[Python-Dev] Method __func__ objects are unpicklable

[haael]

During some sophisticated pickling I noticed that method `__func__`
objects are unpicklable, because they share the name with the bound
method object itself.


```
from pickle import dumps

class A:
@classmethod
def b(cls):
pass

print(A.b) # <bound method A.b of <class '__main__.A'>>
print(A.b.__func__) # <function A.b at 0x7f5574570bf8>

dumps(A.b) # works
dumps(A.b.__func__) # breaks
# >Traceback (most recent call last):
# > File "<stdin>", line 1, in <module>
# >_pickle.PicklingError: Can't pickle <function A.b at 0x7fd1d50d8bf8>:
# >it's not the same object as __main__.A.b
```


The last call compains that global symbol "A.b" is not the same object
as `A.b.__func__`.

Everything would work if the `__func__` objects had the suffix
".__func__" in their qualname.

Actually setting the qualname of the `__func__` object makes it
picklable, but then the bound method object is unpicklable, as it
inherits the name from the `__func__`. It would be good if they were
separate.

This is an attempt at emulating the desired behavior:


```
from pickle import dumps

class A:
pass

def c(cls):
pass

A.c = lambda: c(A)
A.c.__qualname__ = "A.c"
A.c.__name__ = "c"
A.c.__self__ = A
A.c.__func__ = c
A.c.__func__.__qualname__ = "A.c.__func__"

print(A.c) # <function A.c at 0x7f83ceabcbf8>
print(A.c.__func__) # <function A.c.__func__ at 0x7f83ceabcb70>

dumps(A.c) # works
dumps(A.c.__func__) # works
```


Can we make the `__func__` objects picklable this way?


haael

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[Joao S. O. Bueno]

This is not just about the content of the name attributes: ;bound methods do contain a reference to the specific instance

they are bound too - which is not retrievable (or maybe is through some dunder attribute) - this instance has to be pickled, transported

and its reference updated on unpickling.

[Brett Cannon]

I believe a lot of people use https://pypi.org/project/cloudpickle/ thanks to its support for pickling functions.