functools.update_wrapper does not work with type

[nikola-benes]

Bug report

Bug description:

from functools import update_wrapper

def my_type(*args): pass

t = update_wrapper(my_type, type)

This works on Python ≤ 3.11, and I think this is expected because the docs for update_wrapper claim that it may be used with callables other than functions.

However, with 3.12, I get

TypeError: __type_params__ must be set to a tuple

(Similarly for functools.wraps.)

The problem is that update_wrapper includes __type_params__ by default (which is undocumented, see related issue) and type.__type_params__ is a descriptor.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-119011: type.__type_params__ now return an empty tuple #119296
• gh-119011: Return an empty tuple on type_get_type_params as special case for PyType_Type #119313
• [3.13] gh-119011: type.__type_params__ now return an empty tuple (GH-119296) #119678
• [3.12] gh-119011: type.__type_params__ now return an empty tuple (GH-119296) #119681

[sobolevn]

3.11:

Python 3.11.5 (main, Sep  5 2023, 10:34:31) [Clang 14.0.3 (clang-1403.0.22.14.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from functools import update_wrapper
>>> 
>>> def my_type(*args): pass
... 
>>> t = update_wrapper(my_type, type)
>>> 
>>> t
<function type at 0x104963060>

3.12+:

>>> from functools import update_wrapper
... 
... def my_type(*args): pass
... 
... t = update_wrapper(my_type, type)
Traceback (most recent call last):
  File "<python-input-8>", line 5, in <module>
    t = update_wrapper(my_type, type)
  File "/Users/sobolev/Desktop/cpython2/Lib/functools.py", line 57, in update_wrapper
    setattr(wrapper, attr, value)
    ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
TypeError: __type_params__ must be set to a tuple

[dwt]

This was reported to me at dwt/fluent#14

I'm seeing this when wrapping type() to make it useable in a fluent way.

Is there something I need to do differently for python 3.12, or is this a bug in the python stdlib?

[AlexWaygood]

Is there something I need to do differently for python 3.12, or is this a bug in the python stdlib?

This is a CPython bug

[mdkcore0]

hi @AlexWaygood, I'm trying to figure out what is happening here; what CPython should do with __type_params__? Should it be skipped?

@encukou is also helping me to understand it :)

[JelleZijlstra]

Some notes from in-person conversation:

The problem is this:

>>> type.__type_params__
<attribute '__type_params__' of 'type' objects>

We would instead want this to be an empty tuple, because that's the normal behavior for classes that are not PEP 695 generics.

The way to achieve this would be to use a special case for &PyType_Type in the type_get_type_params getter. There is a similar check in the __abstractmethods__ getter.