invalid-format-returned / E0311

Message emitted:

__format__ does not return str

Description:

Used when a __format__ method returns something which is not a string

Problematic code:

class CustomFormat:
    """__format__ returns <type 'int'>"""

    def __format__(self, format_spec):  # [invalid-format-returned]
        return 1

Correct code:

class CustomFormat:
    """__format__ returns <type 'str'>"""

    def __format__(self, format_spec):
        return "hello!"

Created by the classes checker.