Source code for spinn_utilities.require_subclass

# Copyright (c) 2021 The University of Manchester
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# GNU General Public License for more details.
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class _RequiresSubclassTypeError(TypeError):
    A special exception to handle just the case where the decorator is applied
    so that we can easily figure out when to pass on the extra meta-arguments.

    Do not use outside this file. Outsiders should just see ``TypeError``.

[docs]def require_subclass(required_class): """ Decorator that arranges for subclasses of the decorated class to\ require that they are also subclasses of the given class. :param type required_class: The class that the subclass of the decorated class must be an instance of (if that subclass is concrete). """ # Beware! This is all deep shenanigans! # # The __init_subclass__ stuff is from # # The setattr() call is from: # # The classmethod() call is from: # # The use of __class__ to enable super() to work is from: # # The need to do this as a functional decorator is my own discovery; # without it, some very weird interactions with metaclasses happen and I # really don't want to debug that stuff. def decorate(target_class): # pylint: disable=unused-variable __class__ = target_class # @ReservedAssignment # noqa: F841 def __init_subclass__(cls, allow_derivation=False, **kwargs): if not issubclass(cls, required_class) and not allow_derivation: raise _RequiresSubclassTypeError( f"{cls.__name__} must be a subclass " f"of {required_class.__name__} and the derivation was not " "explicitly allowed with allow_derivation=True") try: super().__init_subclass__(**kwargs) except _RequiresSubclassTypeError: super().__init_subclass__( allow_derivation=allow_derivation, **kwargs) setattr(target_class, '__init_subclass__', classmethod(__init_subclass__)) return target_class return decorate