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This issue will serve to track development and PRs for the implementation of PEP 570: Python Positional-Only Parameters.

I will separate the work in two Pull Request (maybe more). The implementation of the PEP will be done in PR12701 and the documentation and other additions will be done in future Pull Requests to keep things more manageable.

PR 12701 makes breaking changes in the inspect module.
Currently we have two sets of APIs in the inspect module: old deprecated API that do not support keyword-only arguments, and new *full* API that supports them. Even many years since 3.0 the old API still has not been removed. Adding support for positional-only arguments needs changes of the same order as adding support of keyword-only arguments. It means deprecating the current API and introducing a new (*fuller*?) API. This is a slow process for many releases. We could also take opportunity to redesign APIs completely instead of just adding new parameters and fields.
I suggest to remove all breaking changes in the inspect module and defer them to the separate issue. This perhaps will need a discussion on Python-Dev, and maybe not one.

It seems like deprecating or removing "old" inspect functions is an issue. An option would be to raise an error if these functions is used on a function which has at least one positional argument.

Łukasz, are you ok if we include this one in alpha4 as we were discussing in the PRs? If you are happy with that I will merge it so there is early testing and then we can keep working on the documentation and minor details for the next stage. If not, please, unmark it as a release blocker.

Opened https://bugs.python.org/issue36751 to track changes in the inspect module.

New changeset 8c77b8cb9188165a123f2512026e3629bf03dc9b by Pablo Galindo in branch 'master':
bpo-36540: PEP 570 -- Implementation (GH-12701)
https://github.com/python/cpython/commit/8c77b8cb9188165a123f2512026e3629bf03dc9b

The implementation has been merged, so I will remove the "release blocker". We are still missing the documentation and the final changes to the inspect module.

+1

New changeset b76302ddd0896cb39ce69909349b53db6e7776e2 by Carol Willing (Pablo Galindo) in branch 'master':
bpo-36540: Documentation for PEP570 - Python positional only arguments (#13202)
https://github.com/python/cpython/commit/b76302ddd0896cb39ce69909349b53db6e7776e2

Closing this. Will reopen if we realize we missed something.

New changeset 58d72cab89cf9652acc0bf0007aa20b2bcc98499 by Saiyang Gou in branch 'master':
bpo-36540: Improve doc of function definition regarding positional-only arguments (GH-25235)
https://github.com/python/cpython/commit/58d72cab89cf9652acc0bf0007aa20b2bcc98499

New changeset 4554ab435727a0f3b6441c9c1a8d3fe6534433c7 by Saiyang Gou in branch '3.8':
[3.8] bpo-36540: Improve doc of function definition regarding positional-only arguments (GH-25235) (GH-25260)
https://github.com/python/cpython/commit/4554ab435727a0f3b6441c9c1a8d3fe6534433c7

New changeset 3b1cf202976d9590d5b453c47b883f2e7cd1a7a5 by Saiyang Gou in branch '3.9':
[3.9] bpo-36540: Improve doc of function definition regarding positional-only arguments (GH-25235) (GH-25259)
https://github.com/python/cpython/commit/3b1cf202976d9590d5b453c47b883f2e7cd1a7a5