Break reference cycle Line2D <-> Line2D._lineFunc.

[anntzer]

Upon drawing, Line2D objects would store a reference to one of their own
bound methods as their _lineFunc argument. This would lead to them
being gc'ed not when going out of scope, but only when the "true" gc
kicks in; additionally this led to some pickle-related bugs (#3627).

One can easily sidestep this problem by not storing this bound method.

To check the behavior, try (py3.4+ only):

import gc
import weakref
from matplotlib import pyplot as plt

def f():
    fig, ax = plt.subplots()
    img = ax.imshow([[0, 1], [2, 3]])
    weakref.finalize(img, print, "gc'ing image")
    l, = plt.plot([0, 1])
    weakref.finalize(l, print, "gc'ing line")
    fig.canvas.draw()
    img.remove()
    l.remove()

f()
print("we have left the function")
gc.collect()
print("and cleaned up our mess")

Before the patch, the AxesImage is gc'ed when the function exits but the
Line2D only upon explicit garbage collection. After the patch, both are
collected immediately.

[efiring]

Looks good to me. Why didn't Travis run?

[anntzer]

Not sure why... it built on my side: https://travis-ci.org/anntzer/matplotlib/builds/146967616 (docs never build for me, btw... suggestions welcome).

[Kojoley]

_lineFunc

[anntzer]

?

[efiring]

Probably best to delete the whole comment; it is no longer relevant, right?

[anntzer]

Should we just get rid of the whole test? Given that it is exactly there to test the workaround that this PR removes.

[efiring]

Good point--I didn't notice that this was in a test. Yes, I think removing the test makes perfect sense.

[anntzer]

done.

[jenshnielsen]

The docs build fails for you on travis because the install of basemap causes matplotlib to be downgraded to 1.5.1. This is most likely because your fork does not have all the tags which confuses the versioneer version script. I suggest pushing the tags to your branch. git push <REMOTENAME> --tags should do the trick https://help.github.com/articles/pushing-to-a-remote/
See the following in the logs

Installing collected packages: matplotlib, pyproj, pyshp, basemap
  Found existing installation: matplotlib 0+untagged.3118.g353275c
    Uninstalling matplotlib-0+untagged.3118.g353275c:
      Successfully uninstalled matplotlib-0+untagged.3118.g353275c
  Running setup.py install for basemap ... done
Successfully installed basemap-1.0.8 matplotlib-1.5.1 pyproj-1.9.5.1 pyshp-1.2.3

[jenshnielsen]

I opened and closed the pr to re trigger travis

[anntzer]

Thanks for the help re: docs.

[WeatherGod]

The _drawStyles_l dictionary still references _draw_lines, and so I would imagine that by removing this method would cause problems (but I am not sure I am clear how it would have worked properly in the first place?).

[anntzer]

The entire drawStyles dict is now unused, and only left for backcompatibility purposes (mostly to have a list of drawstyles available as matplotlib.lines.drawStyles). With this change none of the values actually refer to a method name anymore. I killed the other entries in #6497 for another purpose.

(That's also the reason why this can't go in 2.x: this depends on #6497 (and its followup, #6645)).

[WeatherGod]

btw, in case it isn't clear, this can not be backported to v2.x branch as-is because there are more usages of _lineFunc in the v2.x branch.

[mdboom]

👍 from me. Thanks for cleaning out our cruft.