Bug report

In

@cbook.deprecated("3.4")
class Mixin:
    pass

class Base:
    def __init__(self, x): self.x = x

class Mixed(Mixin, Base):
    pass

Mixed(x=1)

the last line raises a rather puzzling

TypeError: object.__init__() takes exactly one argument (the instance to initialize)

because the deprecation machinery grabs object.__init__ to install the deprecation emitter on Mixin. (This is not a theoretical case, see e.g. #18675.)

There's a few possible solutions:

• "it's too rare, let's not bother"
• forbid deprecation of classes where __init__ is not defined (which I think is the main failing case)
• make cbook.deprecated actually create another class that inherits from Mixin and has its own __init__ (which gets wrapped in cbook.deprecated() and just calls super().__init__), instead of monkey-patching Mixin's original __init__.

Matplotlib version

• Operating system: n/a
• Matplotlib version: HEAD
• Matplotlib backend (print(matplotlib.get_backend())): n/a
• Python version: n/a
• Jupyter version (if applicable):
• Other libraries: