Fix for unpickling polar plot issue #4068

[olanmatt]

This implements a fix for the bug documented in issue #4068

PolarAffine of lib/matplotlib/projections/polar.py returned an empty state when being pickled because of an unimplemented __getstate__ function. Removing this function resolved the reported exception and restored the expected behaviour.

[jenshnielsen]

Travis failure in docs build is not related to this Pull request. Fixed by #4267

[ghost]

@jenshnielsen Thanks for the heads up! Would it be possible to get the tests restarted?

[tacaswell]

cc @pelson

It looks like Phil added this code in 7885d94 in PR #1175

[ghost]

cc @pelson @tacaswell

Here's what we've noticed.

The problem arises because PolarAffine is overriding the __getstate__() function of Transform, but all it does is return an empty dictionary currently. As such, when Pickle is used to restore the instance of a PolarAffine, all of its attributes
are lost and not set. Therefore, the AttributeError exception was being raised.

It seems like other children of Transform seem to just use their parent's __getstate__ and __setstate__ functions - which seem to work well with pickle.

[tacaswell]

Can you add a test? https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/tests/test_pickle.py has some tests already (and while you are in there can you add an @cleanup to all of the tests?).

[pelson]

Nit-pick: We leave 2 newlines between top level module objects. (I think the PEP8 tests will probably catch this)

[pelson]

I'm OK with the change. We can't be certain that this wont break other classes, which may have attributes which aren't picklable, but we can deal with that as the occur (should be a simple fix when it crops up).

👍

[dkua]

Whitespace issue fixed, @cleanup decorator added to tests, and a minimum test created.

There was a funny error in one of the Travis test suites, does it simply need a restart?

[tacaswell]

Yes, I restarted that test. Looks like apt timed out or some-such.

[tacaswell]

Thank you!

[tacaswell]

cherry-picked to color_overhaul as 592818a