gh-138859: Account for `ParamSpec` defaults that are not lists … by bzoracler · Pull Request #138868 · python/cpython

bzoracler · 2025-09-13T12:12:58Z

…when converting type argument list to tuple

Allows omitting common ParamSpec defaults, like ... or another ParamSpec.

Issue: Old-style generic ParamSpec(default=...) fails if not provided as a type argument and not in the last position of a type list when passing type arguments #138859

Issue: Old-style generic ParamSpec(default=...) fails if not provided as a type argument and not in the last position of a type list when passing type arguments #138859

python-cla-bot · 2025-09-13T12:13:03Z

All commit authors signed the Contributor License Agreement.

bedevere-app · 2025-09-13T12:13:04Z

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

sobolevn · 2025-09-13T12:21:07Z

Lib/typing.py

    elif isinstance(args[i], list):
        args = (*args[:i], tuple(args[i]), *args[i+1:])
+    else:
+        args = (*args[:i], args[i], *args[i + 1:])


this can be simplified to args = tuple(args)

sobolevn · 2025-09-13T12:21:33Z

Misc/NEWS.d/next/Library/2025-09-13-12-19-17.gh-issue-138859.PxjIoN.rst

@@ -0,0 +1 @@
+Fix failure during parameterization of generics when a non-type-list `ParamSpec` default is followed by another type variable default, and the `ParamSpec` is not provided as a type argument.


We use rst, not md in NEWS, so please use double backticks.

JelleZijlstra · 2025-09-15T01:27:22Z

Lib/typing.py

    # Convert lists to tuples to help other libraries cache the results.
    elif isinstance(args[i], list):
        args = (*args[:i], tuple(args[i]), *args[i+1:])
+    else:


Should we create a tuple directly on line 1103 above instead?

sobolevn · 2025-09-15T09:12:55Z

Misc/NEWS.d/next/Library/2025-09-13-12-19-17.gh-issue-138859.PxjIoN.rst

@@ -0,0 +1 @@
+Fix failure during parameterization of generics when a non-type-list ``ParamSpec`` default is followed by another type variable default, and the ``ParamSpec`` is not provided as a type argument.


Sorry, I don't understand what "non-type-list" is. I propose this wording:

Suggested change

Fix failure during parameterization of generics when a non-type-list ``ParamSpec`` default is followed by another type variable default, and the ``ParamSpec`` is not provided as a type argument.

Fixes a :exc:`TypeError` when substituting a :class:`ParamSpec` with a default

that is followed by another :class:`TypeVar` with a default.

Thanks, the original description was unclear. Unless there are other objections, I'll reduce it to

Fix generic type parameterization raising a :exc:TypeError when omitting a :class:ParamSpec that has a non-type-list default.

"Non-type-list default" refers to the bug occurring if the ParamSpec default isn't a list of types, such as ... or P (another ParamSpec in scope). The bug would not be encountered if the default was a list of types (like [int, str]).

I think the closest wording for this construct in the documentation is actually "list of types" (see e.g. ParamSpec Defaults), but I don't think "list of types" is necessarily better than "type list".

Let's say something like "default that is not a list of types". "type list" is definitely less clear to me than "list of types".

Rewording suggestions done in 9669479

sergey-miryanov · 2025-10-16T07:30:09Z

@bzoracler Something left to do here? Is it ready for review?

…when converting type argument list to tuple

bzoracler · 2025-10-16T11:05:11Z

@sergey-miryanov I think I've addressed everything.

@JelleZijlstra Are there any other concerns?

miss-islington-app · 2025-10-16T12:30:39Z

Thanks @bzoracler for the PR, and @JelleZijlstra for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

pythonGH-138868) (cherry picked from commit 379fd02) Co-authored-by: bzoracler <50305397+bzoracler@users.noreply.github.com>

bedevere-app · 2025-10-16T12:30:54Z

GH-140207 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2025-10-16T12:30:59Z

GH-140208 is a backport of this pull request to the 3.13 branch.

JelleZijlstra · 2025-10-16T12:50:32Z

@bzoracler it looks like you'll have to re-sign the CLA under a different email address for the backports (#140208 and #140207) to go through. As I understand it this can happen if you have different email addresses associated with your GitHub account and your commits.

bzoracler · 2025-10-16T18:46:29Z

@JelleZijlstra oh, it looks like rebasing straight from the GitHub UI made the commits under a different email address (@users.noreply.github.com) because of GitHub email privacy settings. I've signed the CLA with that one as well.

Thanks all!

… … (GH-138868) (#140208) gh-138859: Account for `ParamSpec` defaults that are not lists … (GH-138868) (cherry picked from commit 379fd02) Co-authored-by: bzoracler <50305397+bzoracler@users.noreply.github.com>

… … (GH-138868) (#140207) gh-138859: Account for `ParamSpec` defaults that are not lists … (GH-138868) (cherry picked from commit 379fd02) Co-authored-by: bzoracler <50305397+bzoracler@users.noreply.github.com>

python#138868)

bzoracler requested review from AlexWaygood and JelleZijlstra as code owners September 13, 2025 12:12

bedevere-app bot added the awaiting review label Sep 13, 2025

bedevere-app bot mentioned this pull request Sep 13, 2025

Old-style generic ParamSpec(default=...) fails if not provided as a type argument and not in the last position of a type list when passing type arguments #138859

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sobolevn reviewed Sep 13, 2025

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JelleZijlstra reviewed Sep 15, 2025

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bzoracler requested review from JelleZijlstra and sobolevn September 15, 2025 07:25

sobolevn reviewed Sep 15, 2025

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bzoracler and others added 6 commits October 16, 2025 20:42

pythongh-138859: Account for ParamSpec defaults that are not lists …

22235b4

…when converting type argument list to tuple

📜🤖 Added by blurb_it.

da0d060

Address code review

bb0ad9b

Fix news entry

120b9bb

Simplify args tuple creation

1d35e9b

Reword news entry

bd6d90f

bzoracler force-pushed the issue-138859 branch from 9669479 to bd6d90f Compare October 16, 2025 07:42

JelleZijlstra added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Oct 16, 2025

JelleZijlstra approved these changes Oct 16, 2025

View reviewed changes

bedevere-app bot added awaiting merge and removed awaiting review labels Oct 16, 2025

JelleZijlstra merged commit 379fd02 into python:main Oct 16, 2025
53 checks passed

bedevere-app bot removed the awaiting merge label Oct 16, 2025

bedevere-app bot removed the needs backport to 3.14 bugs and security fixes label Oct 16, 2025

bedevere-app bot removed the needs backport to 3.13 bugs and security fixes label Oct 16, 2025

bzoracler deleted the issue-138859 branch October 16, 2025 18:40

StanFromIreland pushed a commit to StanFromIreland/cpython that referenced this pull request Dec 6, 2025

pythongh-138859: Account for ParamSpec defaults that are not lists … (

55c8f71

python#138868)

		@@ -0,0 +1 @@
		Fix failure during parameterization of generics when a non-type-list `ParamSpec` default is followed by another type variable default, and the `ParamSpec` is not provided as a type argument.

	Fix failure during parameterization of generics when a non-type-list ``ParamSpec`` default is followed by another type variable default, and the ``ParamSpec`` is not provided as a type argument.
	Fixes a :exc:`TypeError` when substituting a :class:`ParamSpec` with a default
	that is followed by another :class:`TypeVar` with a default.

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