version, which is the topic of the next post.

A block can’t be empty. If you need a placeholder, write pass, a statement that does nothing:

x = 3
if x > 0:
    pass
print("ok")

ok

The conditional expression

When all you need is to choose between two values, there’s a one-line form. The docs call it a conditional expression, “sometimes called a ternary operator”:

age = 20
status = "adult" if age >= 18 else "minor"
print(status)

n = 1
print(f"{n} item{'' if n == 1 else 's'}")

adult
1 item

Read it as “adult if the condition is true, else minor”. The condition is evaluated first, then only one of the two values. Being an expression, it fits inside an assignment, a call or an f-string. Keep it for short choices; if either side needs more than a few words, use a normal if/else.

When there are many values: match

If you’re comparing one value against many constants, Python 3.10 added the match statement:

def describe(command):
    match command:
        case "start" | "go":
            return "Starting"
        case "stop":
            return "Stopping"
        case _:
            return "Unknown command"

print(describe("go"), describe("stop"), describe("jump"))

Starting Stopping Unknown command

Only the first matching case runs. | means “or”, and _ matches anything, like else. match can do much more, such as unpacking lists and objects (see the tutorial), but for a few conditions if/elif is simpler.

Common mistakes

  • = instead of ==. if x = 5: is a SyntaxError. Python even suggests the fix: Maybe you meant '==' or ':=' instead of '='?
  • Forgetting the colon. if x == 5 without : gives SyntaxError: expected ':'.
  • Comparing text to a number. 5 == "5" is False, not an error. Convert the value from input() before comparing.

What’s next

Next: the logical operators and, or and not, which combine several conditions into one.

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