[New-bugs-announce] [issue37297] function changed when pickle bound method object

[George Xie]

New submission from George Xie <georgexsh at gmail.com>:

if we create a bound method object `f` with function object `A.f` and instance object `B()`,
when pickling this bound method object:

	import pickle

	class A():
	    def f(self):
	        pass

	class B():
	    def f(self):
	        pass

	o = B()
	f = A.f.__get__(o)
	pf = pickle.loads(pickle.dumps(f))
	print(f)
	print(pf)

we get:

	<bound method A.f of <__main__.B object at 0x10b82ac18>>
	<bound method B.f of <__main__.B object at 0x10b82ac88>>

the underlaying function are lost, changed from `A.f` to `B.f`.

as pickle calls `__reduce__` method of `method object`, IMO [its implementation][1] simply ignored the real function, whcih is not right.

I have tried a [wordaround][2]:

	import types
	import copyreg

	def my_reduce(obj):
	    return (obj.__func__.__get__, (obj.__self__,))

	copyreg.pickle(types.MethodType, my_reduce)


[1]: https://github.com/python/cpython/blob/v3.7.3/Objects/classobject.c#L75-L89
[2]: https://stackoverflow.com/a/56614748/4201810

----------
components: Library (Lib)
messages: 345721
nosy: georgexsh
priority: normal
severity: normal
status: open
title: function changed when pickle bound method object
type: behavior
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37297>
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