8000 Deprecate non-string values as legend labels by timhoffm · Pull Request #16771 · matplotlib/matplotlib · GitHub
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Deprecate non-string values as legend labels #16771

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Deprecate non-string values as legend labels #16771

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4 changes: 4 additions & 0 deletions doc/api/next_api_changes/deprecations.rst
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Expand Up @@ -333,6 +333,10 @@ Support for passing ``None`` as base class to `.axes.subplot_class_factory`,
``axes_grid1.parasite_axes.parasite_axes_auxtrans_class_factory`` is deprecated.
Explicitly pass the correct base ``Axes`` class instead.

Legend labels must be strings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Passing non-string objects as legend labels is deprecated.

``axes_rgb``
~~~~~~~~~~~~
In :mod:`mpl_toolkits.axes_grid1.axes_rgb`, ``imshow_rgb`` is deprecated (use
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5 changes: 5 additions & 0 deletions lib/matplotlib/legend.py
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Expand Up @@ -395,6 +395,11 @@ def __init__(self, parent, handles, labels,
# trim handles and labels if illegal label...
_lab, _hand = [], []
for label, handle in zip(labels, handles):
if not isinstance(label, str):
cbook.warn_deprecated(
"3.3",
message="Passing non-string objects as legend "
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Can you add a note suggesting that they probably wanted to pass in two lists?

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@timhoffm timhoffm Mar 15, 2020 •
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I'd rather not. This check is now implemented in Legend with signature Legend(parent, handles, labels, ..). So the "two lists" does not apply anymore. Otherwise I'd have to make the check in mlegend._parse_legend_args(). But then the parameter in Legend is not checked explicitly - was not before, but I think it should anyway. Furthermore, you can pass labels positionally or by kwarg to legend(); while the str-check would apply to both, the proposed addition would only make sense for the former.

On the other hand, IMHO it's clear enough to respond on a call legend(lines) with the message Passing non-string objects as legend labels is deprecated. It's rather obvious from this that we interpret the only parameter as labels.

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I suspect that we are still going to get some confused users, but this is certainly clearer than it currently is!

"labels is deprecated.")
if isinstance(label, str) and label.startswith('_'):
cbook._warn_external('The handle {!r} has a label of {!r} '
'which cannot be automatically added to'
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10 changes: 10 additions & 0 deletions lib/matplotlib/tests/test_legend.py
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Expand Up @@ -5,6 +5,7 @@
import numpy as np
import pytest

from matplotlib.cbook import MatplotlibDeprecationWarning
from matplotlib.testing.decorators import image_comparison
import matplotlib.pyplot as plt
import matplotlib as mpl
Expand Down Expand Up @@ -234,6 +235,15 @@ def test_legend_handle_label(self):
plt.legend(lines, ['hello world'])
Legend.assert_called_with(plt.gca(), lines, ['hello world'])

def test_legend_handles_only(self):
lines = plt.plot(range(10))
with pytest.warns(MatplotlibDeprecationWarning,
match="Passing non-string objects as legend labels "
"is deprecated"):
# a single arg is interpreted as labels
# it's a common error to just pass handles
plt.legend(lines)

def test_legend_no_args(self):
lines = plt.plot(range(10), label='hello world')
with mock.patch('matplotlib.legend.Legend') as Legend:
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