Booleans

Two values

True and False are the whole type. Both are capitalised, and both are keywords rather than strings:

is_open = True
is_locked = False
print(is_open, is_locked)
print(type(is_open))

True False
<class 'bool'>

Comparisons produce booleans

Any comparison evaluates to one of the two:

print(10 > 3)
print(10 == 3)
print("a" < "b")
print(3 in [1, 2, 3])

True
False
True
True

Truthiness

Every value can stand in for a condition. The rule is short: empty and zero are false, everything else is true.

Falsy values, in full:

• False and None
• zero of any numeric type: 0, 0.0
• empty sequences and collections: "", [], (), {}, set()

print(bool(0), bool(0.0), bool(None))
print(bool(""), bool([]), bool({}))
print(bool(1), bool(-1), bool("no"), bool([0]))

False False False
False False False
True True True True

Note bool([0]) is True: the list holds one item, so it is not empty. What is in it does not matter.

Using truthiness

This is why Python code checks emptiness directly rather than comparing lengths:

items = []

if items:
    print("we have items")
else:
    print("nothing here")

nothing here

if items: is preferred over if len(items) > 0: — shorter, and it works for any container.

and, or, not

print(True and False)
print(True or False)
print(not True)

False
True
False

These do not return True or False — they return one of the operands, and stop as soon as the answer is settled:

print(0 or "fallback")
print("given" or "fallback")
print(1 and "second")

fallback
given
second

That short-circuit is used deliberately for defaults, and it is also why name or "Anonymous" is such a common line.

Booleans are integers

bool is a subclass of int, with True worth 1 and False worth 0:

print(True + True)
print(sum([True, False, True]))

2
2

Summing a list of comparisons to count matches is a genuinely useful trick:

scores = [45, 80, 62, 95]
print(sum(score > 60 for score in scores), "passed")

3 passed

any and all

print(any([False, False, True]))
print(all([True, True, False]))
print(all([]))

True
False
True

all([]) is True: there is no item that fails, so the claim holds. It catches people out, and it is the mathematically consistent answer.