Add `ModificationContext` ABC to `contextlib`.

[jb2170]

Feature or enhancement

Proposal:

We add an ABC ModificationContext to contextlib:

ModificationContext specializes the functionality of AbstractContextManager, for context managers designed to change the value of a state or attribute on entry, and revert it on exit. We have noticed that this style is used by many existing and proposed context managers.

The following classes already present in contextlib are to be refactored to inherit from ModificationContext (with no compatibility issues)

• contextlib._RedirectStream, parent of redirect_stdout and redirect_stderr
• contextlib.chdir

After this addition, the following issues are rebased, so that their additions inherit from ModificationContext:

• gh-128432: Add shutil.umask
• TBC: Add shutil.seteuid and shutil.setegid
• gh-128432: Move contextlib.chdir to shutil.chdir

This addition makes those first two PRs trivial, and contextlib.chdir is refactored whether it is moved to shutil or not.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

Thread started by @ncoghlan

https://discuss.python.org/t/adding-a-contextlib-modificationcontext-base-class/76322/2

[ncoghlan]

It's worth noting that adding this weakens the case for adding the additional more specific subclasses in shutil (which in turn weakens the case for moving chdir), since it makes it easier for folks with a use case for that context manager to write it themselves. However, I'll discuss that more on gh-128432

[jb2170]

Suggested cancelling in the discussion due to the unnecessary / confusing number of ABCs it introduces.

The code is still available on my fork here if desired.

I'll close this issue for now 🙂