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Clarify warning about labels with leading underscores. #22440

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Merged
merged 1 commit into from
Feb 12, 2022
Merged

Clarify warning about labels with leading underscores. #22440

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6 changes: 3 additions & 3 deletions lib/matplotlib/legend.py
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Original file line number Diff line number Diff line change
Expand Up @@ -406,9 +406,9 @@ def val_or_rc(val, rc_name):
_lab, _hand = [], []
for label, handle in zip(labels, handles):
if isinstance(label, str) and label.startswith('_'):
_api.warn_external('The handle {!r} has a label of {!r} '
'which cannot be automatically added to'
' the legend.'.format(handle, label))
_api.warn_external(f"The label {label!r} of {handle!r} starts "
"with '_'. It is thus excluded from the "
"legend.")
else:
_lab.append(label)
_hand.append(handle)
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14 changes: 14 additions & 0 deletions lib/matplotlib/tests/test_legend.py
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,20 @@ def test_various_labels():
ax.legend(numpoints=1, loc='best')


def test_legend_label_with_leading_underscore():
"""
Test that artists with labels starting with an underscore are not added to
the legend, and that a warning is issued if one tries to add them
explicitly.
"""
fig, ax = plt.subplots()
line, = ax.plot([0, 1], label='_foo')
with pytest.warns(UserWarning,
match=r"starts with '_'.*excluded from the legend."):
legend = ax.legend(handles=[line])
assert len(legend.legendHandles) == 0


@image_comparison(['legend_labels_first.png'], remove_text=True)
def test_labels_first():
# test labels to left of markers
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