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ngoldbaum opened this issue Aug 13, 2018 · 8 comments
Closed

get_backend() backward compatibility #11852

ngoldbaum opened this issue Aug 13, 2018 · 8 comments
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Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions.
Milestone
v3.0.0

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@ngoldbaum
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Under matplotlib 3.0.0rc1, the return value of matplotlib.get_backend() is a sentinel object:

In [2]: matplotlib.get_backend()
Out[2]: <object at 0x111292d50>

In [3]: backend = matplotlib.get_backend()

In [4]: backend.lower()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-f3c7164fcd07> in <module>()
----> 1 backend.lower()

AttributeError: 'object' object has no attribute 'lower'

Under matplotlib 2.2.3 I get:

In [1]: import matplotlib

In [2]: matplotlib.get_backend()
Out[2]: 'MacOSX'

In [3]: backend = matplotlib.get_backend()

In [4]: backend.lower()
Out[4]: 'macosx'

For backward compatibility purposes it would be really nice if matplotlib 3.0.0 could return an object that at least acted like a string. In particular right now I'd like it to have a lower() method. I'd also like it if it had a nice __repr__ or __str__ implementation instead of printing out as an opaque object of type object.

The former is for backward compatibility, as the change breaks some of the yt plotting code --- in particular here's a failure from the test suite:

======================================================================
ERROR: Failure: AttributeError ('object' object has no attribute 'lower')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/nose/failure.py", line 39, in runTest
    raise self.exc_val.with_traceback(self.tb)
  File "/usr/local/lib/python3.7/site-packages/nose/loader.py", line 418, in loadTestsFromName
    addr.filename, addr.module)
  File "/usr/local/lib/python3.7/site-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/usr/local/lib/python3.7/site-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/imp.py", line 235, in load_module
    return load_source(name, filename, file)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/imp.py", line 172, in load_source
    module = _load(spec)
  File "<frozen importlib._bootstrap>", line 696, in _load
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/goldbaum/Documents/yt-git-fixes/yt/visualization/tests/test_color_maps.py", line 23, in <module>
    class TestColorMaps(unittest.TestCase):
  File "/Users/goldbaum/Documents/yt-git-fixes/yt/visualization/tests/test_color_maps.py", line 34, in TestColorMaps
    @requires_backend('Agg')
  File "/Users/goldbaum/Documents/yt-git-fixes/yt/testing.py", line 1143, in requires_backend
    if backend.lower() == matplotlib.get_backend().lower():
AttributeError: 'object' object has no attribute 'lower'

This is code that's internal to yt's test suite, but I wouldn't be at all surprised if there's other code out there that assumes matplotlib.get_backend() returns a string. For example:

https://github.com/search?q=matplotlib+get_backend+lower&type=Code
@jklymak jklymak added this to the v3.0 milestone Aug 13, 2018
@jklymak jklymak added the Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. label Aug 13, 2018
@anntzer
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anntzer commented Aug 13, 2018

The issue is #11600 (comment) and I think (quite strongly) that it makes sense for get_backend() to, well, perform backend resolution and return an actual backend.
Giving the sentinel object a .lower() method is only going to make things worse IMO.

@ngoldbaum
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I don't really care too much what type get_backend returns. I'd kinda prefer it to return something stringlike just on the principle that it would be better if old code continued working, but I also acknowledge that you're doing a major version number increment so some level of breakage is to be expected.

@fredrik-1
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I don't understand this. I am on v3.0.0rc1 on my git version but matplotlib.get_backend() returns a string for me and the docstring says that it should return the name.

@anntzer
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anntzer commented Aug 14, 2018

That's likely because this should only happen if you don't have a backend set in your matplotlibrc.

@fredrik-1
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That seems like a strange api to me. Why do it return an object when the backend is not set and not for example '' and a string when it is set? (that was the behavior that I got when I tested).

that it makes sense for get_backend() to, well, perform backend resolution and return an actual backend.

Why? The docstring for matplotlib.get_backend is "Return the name of the current backend."

@anntzer
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anntzer commented Aug 14, 2018

As explained above, it's a bug (IMO) and should be fixed.

@tacaswell
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The simplest fix may be to switch the sentinel to the string 'auto'? That way it can go in the .matplotlibrc and returns the expected type from get_backend()

I would prefer that if we keep the non-string sentinel that the backend resolution logic moves up to __init__ rather than adding another buried pyplot import.

@tacaswell
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Closing this in favor of #11844 which is addressing a slightly larger set of issues to keep the discussion from getting too fragmented.

@anntzer anntzer mentioned this issue Aug 14, 2018
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