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Defaulted positional-only hook parameters are forwarded as keywords

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@adenzhou1350 is already working on this.

Since Oct 11, 2026.

  • #775 by @adenzhou1350 — open

Assessment

Difficulty
4/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
python
Domain
tooling

Research direction

Start from the reproduction script in the issue: a hookimpl with a defaulted positional-only parameter, called as pm.hook.hello(value="caller"), currently raises a TypeError about positional-only arguments passed as keywords. Read how pluggy forwards hookimpl defaults and binds arguments for both old- and new-style wrappers, and check the explicit-value precedence described for #738. Done when the script prints ["caller"], a later optional positional-only argument leaves the earlier default intact, and the regression tests pass.

Written by the indexing model from the issue text.

Description

On current main 87f130aa18d8fdf9c92585e72423eca7f3de727d, the new hookimpl-default forwarding behavior passes defaulted positional-only arguments as keywords:

import pluggy

hookspec = pluggy.HookspecMarker("example")
hookimpl = pluggy.HookimplMarker("example")

class Spec:
    @hookspec
    def hello(self, value):
        pass

class Plugin:
    @hookimpl
    def hello(self, value="fallback", /):
        return value

pm = pluggy.PluginManager("example")
pm.add_hookspecs(Spec)
pm.register(Plugin())
print(pm.hook.hello(value="caller"))

Expected: ["caller"], following the explicit-value precedence introduced in #738. Actual: TypeError: ... got some positional-only arguments passed as keyword arguments: 'value'.

The same failure occurs in new- and old-style wrappers. Removing the default makes positional-only delivery work. If only a later optional positional-only argument is supplied, the earlier argument must retain its implementation default rather than shifting the supplied value.

Reproduced on native Windows and Linux CPython 3.12. The unmodified full suite passes 219 tests on both platforms; a focused added matrix has 13 failures and 9 passing controls on each. This concerns positional-only arguments, not the keyword-only API design in #742. I have a small fix and regression tests prepared.

AI assistance: OpenAI Codex; source inspection and native baseline/candidate tests were used to validate the finding.

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Merged PRs (30d)
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