Bool

True means "yes" and false means "no." In Python we rarely need to use the bool built-in. Often expressions can just be used directly as bool expressions.

In a class, we can provide a bool method that returns True or False. This method indicates how an instance of the class evaluates when used with bool.

Here, we use the bool built-in with some expressions and constants. We see how Python evaluates the truth of things. A nonempty string is True, but None is False.

# Compute bools based on expressions and constants.
value1 = bool(2 == 3)
print(value1)

value2 = bool(None)
print(value2)

value3 = bool("cat")
print(value3)
print()

# Compute bools based on these variables.
a = 10
b = 10
value4 = bool(a == b)
print(value4)

value5 = bool(a != b)
print(value5)

value6 = bool(a > b)
print(value6)
False
False
True

True
False
False

Class, def bool

Let us continue with a class. We implement the "__bool__" method. This method must return True or False (this is the bool value a class instance evaluates to).

class Box:
    def __init__(self, value):
        # Initialize our box.
        self.value = value

    def __bool__(self):
        # This returns true only if value is 1.
        if self.value == 1:
            return True
        else:
            return False

# Create Box instances.
# ... Use bool on them.
box = Box(0)
result = bool(box)
print(result)

box = Box(1)
result = bool(box)
print(result)

box = Box(2)
result = bool(box)
print(result)
False
True
False

Bool expressions

Often we can omit the "bool" keyword entirely—this can lead to clearer, shorter code. Here we store a bool in the correct and correct_bool variables.

value = 5

# Both of these expressions evaluate to a bool.
# ... The bool keyword is not required.
correct = value == 5
correct_bool = bool(value == 5)

# Print results.
print(correct)
print(correct_bool)
True
True

Argument examples

Let us be thorough with our examination of bool. When called with no arguments, bool() returns False. I have never used this in a Python program.

# These evaluate to false.
print(bool())
print(bool(None))
# The inner bool returns value.
print(bool(bool()))

# A string evaluates to true.
print(bool("a"))
False
False
False
True

Notes, int

A bool might look like a separate type. But it really is just an int—a subclass of an int. A bool can be only True or False.

Using bool() has been, in my experience, rare in simple Python programs. But it is important to know about it, for when it is needed.