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Deprecate mixing positional and keyword args for legend(handles, labels) #27175

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Oct 26, 2023
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Deprecate mixing positional and keyword args for legend(handles, labels) #27175

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5 changes: 5 additions & 0 deletions doc/api/next_api_changes/deprecations/27175-AL.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
Mixing positional and keyword arguments for ``legend`` handles and labels
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This previously only raised a warning, but is now formally deprecated. If
passing *handles* and *labels*, they must be passed either both positionally or
both as keyword.
8 changes: 5 additions & 3 deletions lib/matplotlib/legend.py
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Expand Up @@ -1301,7 +1301,7 @@ def _parse_legend_args(axs, *args, handles=None, labels=None, **kwargs):
legend(handles=handles, labels=labels)

The behavior for a mixture of positional and keyword handles and labels
is undefined and issues a warning.
is undefined and issues a warning; it will be an error in the future.

Parameters
----------
Expand Down Expand Up @@ -1334,8 +1334,10 @@ def _parse_legend_args(axs, *args, handles=None, labels=None, **kwargs):
handlers = kwargs.get('handler_map')

if (handles is not None or labels is not None) and args:
_api.warn_external("You have mixed positional and keyword arguments, "
"some input may be discarded.")
_api.warn_deprecated("3.9", message=(
"You have mixed positional and keyword arguments, some input may "
"be discarded. This is deprecated since %(since)s and will "
"become an error %(removal)s."))

if (hasattr(handles, "__len__") and
hasattr(labels, "__len__") and
Expand Down
8 changes: 4 additions & 4 deletions lib/matplotlib/tests/test_legend.py
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Expand Up @@ -405,10 +405,10 @@ def test_warn_mixed_args_and_kwargs(self):
th = np.linspace(0, 2*np.pi, 1024)
lns, = ax.plot(th, np.sin(th), label='sin')
lnc, = ax.plot(th, np.cos(th), label='cos')
with pytest.warns(UserWarning) as record:
with pytest.warns(DeprecationWarning) as record:
ax.legend((lnc, lns), labels=('a', 'b'))
assert len(record) == 1
assert str(record[0].message) == (
assert str(record[0].message).startswith(
"You have mixed positional and keyword arguments, some input may "
"be discarded.")

Expand Down Expand Up @@ -474,10 +474,10 @@ def test_warn_args_kwargs(self):
fig, axs = plt.subplots(1, 2)
lines = axs[0].plot(range(10))
lines2 = axs[1].plot(np.arange(10) * 2.)
with pytest.warns(UserWarning) as record:
with pytest.warns(DeprecationWarning) as record:
fig.legend((lines, lines2), labels=('a', 'b'))
assert len(record) == 1
assert str(record[0].message) == (
assert str(record[0].message).startswith(
"You have mixed positional and keyword arguments, some input may "
"be discarded.")

Expand Down