Python Booleans

3Python Booleans: explained step by step

A boolean is the simplest value there is: it is either True or False. Python's type for it is bool. Booleans are the answers to yes/no questions, and they are what if statements and loops use to decide what happens next.

Both names start with a capital letter. true and false are not Python; they are just undefined names and cause a NameError.

Comparisons produce booleans

Every comparison operator (==, !=, <, >, <=, >=) and the membership test in evaluates to a bool. You rarely type True yourself; you let a comparison create it.

age = 17
print(age >= 18)
print(age != 20)
print("a" in "banana")
print(type(age >= 18))

can_vote = age >= 18
print(can_vote)

Storing a comparison in a variable such as can_vote gives the condition a name, which makes later code read like English.

bool is a kind of int

Surprising but useful: True is 1 and False is 0. bool is a subclass of int, so booleans work in arithmetic. The most practical trick is counting: add up a list of booleans to count how many are True.

print(True + True)
print(True * 10)
print(isinstance(True, int))

scores = [45, 80, 92, 38, 71]
passed = sum(score >= 50 for score in scores)
print("Passed:", passed)

If you need to know whether something is specifically a boolean and not just a number, use isinstance(x, bool). Check for bool first, because isinstance(True, int) is also true.

and, or and not

Three keywords combine booleans. and needs both sides true, or needs at least one, and not flips the value. The logical conditions page shows how to build longer conditions, and operators lists their precedence: not first, then and, then or.

logged_in = True
is_admin = False
print(logged_in and is_admin)
print(logged_in or is_admin)
print(not is_admin)

and / or return an operand, not just True or False

This detail trips up many learners. and and or short-circuit: they stop as soon as the answer is known, and the value they return is the operand that decided it, not necessarily a bool.

• a or b returns a if a is truthy, otherwise b.
• a and b returns a if a is falsy, otherwise b.

username = ""
print(username or "Guest")
print(0 and 100)
print("Asha" and "Welcome")

items = []
print(len(items) > 0 and items[0])

The last line is why short-circuiting is useful: when the list is empty, items[0] is never evaluated, so no IndexError happens. The idea of "which values count as false" is covered on the truthy and falsy page.

Chained comparisons

Python lets you write a range check the way you would on paper. 1 < x < 10 means 1 < x and x < 10, and x is only evaluated once.

x = 7
print(1 < x < 10)
print(10 <= x <= 20)
print(0 < x != 5)

== versus is

== asks "are these values equal?" while is asks "are these the very same object?" Because True and False each exist only once, is True works for real booleans, but it is stricter than most code needs. Note that 1 == True is True, yet 1 is True is False.

flag = 1
print(flag == True)
print(flag is True)

is_ready = True
if is_ready:
    print("go")

The plain if is_ready: is the idiomatic form. Writing if is_ready == True: adds nothing and breaks for truthy values that are not exactly True.

Booleans as flags, and naming them well

A flag is a boolean variable that remembers whether something has happened, such as found = False before a loop and found = True inside it. Start names with is_, has_, can_ or should_ so the variable reads like a question: is_empty, has_discount. For checking many conditions at once, see any() and all(): all(flags) is true when every item is true, and any(flags) when at least one is.

Converting other values into booleans is covered in Python casting; the short version is that bool(x) follows the same rules an if does.