What happens if you don’t implement __bool__ in a class?

All Questions From This Chapter

If you don’t implement __bool__ in a Python class, Python falls back to another method to determine the truthiness of your object.

🔁 Fallback Behavior:

1. If __bool__ is not defined, Python checks for __len__.

2. If __len__ is defined:

   • if obj: will evaluate to False if len(obj) == 0.

   • Otherwise, it evaluates to True.

3. If neither __bool__ nor __len__ is defined, the object is considered truthy by default (i.e., True).

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🔍 Example Without __bool__:

python
class Empty:
    def __len__(self):
        return 0

e = Empty()
print(bool(e))  # False, because __len__ returns 0

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🧱 Example With Neither:

python
class Thing:
    pass

t = Thing()
print(bool(t))  # True, because no __bool__ or __len__

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✅ Summary

You define…	bool(obj) is…
__bool__	Uses the returned Boolean value
__len__, no __bool__	False if len(obj) == 0, True otherwise
Neither	Always True

🧠 Tip: Define __bool__ if you want explicit control over an object’s truthiness.

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