til/python/override-the-boolean-context-of-a-class.md

Override The Boolean Context Of A Class

Everything in Python has a truthiness that can be checked with bool(). An empty list ([]) is falsy. A non-empty list ([1,2,3]) is truthy. Similar with numbers:

>>> bool(0)
False
>>> bool(1)
True

Any instance of an object is going to be truthy by default. If you want to control in what context an instance is considered truthy or falsy, you can override __bool__(). If that's not implemented, but __len__() is, then it will fallback to that.

Let's look at a few example classes:

class CartZero:
    def __init__(self, items=[]):
        self.items = items or []

class CartBool:
    def __init__(self, items=[]):
        self.items = items or []

    def __bool__(self):
        print("__bool__() override")
        return bool(self.items)

class CartLen:
    def __init__(self, items=[]):
        self.items = items or []

    def __len__(self):
        print("__len__() override")
        return len(self.items)

class CartBoolAndLen:
    def __init__(self, items=[]):
        self.items = items or []

    def __len__(self):
        print("__len__() override")
        return len(self.items)

    def __bool__(self):
        print("__bool__() override")
        return bool(self.items)

cart1 = CartZero()
cart2 = CartBool()
cart3 = CartLen()
cart4 = CartBoolAndLen()

print("CartZero() -> %s" %(bool(cart1)))
print('')
print("CartBool() -> %s" %(bool(cart2)))
print('')
print("CartLen() -> %s" %(bool(cart3)))
print('')
print("CartBoolAndLen() -> %s" %(bool(cart4)))

An 'empty' Cart be default is truthy. However, we can override some combination of __bool__() or __len__() to give it a boolean context that goes false when "empty".

CartZero() -> True

__bool__() override
CartBool() -> False

__len__() override
CartLen() -> False

__bool__() override
CartBoolAndLen() -> False