Since 3.6: TypeError: cannot create weak reference to '_Any' object

[Tinche]

Hi,

I'm the author of cattrs, which is an experimental library for converting Python data (to keep it simple, it's a little more complex than that, and docs are at rtd).

Anyway, cattrs uses typing.py for runtime conversions, along with singledispatch. Looks like the version of typing.py in 3.6 broke it in several ways. :) Here's the first:

>>> import typing, weakref
>>> weakref.ref(typing.Any)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create weak reference to '_Any' object

This used to work. It's a problem when used together with singledispatch, since singledispatch has a WeakKeyDictionary cache in it.

>>> import typing
>>> from functools import singledispatch
>>> s = singledispatch(lambda x: x)
>>> s.register(typing.Any, lambda x: x+1)
<function <lambda> at 0x10d89a268>
>>> s.dispatch(typing.Any)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/functools.py", line 776, in dispatch
    impl = dispatch_cache[cls]
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/weakref.py", line 365, in __getitem__
    return self.data[ref(key)]
TypeError: cannot create weak reference to '_Any' object

This is kind of unfortunate for the library. Any chance of this being fixed on this end?

[gvanrossum]

It is indeed unfortunate. Perhaps you can submit a PR? Note that we won't be able to fix this in 3.5.3 (it's too close to the release date) so it'll have to go into 3.5.4 as well as 3.6.1.

[Tinche]

I can take a look but off the cuff I don't actually know how to fix this. What's the criterion for Python being able to create weak references to things?

[gvanrossum]

It may be as easy as adding a __weakref__ slot to the appropriate base class: http://stackoverflow.com/questions/19526340/weakref-and-slots

Also it would need tests of course.

[ilevkivskyi]

I think we should enable this.

@gvanrossum You are right. Here is a PR #348 that adds __weakref__.