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Deprecate removal of explicit legend handles whose label starts with _. #26190

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Jun 26, 2023
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Deprecate removal of explicit legend handles whose label starts with _. #26190

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7 changes: 7 additions & 0 deletions doc/api/next_api_changes/deprecations/26190-AL.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
Artists explicitly passed in will no longer be filtered by legend() based on their label
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Currently, artists explicitly passed to ``legend(handles=[...])`` are filtered
out if their label starts with an underscore. This behavior is deprecated;
explicitly filter out such artists
(``[art for art in artists if not art.get_label().startswith('_')]``) if
necessary.
9 changes: 6 additions & 3 deletions lib/matplotlib/legend.py
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Expand Up @@ -465,9 +465,12 @@ 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(f"The label {label!r} of {handle!r} starts "
"with '_'. It is thus excluded from the "
"legend.")
_api.warn_deprecated("3.8", message=(
"An artist whose label starts with an underscore was passed to "
"legend(); such artists will no longer be ignored in the future. "
"To suppress this warning, explicitly filter out such artists, "
"e.g. with `[art for art in artists if not "
"art.get_label().startswith('_')]`."))
else:
_lab.append(label)
_hand.append(handle)
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5 changes: 2 additions & 3 deletions lib/matplotlib/tests/test_legend.py
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Expand Up @@ -17,7 +17,7 @@
import matplotlib.lines as mlines
from matplotlib.legend_handler import HandlerTuple
import matplotlib.legend as mlegend
from matplotlib import rc_context
from matplotlib import _api, rc_context
from matplotlib.font_manager import FontProperties


Expand Down Expand Up @@ -144,8 +144,7 @@ def test_legend_label_with_leading_underscore():
"""
fig, ax = plt.subplots()
line, = ax.plot([0, 1], label='_foo')
with pytest.warns(UserWarning,
match=r"starts with '_'.*excluded from the legend."):
with pytest.warns(_api.MatplotlibDeprecationWarning, match="with an underscore"):
legend = ax.legend(handles=[line])
assert len(legend.legend_handles) == 0

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