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BUG: Fix UnboundLocalError in contour labelling #10710

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Merged
merged 1 commit into from
Mar 7, 2018
Merged

BUG: Fix UnboundLocalError in contour labelling #10710

merged 1 commit into from
Mar 7, 2018

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dopplershift
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@dopplershift dopplershift commented Mar 7, 2018 •
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This fixes hitting the following traceback with 2.2.0 (problem also exists on master):

  File "/home/travis/miniconda/envs/gallery/lib/python3.5/site-packages/matplotlib/pyplot.py", line 2790, in clabel
    ret = ax.clabel(CS, *args, **kwargs)
  File "/home/travis/miniconda/envs/gallery/lib/python3.5/site-packages/matplotlib/axes/_axes.py", line 6173, in clabel
    return CS.clabel(*args, **kwargs)
  File "/home/travis/miniconda/envs/gallery/lib/python3.5/site-packages/matplotlib/contour.py", line 213, in clabel
    self.labels(inline, inline_spacing)
  File "/home/travis/miniconda/envs/gallery/lib/python3.5/site-packages/matplotlib/contour.py", line 646, in labels
    inline_spacing)
  File "/home/travis/miniconda/envs/gallery/lib/python3.5/site-packages/matplotlib/contour.py", line 445, in calc_label_rot_and_inline
    nlc.append(np.row_stack([xy2, lc[I[1]:I[0]+1], xy1]))
UnboundLocalError: local variable 'xy2' referenced before assignment

That traces back to this block:

matplotlib/lib/matplotlib/contour.py

Lines 433 to 444 in d2f7bb1

I = np.interp(xi, pl, np.arange(len(pl)), left=-1, right=-1)
I = [np.floor(I[0]).astype(int), np.ceil(I[1]).astype(int)]
if I[0] != -1:
xy1 = [np.interp(xi[0], pl, lc_col) for lc_col in lc.T]
if I[1] != -1:
xy2 = [np.interp(xi[1], pl, lc_col) for lc_col in lc.T]
# Actually break contours
if closed:
# This will remove contour if shorter than label
if np.all(I != -1):
nlc.append(np.row_stack([xy2, lc[I[1]:I[0]+1], xy1]))

The code was trying to protect the access of xy1 and xy2 to valid cases, using np.all(). Unfortunately, in this case I is a list, so I != -1 doesn't do element-by-element or
return an error--it just always returns True. That caused the if to not actually guard anything. Replaced with all and a generator expression.

I tried making a test, but I couldn't find any sensible set of data to reproduce. We hit this in an example of ours that was using cartopy incorrectly (fixing that problem makes the error disappear). It appears to only trigger when you have a really tiny contour (in display coords) that can't be properly interpolated to create a spot for the contour label.

cc @anntzer since you introduced this code according to git.

@dopplershift
BUG: Fix UnboundLocalError in contour labelling
ca9502e 
The code was trying to protect the access of xy1 and xy2 to valid cases,
but unfortunately, for a list, l != -1 doesn't do element-by-element or
return an error--it just always returns True. Therefore it wasn't
actually guarding.
@dopplershift dopplershift added this to the v2.2.1 milestone Mar 7, 2018
anntzer
anntzer approved these changes Mar 7, 2018
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Sorry about that...

@anntzer anntzer added the Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. label Mar 7, 2018
@jklymak jklymak merged commit ef5d01a into matplotlib:master Mar 7, 2018
lumberbot-app bot pushed a commit that referenced this pull request Mar 7, 2018
@jklymak
Backport PR #10710: BUG: Fix UnboundLocalError in contour labelling
1749c2e
@lumberbot-app lumberbot-app bot mentioned this pull request Mar 7, 2018
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@dopplershift
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No worries @anntzer ! Making our own code actually work correctly was a suitable workaround. 😁 If it weren't for this bug, who knows when we would have realized...

@dopplershift dopplershift deleted the fix-error branch March 7, 2018 21:39
jklymak added a commit that referenced this pull request Mar 8, 2018
@jklymak
Merge pull request #10712 from matplotlib/auto-backport-of-pr-10710
7046022 
Backport PR #10710 on branch v2.2.x
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@anntzer anntzer anntzer approved these changes

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