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3.5.0b1: ax.contour generates different artists #20906

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mwaskom opened this issue Aug 25, 2021 · 5 comments
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3.5.0b1: ax.contour generates different artists #20906

mwaskom opened this issue Aug 25, 2021 · 5 comments
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Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions.
Milestone
v3.5.0

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@mwaskom
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mwaskom commented Aug 25, 2021 •
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Bug summary

The artists added when using ax.contour have changed abruptly from 3.4 to the 3.5 beta. This causes issues for workflows that use the artist properties.

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt

xx, yy = np.mgrid[-3:3:.01, 0:1:.05]
zz = np.sin(xx * yy)

ax = plt.figure().subplots()
ax.contour(xx, yy, zz)
print(ax.collections[0].__class__)

Actual outcome

The result on 3.5.0b1:

<class 'matplotlib.collections.PathCollection'>

Expected outcome

The result on 3.4.1:

<class 'matplotlib.collections.LineCollection'>
@tacaswell tacaswell added this to the v3.5.0 milestone Aug 25, 2021
@tacaswell
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@mwaskom What are you doing with the return of get_segments? I am 👎🏻 on backing out these changes, but we need a better mitigation / migration story for the people we have broken.

attn @ianthomas23

@tacaswell tacaswell added the Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. label Aug 25, 2021
@mwaskom
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mwaskom commented Aug 25, 2021

In seaborn, just accessing them to test the 2D kdeplot results.

Like #20895, I don't think this affects any user-facing code, so it's not especially critical for me.

But since my test suite turned up an unexpected breakage, I thought it was good to report anyway.

@ianthomas23
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@mwaskom The change in class returned by contour from LineCollection to PathCollection is intended. The PR (#19623) fixes a bug that has been around for many years in that closed line loops were rendered as open line strips so the ends were not joined correctly. To fix this it was necessary internally to use the CLOSEPOLY Path code. LineCollection doesn't support the use of codes whereas PathCollection does, hence the change.

There should certainly have been a note about the API change. If someone else wants to pick this up that would be great - I am on holiday at the moment and start a new job next week so mpl is not my highest priority in the short term.

@mwaskom
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mwaskom commented Aug 26, 2021

I appreciate that it was intentional. FWIW, forward guidance on such changes a version or so in advance (either just in release notes, or ideally with a future warning) would reduce the change of abrupt breakage.

QuLogic added a commit to QuLogic/matplotlib that referenced this issue Sep 28, 2021
@QuLogic
Document API change in ContourSet
348478d 
Also, update documentation for Attributes.

Followup to matplotlib#19623; fixes matplotlib#20906.
QuLogic added a commit to QuLogic/matplotlib that referenced this issue Sep 29, 2021
@QuLogic
Document API change in ContourSet
36aa609 
Also, update documentation for Attributes.

Followup to matplotlib#19623; fixes matplotlib#20906.
@timhoffm
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Is @QuLogic ‘s documentation enough to close this issue?

tacaswell pushed a commit to tacaswell/matplotlib that referenced this issue Oct 12, 2021
@QuLogic
Document API change in ContourSet
c13cc76 
Also, update documentation for Attributes.

Followup to matplotlib#19623; fixes matplotlib#20906.
tacaswell pushed a commit that referenced this issue Oct 20, 2021
@QuLogic
Document API change in ContourSet
a9a7e5c 
Also, update documentation for Attributes.

Followup to #19623; fixes #20906.
This was referenced Nov 21, 2021
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@rcomer rcomer mentioned this issue Jun 20, 2022
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Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions.
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v3.5.0
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@tacaswell @mwaskom @ianthomas23 @timhoffm