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PEP 827: Wrap the new-style extended callables in a Params argument #4866

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Mar 19, 2026
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PEP 827: Wrap the new-style extended callables in a Params argument #4866

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PEP 827: Wrap the new-style extended callables in a Params argument
msullivan Mar 18, 2026
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Merge branch 'main' into parameters
msullivan Mar 18, 2026
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24 changes: 17 additions & 7 deletions peps/pep-0827.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -389,7 +389,17 @@ We introduce a ``Param`` type that contains all the information about a function
type ArgsParam[T] = Param[Literal[None], T, Literal["*"]]
type KwargsParam[T] = Param[Literal[None], T, Literal["**"]]

And then, we can represent the type of a function like::
We also introduce a ``Params`` type that wraps a sequence of ``Param``
types, serving as the first argument to ``Callable``::

class Params[*Ps]:
pass

The presence of ``Params`` as the first argument to ``Callable``
distinguishes the extended callable format from the standard format.
``Params`` also serves as a natural bound for ``ParamSpec``.

We can then represent the type of a function like::

def func(
a: int,
Expand All @@ -406,7 +416,7 @@ And then, we can represent the type of a function like::
as::

Callable[
[
Params[
Param[Literal["a"], int, Literal["positional"]],
Param[Literal["b"], int],
Param[Literal["c"], int, Literal["default"]],
Expand All @@ -422,7 +432,7 @@ as::
or, using the type abbreviations we provide::

Callable[
[
Params[
PosParam[Literal["a"], int],
Param[Literal["b"], int],
DefaultParam[Literal["c"], int],
Expand Down Expand Up @@ -1257,6 +1267,10 @@ We present implementations of a selection of them::
]
]

# KeyOf[T]
# Constructs a union of the names of every member of T.
type KeyOf[T] = Union[*[p.name for p in typing.Iter[typing.Members[T]]]]

# Exclude<T, U>
# Constructs a type by excluding from T all union members assignable to U.
type Exclude[T, U] = Union[
Expand Down Expand Up @@ -1884,10 +1898,6 @@ Open Issues
``readonly`` had been added as a parameter to ``TypedDict`` we would
use that, but it wasn't.

* :ref:`Extended Callables <pep827-extended-callables-prereq>`: Should the extended
argument list be wrapped in a ``typing.Parameters[*Params]`` type (that
will also kind of serve as a bound for ``ParamSpec``)?

* :ref:`Extended Callables <pep827-extended-callables-prereq>`: Currently the
qualifiers are short strings for code brevity, but an alternate approach
would be to mirror ``inspect.Signature`` more directly, and have an enum
Expand Down
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