Question about the representation of ParamSpec Generic classes

@erictraut

While working on PEP 696 (TypeVarLike defaults) one point I've been particularly confused about how was the types that should be used for Generics with ParamSpecs in __parameters__ subscription.

Should you pass a list to the list as suggested in the PEP, types.GenericAlias and by mypy

class X(Generic[P]): ...

# from pep 612
def f(x: X[[int, bool]]) -> str: ...
reveal_type(X[[int, bool]]())  # mypy: revealed type is __main__.X[[builtins.int, builtins.bool]]

# using types.GenericAlias
class X_types:
    __class_getitem__ = classmethod(types.GenericAlias)
reveal_type(X_types[[int, bool]])  # runtime: X_types[[<class 'int'>, <class 'bool'>]]
reveal_type(X_types[(int, bool)])  # runtime: X_types[<class 'int'>, <class 'bool'>]

or use a tuple as the typing runtime suggests and by pyright

reveal_type(X[[int, bool]])  # runtime: revealed type is X[(<class 'int'>, <class 'bool'>)]
reveal_type(X[[int, bool]])  # pyright: revealed type is type[X[(int, bool)]]

This may seem relatively inconsequential and at runtime it probably is but after talking to @erictraut he brought up that PEP 696 supports both lists and tuples as the defaults for ParamSpec (https://peps.python.org/pep-0696/#paramspec-defaults) and this will be harder to support in pyright but I'd really like symmetry with the type of default and the type that you should pass to a subscription. Ideally I think I tuples would be the best type for for default just cause they should be the easiest to analyse and they are the closest in style to a PEP 677 (Callable Type Syntax) syntax even if the current syntax does look more like Callable's current syntax. Does anyone from the pyre team or from mypy have any particular opposition to changing to just supporting tuples in default?

I think a list representation is more consistent with the text of PEP 612 and the runtime representation of Callable. We should change the runtime repr of ParamSpec generics to be more like Callable, because right now they're inconsistent:

>>> Callable[[int, str], bool]
typing.Callable[[int, str], bool]
>>> P = ParamSpec("P")
>>> T = TypeVar("T")
>>> class MyCallable(Generic[P, T]): pass
... 
>>> MyCallable[[int, str], bool]
__main__.MyCallable[(<class 'int'>, <class 'str'>), bool]
>>> get_args(Callable[[int, str], bool])
([<class 'int'>, <class 'str'>], <class 'bool'>)
>>> get_args(MyCallable[[int, str], bool])
((<class 'int'>, <class 'str'>), <class 'bool'>)

IIRC there was a specific reason to use a list for ParamSpec. The runtime cannot tell the difference between X[Y, Z] and X[(Y, Z)] (this is independent of the type of X -- it's a general problem with __getitem__).

To avoid this ambiguity I recommend using a list, as Jelle wrote.

This has been changed as of python/peps#3052. I'll create an issue for the internal representations of these classes on cpython.

#2099

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Question about the representation of ParamSpec Generic classes #1274

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Question about the representation of ParamSpec Generic classes #1274

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JelleZijlstra commented on Oct 24, 2022

gvanrossum commented on Nov 4, 2022

Gobot1234 commented on Mar 12, 2023

jeffbrown7816-collab commented on Oct 12, 2025

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