Dunder __bool__

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Used for built-in truth value testing and invoked by the builtin bool(...) method. This method should return either False or True booleans respectively and if a dunder __bool__ is omitted from a class, instances of such class will be checked based on a __len__. In the event that __len__ exists, anything other than nonzero is considered True. If neither a __bool__ or __len__ are defined, instances of such classes are determined to be True in a boolean context.`

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Examples

User defined classes by default will return True when evaluated in a boolean context. This is because neither __bool__ or __len__ are defined.

class Default:
    ...

bool(Default())  # True

In order to override the boolean behaviour, implement __bool__ and return True|False accordingly.

class DunderBool:

    def __init__(self, x: int) -> None:
        self.x = x

    def __bool__(self) -> bool:
        return self.x > 100

bool(DunderBool(100))  # False
bool(DunderBool(101))  # True

If __len__ is implemented and __bool_ is not, __len__ is used as a fallback, where non-zero values are considered True.

from typing import Optional
from typing import List

class FallbackToLen:

    def __init__(self, container: Optional[List[int]] = None) -> None:
        self.container = container or []

    def __len__(self) -> int:
        return len(self.container)

bool(FallbackToLen([1]))  # True
bool(FallbackToLen()) # False

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