Make __bases__ mutable

[dchiquito]

Assigning __bases__ allows you to alter the class inheritance tree at any point. The updated typing.py uses this feature when constructing NamedTuples, for reasons unknown to me.

I had to make PyType.bases and PyType.mro mutable behind a PyRwLock in order to enable this feature.

Assigning __bases__ has several features I wasn't sure how to implement. I left TODOs for these:

• Making sure the new method resolution order is actually a tree, i.e. it contains no cycles
• Removing the modified class from any __subclass__ lists it used to belong to
• Removing any slots that no longer apply (not sure if this is required?)

[dchiquito]

The test failures don't seem related to my changes. @youknowone any insight?

[youknowone]

The failing test is about threading, it might be flaky. re-triggered failed jobs.

[youknowone]

Making sure the new method resolution order is actually a tree, i.e. it contains no cycles

Leaving a comment will be enough for now.

Removing the modified class from any subclass lists it used to belong to

How is this done in CPython? Does it maintain weakrefs to subclasses?

Removing any slots that no longer apply (not sure if this is required?)

We don't have this feature yet. To do this, we have to implement slot copy first, which will remove most of iter_mro calls in our codebase.

[dchiquito]

Removing the modified class from any subclass lists it used to belong to

How is this done in CPython? Does it maintain weakrefs to subclasses?

Yes, the subclass list is a PyRwLock<Vec<PyRef<PyWeak>>>, and CPython is similar-ish. The complicated part is that CPython maintains a dict of subclasses so that they are uniquely identifiable, presumably just so they can be removed or modified. The key function looks complicated to get right, so I left the TODO.

[youknowone]

👍