Alexey Muranov <alexey.mura...@gmail.com> added the comment:
Here is a use case for writable bases: https://stackoverflow.com/q/56007866 class Stateful: """ Abstract base class for "stateful" classes. Subclasses must implement InitState mixin. """ @staticmethod def __new__(cls, *args, **kwargs): super_new = super(__class__, __class__).__new__ # XXX: see https://stackoverflow.com/a/9639512 class CurrentStateProxy(cls.InitState): @staticmethod def _set_state(state_cls=cls.InitState): __class__.__bases__ = (state_cls,) class Eigenclass(CurrentStateProxy, cls): @staticmethod def __new__(cls, *args, **kwargs): cls.__new__ = None # just in case return super_new(cls, *args, **kwargs) return Eigenclass(*args, **kwargs) class StatefulThing(Stateful): class StateA: """First state mixin.""" def say_hello(self): print("Hello!") self.hello_count += 1 self._set_state(self.StateB) return True def say_goodbye(self): print("Another goodbye?") return False class StateB: """Second state mixin.""" def say_hello(self): print("Another hello?") return False def say_goodbye(self): print("Goodbye!") self.goodbye_count += 1 self._set_state(self.StateA) return True # This one is required by Stateful. class InitState(StateA): """Third state mixin -- the initial state.""" def say_goodbye(self): print("Why?") return False def __init__(self): self.hello_count = self.goodbye_count = 0 def say_hello_followed_by_goodbye(self): self.say_hello() and self.say_goodbye() # ---------- # ## Demo ## # ---------- if __name__ == "__main__": t1 = StatefulThing() t2 = StatefulThing() print("> t1, say hello:") t1.say_hello() print("> t2, say goodbye:") t2.say_goodbye() print("> t2, say hello:") t2.say_hello() print("> t1, say hello:") t1.say_hello() print("> t1, say hello followed by goodbye:") t1.say_hello_followed_by_goodbye() print("> t2, say goodbye:") t2.say_goodbye() print("> t2, say hello followed by goodbye:") t2.say_hello_followed_by_goodbye() print("> t1, say goodbye:") t1.say_goodbye() print("> t2, say hello:") t2.say_hello() print("---") print( "t1 said {} hellos and {} goodbyes." .format(t1.hello_count, t1.goodbye_count) ) print( "t2 said {} hellos and {} goodbyes." .format(t2.hello_count, t2.goodbye_count) ) # Expected output: # # > t1, say hello: # Hello! # > t2, say goodbye: # Why? # > t2, say hello: # Hello! # > t1, say hello: # Another hello? # > t1, say hello followed by goodbye: # Another hello? # > t2, say goodbye: # Goodbye! # > t2, say hello followed by goodbye: # Hello! # Goodbye! # > t1, say goodbye: # Goodbye! # > t2, say hello: # Hello! # --- # t1 said 1 hellos and 1 goodbyes. # t2 said 3 hellos and 2 goodbyes. ---------- nosy: +alexey-muranov _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue32768> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
