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PEP 696: Changes from Pyright implementation #2844

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Merged
merged 15 commits into from
Oct 30, 2022
Merged

PEP 696: Changes from Pyright implementation #2844

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7551033
Fix typo in example
Gobot1234 Aug 24, 2022
159f14d
Revamp the "Using another ``TypeVarLike`` as the default" section amo…
Gobot1234 Aug 26, 2022
f3918f6
Merge branch 'main' into typevar-defaults
Gobot1234 Aug 28, 2022
c54857c
Add extra scoping rules
Gobot1234 Aug 29, 2022
6490a3d
Merge remote-tracking branch 'origin/typevar-defaults' into typevar-d…
Gobot1234 Aug 29, 2022
add591f
Fix grammar and syntax issues and improve ``TypeVarLike``\ s As Param…
Gobot1234 Aug 31, 2022
2112f5a
Textual and clarity improvements
Gobot1234 Sep 14, 2022
5248aa2
Last couple of updates
Gobot1234 Oct 16, 2022
240fb2a
Fix ParamSpecs and TypeVarTuples being missing from their respective …
Gobot1234 Oct 23, 2022
4f6cbd3
Remove function defaults as they are too hard to keep track of
Gobot1234 Oct 23, 2022
a420e26
Remove mention of _DefaultMixin and only describe the end behaviour
Gobot1234 Oct 23, 2022
4b33b5d
Merge remote-tracking branch 'upstream/HEAD' into typevar-defaults
Gobot1234 Oct 23, 2022
e6bfc52
I think this reads better
Gobot1234 Oct 26, 2022
3bcb548
Add a note as to why function defaults are unsupported
Gobot1234 Oct 27, 2022
ad23089
Improve readability of clause
Gobot1234 Oct 27, 2022
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30 changes: 10 additions & 20 deletions pep-0696.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -140,7 +140,7 @@ future, this might be possible (see `Interaction with PEP

``ParamSpec`` defaults are defined using the same syntax as
``TypeVar`` \ s but use a ``list`` or ``tuple`` of types or an ellipsis
literal "``...``".
literal "``...``" or another in-scope ``ParamSpec`` (see :ref:`scoping-rules`).

.. code-block:: py

Expand All @@ -155,7 +155,8 @@ literal "``...``".
'''''''''''''''''''''''''

``TypeVarTuple`` defaults are defined using the same syntax as
``TypeVar`` \ s but use an unpacked tuple of types instead of a single type.
``TypeVar`` \ s but use an unpacked tuple of types instead of a single type
or another in-scope ``TypeVarTuple`` (see :ref:`scoping-rules`).

.. code-block:: py

Expand Down Expand Up @@ -348,31 +349,20 @@ subtype of one of the constraints.
Function Defaults
'''''''''''''''''

``TypeVarLike``\ s currently can only be used where a parameter can go unsolved.

.. code-block:: py

def foo(a: DefaultStrT | None = None) -> DefaultStrT: ...

reveal_type(foo(1)) # type is int
reveal_type(foo()) # type is str

If they are used where the parameter type is known, the defaults
should just be ignored and a type checker can emit a warning.

``TypeVarLike``\ s currently are not supported in the signatures of
functions as ensuring the ``default`` is returned in every code path
where the ``TypeVarLike`` can go unsolved is too hard to implement.

Implementation
--------------

At runtime, this would involve the following changes to the ``typing``
module.

- a new class ``_DefaultMixin`` needs to be added which is a superclass
of ``TypeVar``, ``ParamSpec``, and ``TypeVarTuple``.

- the type passed to default would be available as a ``__default__``
attribute and a sentinel would need to be added that can be
checked against to see if it was supplied.
- The classes ``TypeVar``, ``ParamSpec``, and ``TypeVarTuple`` should
expose the type passed to ``default``. This would be available as
a ``__default__`` attribute, which would be ``None`` if no argument
is passed and ``NoneType`` if ``default=None``.

The following changes would be required to both ``GenericAlias``\ es:

Expand Down