function.__type_params__ and type.__type_params__ should be writable

[JelleZijlstra]

The PEP-695 implementation added a new attribute __type_params__ to functions. I made this field read-only, but I realized there is a use case for writing to it: functools.wraps, when wrapping a generic function, should add the .__type_params__ to the wrapper. Making it writable is also more consistent with other fields on functions, as even the __name__ of functions is writable.

The PEP also adds a __type_params__ attribute to classes and type aliases. For classes it's already writable (it's just stored in the type's __dict__). For type aliases it's readonly, but as I don't see a use case for mutating a type alias's __type_params__, I'd like to keep it that way.

Linked PRs

• gh-104600: Make function.__type_params__ writable #104601
• gh-104600: Make type.__type_params__ writable #104634

[AlexWaygood]

For classes it's already writable (it's just stored in the type's __dict__).

Small correction: as noted in #104616, it's not actually writeable on class objects either at the moment (and probably should be, for similar reasons):

Running Debug|x64 interpreter...
Python 3.12.0a7+ (heads/main:cfa517d5a6, May 18 2023, 17:12:51) [MSC v.1932 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo[T]: ...
...
>>> Foo.__type_params__ = ("bar",)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: attribute '__type_params__' of 'type' objects is not writable
>>> class Bar: ...
...
>>> Bar.__type_params__ = ("bar",)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: attribute '__type_params__' of 'type' objects is not writable

[sobolevn]

Sorry, but now PEP is misleading:

A new read-only attribute called __type_params__ is available on generic classes, functions, and type aliases.

But, since it is now writtable (on classes as well):

>>> class A[X]: ...
... 
>>> A.__type_params__
(X,)
>>> A.__type_params__ = ()
>>> A.__type_params__
()

Shouldn't the PEP be updated? Or do you want to make this "implementation detail" private?

[JelleZijlstra]

Good catch, I missed that the PEP explicitly said this. I think we can just update the PEP, cc @erictraut @gvanrossum. See my message above for the argument why these should be writable.

[erictraut]

Sounds reasonable to me.

[JelleZijlstra]

Reopening to remind myself to do that

[makslevental]

Sorry for commenting on a closed issue but can someone give me a quick word on why this is like this:

def wtfbbq[M, K, N]():
    print(type(M), type(K), type(N))

wtfbbq.__type_params__ = (32, 32, 32)

wtfbbq()

>>> <class 'typing.TypeVar'> <class 'typing.TypeVar'> <class 'typing.TypeVar'>

I expected some transmutation here but I guess in order to actually make the transmutation happen I need to update both wtfbbq.__code__.co_freevars and wtfbbq.__closure__?