[bug] `python_requires_extend` in base class should extend base class but extends sub class

I stumbled upon a little quirk in the implementation of python_requires_extend. I am not sure whether it is worthy to be called a bug, but I think it is unintuitive at the least.

Let’s say that we have a ConanFile subclass ProjectBaseConanFile which serves as the base class for some project variants with their classes VariantAConanFile, VariantBConanFile, VariantCConanFile.

Instances of these VariantXConanFile classes have the following inheritance hierarchy (packages ommitted):

<object>
<class 'ConanFile'>
<class 'ProjectBaseConanFile'>
<class 'VariantXConanFile'>

Now, let’s say that we have a new company-wide Conan base class CompanyConanFile and we want to use python_requires and python_requires_extend in all of our ConanFile implementations (including ProjectBaseConanFile) to inject CompanyConanFile as their respective base class.

Issue description

When we now set python_requires_extend = "companyconan.CompanyConanFile" in ProjectBaseConanFile, then Conan currently injects CompanyConanFile as the base class of self.__class__, so at this position:

<object>
<class 'ConanFile'>
<class 'ProjectBaseConanFile'>
<class 'CompanyConanFile'>      <-- injected here
<class 'VariantXConanFile'>     <-- self

But that means that the custom implementations of ProjectBaseConanFile would be overridden by the default implementations of CompanyConanFile when in fact it should be the other way around.

Expected behavior / workaround

You could work around this and inject the base class at the expected location by adding the following to the ProjectBaseConanFile class:

class ProjectBaseConanFile(ConanFile):
    python_requires = "companyconan/0.1@Company/stable"
    # Intentionally commented-out, injection done manually in init()
    # python_requires_extend = "companyconan.CompanyConanFile"

    def init(self):
        CompanyConanFile = self.python_requires["companyConan"].module.CompanyConanFile
        ProjectBaseConanFile.__bases__ = (CompanyConanFile,) + ProjectBaseConanFile.__bases__

(Workaround deduced from here) https://github.com/conan-io/conan/blob/ebed0370c439a8274c2992ee41e424438eebd455/conans/client/graph/python_requires.py#L103

This injects CompanyConanFile at the correct position:

<object>
<ConanFile>
<CompanyConanFile>      <-- injected here
<ProjectBaseConanFile>
<VariantXConanFile>     <-- self

Environment Details

• Operating System+version: macOS Catalina 10.15.7
• Compiler+version: Apple clang version 12.0.0 (clang-1200.0.32.29)
• Conan version: 1.43.0
• Python version: 3.9.9

[bazizsafi]

Cool! I consider this issue closed then.

[memsharded]

Closed by https://github.com/conan-io/docs/pull/2393