True, False and Questions ⚖️

A third kind of value

sunny = True
raining = False

print(sunny, type(sunny))
print(raining)

True <class 'bool'>
False

Capital T, capital F, no quotes. "True" with quotes is a piece of text and a completely different thing. The type is named bool after George Boole, a Victorian mathematician who worked out the algebra of true and false about ninety years before there was a computer to run it on.

Comparisons produce booleans

print(5 > 3)
print(5 < 3)
print(5 == 5)      # equal? two equals signs!
print(5 != 5)      # not equal
print(5 >= 5)
print("a" < "b")   # alphabetical order

True
False
True
False
True
True

Combining questions

age = 25
has_ticket = True
banned = False

print(age >= 18 and has_ticket)
print(age >= 65 or has_ticket)
print(not banned)
print(age >= 18 and has_ticket and not banned)

True
True
True
True

Python uses the English words, not && and || like most other languages. Read your conditions out loud: if the sentence makes sense in English, it is probably right.

Chained comparisons, a genuine Python nicety

score = 75

print(0 <= score <= 100)          # Python allows this and it means what you think
print(70 < score < 80)

True
True

Most languages force you to write score >= 0 and score <= 100. Python lets you write it the way a mathematician would. It is one of those small touches that make people fond of the language.

Truthiness: everything can answer the question

Python will happily treat any value as a yes or no. The rule is simple and worth committing to memory: empty and zero are false, everything else is true.

print(bool(0), bool(1), bool(-1))
print(bool(""), bool("a"), bool(" "))
print(bool([]), bool([1]))
print(bool({}), bool({"a": 1}))
print(bool(None))

False True True
False True True
False True
False True
False

This is why real Python code looks like this:

name = ""

if not name:
    print("You did not give me a name.")

crew = ["Otis"]
if crew:
    print(f"There are {len(crew)} aboard.")

You did not give me a name.
There are 1 aboard.

if crew: rather than if len(crew) > 0:. Shorter, and every Python programmer reads it instantly. Note the space in bool(" ") is True: a space is a character, so the string is not empty. That distinction has ruined many a form validator.

None: the value that means "nothing here"

winner = None

print(winner)
print(winner is None)
print(type(winner))

None
True
<class 'NoneType'>

None is Python's way of saying "deliberately empty". It is not zero and not an empty string; it is the absence of a value. Functions that do not return anything return None, and it is the standard placeholder for "not decided yet".

== versus is

a = [1, 2, 3]
b = [1, 2, 3]
c = a

print(a == b)     # same contents?
print(a is b)     # the very same object in memory?
print(a is c)

True
False
True

The rule in practice: use == for everything, except when comparing against None, True or False, where the convention is is. So: if value is None:, always.

Short circuits

def expensive_check():
    print("  (this ran)")
    return True

print("First:")
result = False and expensive_check()

print("Second:")
result = True or expensive_check()

print("Neither of those printed the marker, because Python stopped early.")

First:
Second:
Neither of those printed the marker, because Python stopped early.

With and, if the left side is false the answer is already false, so Python never looks at the right side. This is not just an optimisation, it is a tool: it lets you write if name and name[0] == "G": without crashing on an empty name, because the second half never runs when the first half fails.

height = 152
age = 14
holding_grog = False

can_ride = height >= 140 and age >= 12 and not holding_grog
print(f"Cleared to ride: {can_ride}")

# and one who is not
print(f"Second rider:    {130 >= 140 and 30 >= 12 and not False}")

Cleared to ride: True
Second rider:    False

print(bool("False"))
print(bool(0.0))
print(bool([0]))
print(bool(" "))
print(None == False)

password = "grog"
locked = False

if password = "grog" and locked == True:
    print("Welcome")

password = "grog"
locked = False

if password == "grog" and not locked:
    print("Welcome")

Welcome