How does Python determine whether a custom object is truthy or falsy?

Every Python object has a boolean value. In a boolean context (an if condition, a while condition, or passed to bool()), Python calls the object's __bool__ method first. If that is not defined, it falls back to __len__ and returns False if __len__ returns 0. If neither is defined, the object is always truthy.

class Queue:
    def __init__(self):
        self._data = []

    def enqueue(self, item):
        self._data.append(item)

    def __len__(self):
        return len(self._data)

    def __bool__(self):
        return len(self._data) > 0   # explicit

q = Queue()
if not q:
    print('Queue is empty')    # printed — __bool__ returns False

q.enqueue('item')
if q:
    print('Queue has items')   # printed — __bool__ returns True

The built-in falsy values to memorise: None, False, 0, 0.0, 0j (complex zero), '' (empty string), b'' (empty bytes), [], (), {}, set(), and any object whose __bool__ returns False or whose __len__ returns 0.

Practical impact: you can write Pythonic guards like if items:, while queue:, and return value or default instead of verbose length checks. The short-circuit operators and and or return one of their operands, not necessarily a bool: 'alice' or 'default' returns 'alice'; '' or 'default' returns 'default'.

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