Enforce find_package(Python)

[rhaschke]

Fixes #5813

[Copilot]

Pull Request Overview

This PR enforces the use of find_package(Python) by removing conditional checks that would skip Python finding when Python was already found. The change ensures that Python detection logic runs unconditionally rather than being bypassed when Python variables are already set.

• Removes the outer conditional wrapper around Python finding logic
• Ensures Python detection always executes regardless of existing Python_FOUND or Python3_FOUND variables

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[Copilot]

With the removal of the outer conditional check, this Python configuration will now run unconditionally. Consider whether this could overwrite existing Python_FIND_IMPLEMENTATIONS settings or if additional guards are needed to prevent reconfiguration when Python is already properly configured.

[henryiii]

We don't want to call FindPython if it's already been called with the right components present. Or if the user is using the old PythonInterp/Libs support. So I think the check can't be removed, but should instead check for everything that is needed, and trigger the find if everything isn't there.

[rhaschke]

We don't want to call FindPython if it's already been called with the right components present.

What's the problem in doing so? Do you fear performance issues? As far a I know, cmake's find functions cache previous results. So that shouldn't be a big deal.
On the other hand, find_package might have been called with different search-path arguments before, yielding a Python version < 3.8, which would not be compatible to pybind11. I think, pybind11 should ensure its required version and components itself.

Or if the user is using the old PythonInterp/Libs support.

I think this is not relevant here, as the old mode is handled in pybind11Tools.cmake, not pybind11NewTools.cmake.

So I think the check can't be removed, but should instead check for everything that is needed, and trigger the find if everything isn't there.

This would require moving the logic to figure out the required components out of the if.

[henryiii]

cmake's find functions cache previous results

FindPython is the exception (see Python_ARTIFACTS_INTERACTIVE, 3.18+). It was trying to do some weird things, like finding multiple different versions of Python in one build, so it's a really weird find module.

[rhaschke]

I'm not sure about the consequences of your statement, @henryiii. Do you still have objections to call find_package(Python) again? If so, how do you suggest to fix issue #5813?