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Logical operators in Python: and, or, not

How and, or and not combine conditions in Python: truth tables, short-circuit evaluation, why and/or return one of their operands, precedence, and any() and all().

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A single comparison answers one question: is the user an adult? Real conditions often ask several at once: is the user an adult and do they have a ticket? Python has three logical operators for this: and, or and not. They’re words, not symbols like && and || in other languages.

Every example here was run with Python 3.13. The behavior was checked against the Boolean operations reference and the truth value testing docs.

The three operators

  • a and b is true when both sides are true.
  • a or b is true when at least one side is true.
  • not a flips the value.
age = 25
has_ticket = True
print(age >= 18 and has_ticket)
print(age < 12 or age > 65)
print(not has_ticket)

True
False
False

Here are all four combinations of two values:

for a in (True, False):
    for b in (True, False):
        print(a, b, a and b, a or b)

True True True True
True False False True
False True False True
False False False False

They make nested if statements flatter. The ticket check from the previous post becomes:

age = 20
has_ticket = True
if age >= 18 and has_ticket:
    print("Come in")

Come in

Short-circuit evaluation

Python evaluates from left to right and stops as soon as the answer is known. If the left side of and is false, the result is false, so the right side never runs. If the left side of or is true, the right side never runs:

def check(label, result):
    print("checking", label)
    return result

print(check("a", False) and check("b", True))
print(check("c", True) or check("d", True))

checking a
False
checking c
True

This works as a guard: put the safe check first, and the risky one runs only when it’s safe:

items = []
if items and items[0] == "x":
    print("starts with x")
else:
    print("empty or not x")

count = 0
total = 10
if count != 0 and total / count > 2:
    print("big")
print("no ZeroDivisionError")

empty or not x
no ZeroDivisionError

items[0] on an empty list would raise an IndexError, and total / count would raise a ZeroDivisionError. Neither runs, because the left side is already false.

and and or return a value, not just True or False

The docs are precise: x and y returns x if x is false, otherwise y. x or y returns x if x is true, otherwise y. They return one of the operands, not necessarily a bool:

print(0 and 5, 3 and 5, 0 or 5, 3 or 5)
print("" or "guest", [] or "empty", None and 1)
print(not "", not "hi", not 0)

0 5 5 3
guest empty None
True False True

not is different: it always returns True or False.

This makes or handy for defaults. An empty answer from input() is falsy, so or replaces it. Here the user pressed Enter without typing anything:

name = input("Name: ") or "guest"
print("Hello", name)

Name: 
Hello guest

Be careful with numbers: count or 10 also replaces a real 0. When 0 is a valid value, write 10 if count is None else count.

Precedence

not binds tightest, then and, then or. All three come after comparisons, so age >= 18 and has_ticket needs no parentheses:

print(True or False and False)
print((True or False) and False)
print(not 1 == 2)

True
False
True

The first line is True or (False and False). When you mix and and or, add parentheses even when they aren’t needed. The reader shouldn’t have to remember the table.

A common trap is comparing one variable to two values:

x = 5
print(x == 3 or 4)
print(x == 3 or x == 4)
print(x in (3, 4))

4
False
False

x == 3 or 4 means (x == 3) or 4. The comparison is false, so or returns 4, which is truthy. An if with that condition would always run. Repeat the comparison, or better, use in. For a range, a chained comparison such as 10 < x < 20 is shorter than two comparisons joined with and.

Putting it together: leap years

A year is a leap year if it divides by 4, except years that divide by 100, unless they also divide by 400:

def is_leap(year):
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

print(is_leap(2024), is_leap(1900), is_leap(2000), is_leap(2026))

True False True False

The parentheses matter here. % is the remainder operator, which gets its own post later in the series.

any() and all()

For many conditions, the built-ins any() and all() are the or and and of a whole collection:

scores = [72, 88, 95]
print(any(s > 90 for s in scores), all(s > 70 for s in scores))
print(all([]), any([]))

True True
True False

any is true if at least one item is true. all is true if every item is true. Note the empty cases: all([]) is True and any([]) is False. Both stop early, like and and or. See the any() and all() docs.

Common mistakes

  • Writing &&, || or !. Those are other languages. Python uses and, or and not. !x is a SyntaxError.
  • Using & and | instead of and and or. & and | are bitwise operators. 1 & 2 is 0, while 1 and 2 is 2. They also don’t short-circuit.

What’s next

Next: indentation, and the IndentationError you get when the blocks under your conditions are off.

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