WeakMethod does not support builtin methods

[anntzer]

BPO	27865
Nosy	@anntzer

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-08-26.02:46:05.074>
labels = ['library']
title = 'WeakMethod does not support builtin methods'
updated_at = <Date 2016-08-26.02:46:05.074>
user = 'https://github.com/anntzer'

bugs.python.org fields:

activity = <Date 2016-08-26.02:46:05.074>
actor = 'Antony.Lee'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-08-26.02:46:05.074>
creator = 'Antony.Lee'
dependencies = []
files = []
hgrepos = []
issue_num = 27865
keywords = []
message_count = 1.0
messages = ['273688']
nosy_count = 1.0
nosy_names = ['Antony.Lee']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue27865'
versions = ['Python 3.5', 'Python 3.6']

[anntzer]

List subclasses can be weakref'd (as mentioned by the docs), but their methods cannot be wrapped by WeakMethod, even though this makes sense semantically:

In [25]: class L(list): pass

In [26]: weakref.WeakMethod(L().append)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-26-f6246b3b483e> in <module>()
----> 1 weakref.WeakMethod(L().append)

/usr/lib/python3.5/weakref.py in __new__(cls, meth, callback)
     47         except AttributeError:
     48             raise TypeError("argument should be a bound method, not {}"
---> 49                             .format(type(meth))) from None
     50         def _cb(arg):
     51             # The self-weakref trick is needed to avoid creating a reference

TypeError: argument should be a bound method, not <class 'builtin_function_or_method'>

[devdanzin]

Reproduced on main (3.15).

[sergey-miryanov]

I want to work on this, if no one don't mind.

[picnixz]

This is a documented behavior actually:

Not all objects can be weakly referenced. Objects which support weak references include class instances, functions written in Python (but not in C), instance methods, sets, frozensets, some file objects, generators, type objects, sockets, arrays, deques, regular expression pattern objects, and code objects.

In this case append is a built-in function which doesn't have a __func__. So we can't use it.

[sergey-miryanov]

IIUC, append is a method_descriptor. Maybe we can add __func__ and __self__ to it and other descriptors like wrapper_descriptor?

[picnixz]

A method descriptor should behave as an unbound method (that's its purpose) so it doesn't make sense to have a __func__ or a __self__. Note that the conversion from wrapper descriptors to method descriptors for built-in types was done in Python 2.4 for efficiency reasons. I would suggest closing this issue as not planned.

[sergey-miryanov]

Yeah, I saw notes about conversion.

I'm +1 to close this.