[issue32768] object.__new__ does not accept arguments if __bases__ is changed
Alexey Muranov
report at bugs.python.org
Tue May 7 03:40:03 EDT 2019
Alexey Muranov <alexey.muranov at 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
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<https://bugs.python.org/issue32768>
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