Default Truthiness for Instances Without Special Methods

Objects that implement neither __bool__ nor __len__ always evaluate as True in Boolean contexts.

class Packet:
    pass

pkt = Packet()
print(bool(pkt))  # True

Controlling Evaluasion with __bool__

If a class defines __bool__, that method's return value determines the instance's truthiness. Python rqeuires an actual bool return type; otherwise it raises TypeError.

class RetryLimiter:
    def __init__(self, attempts):
        self.attempts = attempts

    def __bool__(self):
        print(f"__bool__ invoked: attempts={self.attempts}")
        self.attempts -= 1
        return self.attempts >= 0

limiter = RetryLimiter(4)
while limiter:
    print("Attempt in progress")
print("No retries left")

# __bool__ invoked: attempts=4
# Attempt in progress
# __bool__ invoked: attempts=3
# Attempt in progress
# __bool__ invoked: attempts=2
# Attempt in progress
# __bool__ invoked: attempts=1
# Attempt in progress
# __bool__ invoked: attempts=0
# No retries left

Flalback to __len__

When __bool__ is missing, the interpreter invokes __len__ if available. A return value of 0 produces False, while any non-zero integer produces True. Like __bool__, this method must return an integer.

class TaskQueue:
    def __init__(self, pending):
        self.pending = pending

    def __len__(self):
        print(f"__len__ invoked: pending={self.pending}")
        self.pending -= 1
        return self.pending

queue = TaskQueue(4)
while queue:
    print("Processing task")
print("Queue empty")

# __len__ invoked: pending=4
# Processing task
# __len__ invoked: pending=3
# Processing task
# __len__ invoked: pending=2
# Processing task
# __len__ invoked: pending=1
# Queue empty

Priority of __bool__ Over __len__

If both methods exist, __bool__ takes precedence and __len__ is ignored during truthiness checks.

class Resource:
    def __init__(self, value):
        self.value = value

    def __bool__(self):
        print(f"__bool__ path: value={self.value}")
        self.value -= 1
        return self.value >= 0

    def __len__(self):
        print(f"__len__ path: value={self.value}")
        self.value -= 1
        return self.value

res = Resource(4)
while res:
    print("Resource available")
print("Resource depleted")

# __bool__ path: value=4
# Resource available
# __bool__ path: value=3
# Resource available
# __bool__ path: value=2
# Resource available
# __bool__ path: value=1
# Resource available
# __bool__ path: value=0
# Resource depleted