invalid-length-hint-returned / E0310

Message emitted:

__length_hint__ does not return non-negative integer

Description:

Used when a __length_hint__ method returns something which is not a non-negative integer

Problematic code:

class CustomLengthHint:
    """__length_hint__ returns non-int"""

    def __length_hint__(self):  # [invalid-length-hint-returned]
        return 3.0

Correct code:

class CustomLengthHint:
    """__length_hint__ returns <type 'int'>"""

    def __length_hint__(self):
        return 10

Created by the classes checker.