Draggable legend results RuntimeError and AttributeError on Mac OS 10.8.1

[sinhrks]

Environment:

• Python 2.7.3
• Matplotlib 1.1.1
• Max OS 10.8.1

Code:

In [1]: import matplotlib.pyplot as plt

In [2]: plt.bar([1, 2, 3], [2, 3, 2], label='label')
Out[2]: 
[<matplotlib.patches.Rectangle at 0x1043415d0>,
 <matplotlib.patches.Rectangle at 0x104341650>,
 <matplotlib.patches.Rectangle at 0x104341ad0>]

In [3]: l = plt.legend()

In [4]: l.draggable()
Out[4]: <matplotlib.legend.DraggableLegend at 0x10434fbd0>

Once I drag the legend, the following errors occurred. The same code works fine in Windows XP.

  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/offsetbox.py", line 376, in get_extent_offsets
    whd_list = [c.get_extent(renderer) for c in self.get_visible_children()]
  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/offsetbox.py", line 639, in get_extent
    "lp", self._text._fontproperties, ismath=False)
  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/backends/backend_macosx.py", line 161, in get_text_width_height_descent
    width, height, descent = self.gc.get_text_width_height_descent(unicode(s), family, size, weight, style)
RuntimeError: CGContextRef is NULL

Traceback (most recent call last):
  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/backend_bases.py", line 1588, in motion_notify_event
    self.callbacks.process(s, event)
  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/cbook.py", line 262, in process
    proxy(*args, **kwargs)
  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/cbook.py", line 188, in __call__
    return mtd(*args, **kwargs)
  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/offsetbox.py", line 1434, in on_motion
    self.update_offset(dx, dy)
  File "/Users/sin/Library/Python/2.7/lib/python/site-packages/matplotlib/offsetbox.py", line 1504, in update_offset
    loc_in_canvas = self.offsetbox_x + dx, self.offsetbox_y + dy
AttributeError: 'DraggableLegend' object has no attribute 'offsetbox_x'

[dmcdougall]

I can recreate this on 10.7.4.

[mdehoon]

The first problem typically occurs when trying to draw outside of the event loop. If so, then the problem may be in the design of the offsetbox.py code. This is a minimal example of this error:

from pylab import *
f = figure()
renderer = f.canvas.renderer
gc = renderer.new_gc()
Traceback (most recent call last):
File "", line 1, in
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/backends/backend_macosx.py", line 97, in new_gc
self.gc.save()
RuntimeError: CGContextRef is NULL

The gist is that the graphics context gc is not properly initialized outside of the event loop, so you cannot grab a renderer and start drawing to it.

[crogre]

I see the same thing with OS X 10.8.2 and matplotlib 1.2.0.

[michaelaye]

It works here with:
10.8.4
mpl 1.2.1
Qt4Agg

[obtitus]

I believe this is a macosx backend issue. Other backends work fine, but the macosx gives the reported runtime error.

[mdehoon]

AFAICT it's not a macosx backend issue, but an issue with offsetbox.py.

[tacaswell]

works fine in linux 1.4.x, 2.7 and Qt4Agg.

It is pretty clearly in the macosx backend.

[mdehoon]

The bug shows up when using the macosx backend. That does not necessarily mean that the bug is located is the macosx backend.

[tacaswell]

This seems to be (as with blitting) an consequence of the quartz not being able to issue draw commands outside of the draw call back.

Should be linked with #1319 and #531

[mdboom]

Fixed by #6178