MNT: Remove AxesStack and deprecated behavior of reuse of existing axes with same arguments

Describe the issue

Summary

In Matplotlib 2.1, the behavior of reusing existing axes when created with the same arguments was deprecated (see #9037). The request is to finally remove this behavior.

My immediate motivation is that I have a collection of projection classes that take as a keyword argument an instance of astropy.coordinates.SkyCoord, which as of Astropy 4.1 are no longer hashable (see also astropy/astropy#9857). This unexpectedly broke my code because I had not realized that projection keyword arguments had to be hashable.

Proposed fix

Remove the AxesStack class, remove the Figure._make_key method, and update the documentation to reflect the removal of the old deprecated behavior.

@anntzer

@anntzer do you already have a branch for this?

IIRC I kind of gave up on that due to unclearness of what we want to keep supporting, e.g. wrt. gca(projection=..., ...) (#10700) (which goes through the same AxesStack info). Probably you need to make some executive decisions on that :)

@anntzer

@anntzer, after studying #10700, my suggestion is:

In the next release of Matplotlib:
- Deprecate kwargs for gca()
- Remove projection kwargs lookup (for gca() and also subplots(), etc.)
- If gca() is called without kwargs, simply return the current axes
- If gca() kwargs matches the kwargs of the current axes, return those axes, otherwise raise an exception
In a future release of Matplotlib:
- Remove kwargs entirely from gca()

What do you think?

@tacaswell

I am personally fine with fully deprecating all kwargs to gca(), but there was some pushback wrt. subplot() in #12513 (and IIRC these basically go through the same machinery) and I had little interest in fighting that fight, which is why I decided to let @tacaswell decide :)

@tacaswell

@tacaswell, what do you think? I am happy to take a stab at the above changes, if you agree with the approach.

How do we want to handle a call to gca() without kwargs when there are not yet any axes? Return None? Or return a new axes?

That should definitely return a new axes (I'd guess that's an extremely common use case (like it or not)).

MNT: Remove deprecated axes kwargs collision detection

Require matplotlib >= 3.4.0

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MNT: Remove AxesStack and deprecated behavior of reuse of existing axes with same arguments #18832

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MNT: Remove AxesStack and deprecated behavior of reuse of existing axes with same arguments #18832

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tacaswell commented on Oct 28, 2020

anntzer commented on Oct 28, 2020

lpsinger commented on Nov 1, 2020

anntzer commented on Nov 1, 2020

lpsinger commented on Nov 8, 2020

lpsinger commented on Nov 18, 2020

anntzer commented on Nov 18, 2020

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