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IndexError thrown by pyplot.legend() when plotting empty bar chart with label #13003

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VanDavv opened this issue Dec 17, 2018 · 3 comments · Fixed by #13551
Closed

IndexError thrown by pyplot.legend() when plotting empty bar chart with label #13003

VanDavv opened this issue Dec 17, 2018 · 3 comments · Fixed by #13551
Milestone
v3.2.0

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@VanDavv
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VanDavv commented Dec 17, 2018 •
edited
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Bug report

Bug summary

When I try to create bar chart with no data, and add a label to this chart, IndexError will be raised when trying to create legend for this plot

Code for reproduction

import matplotlib.pyplot as plt
plt.bar([], [], label='foo')
plt.legend()

Actual outcome

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    plt.legend()
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/pyplot.py", line 3823, in legend
    ret = gca().legend(*args, **kwargs)
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/axes/_axes.py", line 557, in legend
    self.legend_ = mlegend.Legend(self, handles, labels, **kwargs)
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/legend.py", line 699, in __init__
    self._init_legend_box(handles, labels, markerfirst)
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/legend.py", line 954, in _init_legend_box
    fontsize, handlebox))
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/legend_handler.py", line 119, in legend_artist
    fontsize, handlebox.get_transform())
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/legend_handler.py", line 303, in create_artists
    self.update_prop(p, orig_handle, legend)
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/legend_handler.py", line 76, in update_prop
    self._update_prop(legend_handle, orig_handle)
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/legend_handler.py", line 69, in _update_prop
    self._update_prop_func(legend_handle, orig_handle)
  File "/opt/conda/lib/python3.6/site-packages/matplotlib/legend_handler.py", line 42, in update_from_first_child
    tgt.update_from(src.get_children()[0])
IndexError: list index out of range

Expected outcome

Matplotlib version

  • Operating system: Linux 1f8cb2653e0b 4.14.85-1-MANJARO Fix autofmt_xdate() when using in conjunction with twinx() #1 SMP PREEMPT Sat Dec 1 12:18:09 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
  • Matplotlib version: 3.0.2 (installed from PyPi)
  • Matplotlib backend (print(matplotlib.get_backend())): module://backend_interagg
  • Python version: 3.6.7
@timhoffm
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timhoffm commented Dec 17, 2018 •
edited
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Thanks for the report.

Current behavior with bar()

ax = plt.gca()
ax.bar([], [], label='foo')
print(ax.get_legend_handles_labels())
ax.legend()

prints ([<BarContainer object of 0 artists>], ['foo'])

Current behavior with plot() and raises the above error.

ax = plt.gca()
ax.plot([], [], label='foo')
print(ax.get_legend_handles_labels())
ax.legend()

prints ([<matplotlib.lines.Line2D object at 0x7f14fb7c10f0>], ['foo']) and renders an empty plot with a legend.
image

Not sure we can bar() behave similarly.

@choyiny
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choyiny commented Feb 26, 2019

Hi, this is my first time contributing to a large scale open source project, please do pardon my beginner knowledge.

The error originates the function update_from_first_child in legend_handler, which is called by the legend handler of BarContainer defined at legend.py:682

def update_from_first_child(tgt, src):
        tgt.update_from(src.get_children()[0])

In the case where there is no children from the original handler src, it will result in an IndexError. A simple fix like so would fix the issue:

def update_from_first_child(tgt, src):
        try:
            tgt.update_from(src.get_children()[0])
        except IndexError:
            pass

Similarly, we could first check the length of src.get_children() then only update if there is something.

Could anyone please advise if this is the correct approach to this particular issue?

@VanDavv
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VanDavv commented Feb 26, 2019

@choyiny I'd rather do

def update_from_first_child(tgt, src):
    first_child = next(iter(src.get_children()), None)
    tgt.update_from(first_child)

And handle None value either here or in tgt.update_from method

@choyiny choyiny mentioned this issue Feb 28, 2019
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@QuLogic QuLogic added this to the v3.2.0 milestone Mar 3, 2019
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Fix IndexError for pyplot.legend() when plotting empty bar chart with label choyiny/matplotlib
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@QuLogic @choyiny @timhoffm @VanDavv