unused-function-argument (ARG001) - false positive on pytest hookspec functions

[DetachHead]

in pytest, a new hook function is defined using the hookspec decorator, and the function has no implementation as it's basically like an abstract method:

from pytest import hookspec

@hookspec
def pytest_foo(value: int):  # error Unused function argument: `value`
    ...

playground

ruff could detect the use of the hookspec decorator and not report the error here.

related:

• FBT001 and PLR0917 - false positive on pytest functions #7286
• support plugins as a class pytest-dev/pytest#11300

[charliermarsh]

We should be able to support this. Can you link to any relevant docs on hookspec?

[DetachHead]

https://docs.pytest.org/en/7.1.x/how-to/writing_hook_functions.html#declaring-new-hooks

a couple things worth noting:

• the hookspec decorator isn't actually required to declare a hook. instead you have to hook pytest_addhooks and register the whole module:

  # foo/hooks.py
  from pytest import hookspec
  
  def pytest_foo(value: int):
      ...

  # conftest.py
  def pytest_addhooks(pluginmanager):
      from foo import hooks
  
      pluginmanager.add_hookspecs(hooks)

  as far as i can tell the hookspec decorator is mainly used to specify additional options to individual hooks (so it's not needed if you are just using the default options). however using the decorator without any options like in my original example is perfectly valid, and a good way to let ruff know it's a hookspec without needing multi-file analysis (Multi-file analysis #7447)

• pytest' plugin functionality comes from pluggy, so ideally ruff would also recognize hookspecs in any project that uses pluggy. however since decorators are defined like this, it would either need multi-file analysis to work, or just assume any decorator called hookspec is a pluggy hook:

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

[charliermarsh]

Assuming it's defined within the same file (which seems common), we could detect that @hookspec resolves to hookspec = HookspecMarker("...").

[DetachHead]

would still require special casing for pytest, since its hookspec decorators are defined in a different file to where they're used in plugins