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invalid-length-return-type (PLE0303)

Added in 0.6.0 · Related issues · View source

Derived from the Pylint linter.

What it does

Checks for __len__ implementations that return values that are not non-negative integers.

Why is this bad?

The __len__ method should return a non-negative integer. Returning a different value may cause unexpected behavior.

Note: bool is a subclass of int, so it's technically valid for __len__ to return True or False. However, for consistency with other rules, Ruff will still emit a diagnostic when __len__ returns a bool.

Example

class Foo:
    def __len__(self):
        return "2"

Use instead:

class Foo:
    def __len__(self):
        return 2

References