Merge pull request matplotlib#27004 from borgesaugusto/iss_24050

Warning if handles and labels have a len mismatch

Author: rcomer

lib/matplotlib/legend.py

@@ -1337,6 +1337,12 @@ def _parse_legend_args(axs, *args, handles=None, labels=None, **kwargs):
         _api.warn_external("You have mixed positional and keyword arguments, "
                            "some input may be discarded.")
 
+    if (hasattr(handles, "__len__") and
+            hasattr(labels, "__len__") and
+            len(handles) != len(labels)):
+        _api.warn_external(f"Mismatched number of handles and labels: "
+                           f"len(handles) = {len(handles)} "
+                           f"len(labels) = {len(labels)}")
     # if got both handles and labels as kwargs, make same length
     if handles and labels:
         handles, labels = zip(*zip(handles, labels))

lib/matplotlib/tests/test_legend.py

@@ -1357,3 +1357,21 @@ def test_loc_validation_string_value():
     ax.legend(loc='upper center')
     with pytest.raises(ValueError, match="'wrong' is not a valid value for"):
         ax.legend(loc='wrong')
+
+
+def test_legend_handle_label_mismatch():
+    pl1, = plt.plot(range(10))
+    pl2, = plt.plot(range(10))
+    with pytest.warns(UserWarning, match="number of handles and labels"):
+        legend = plt.legend(handles=[pl1, pl2], labels=["pl1", "pl2", "pl3"])
+        assert len(legend.legend_handles) == 2
+        assert len(legend.get_texts()) == 2
+
+
+def test_legend_handle_label_mismatch_no_len():
+    pl1, = plt.plot(range(10))
+    pl2, = plt.plot(range(10))
+    legend = plt.legend(handles=iter([pl1, pl2]),
+                        labels=iter(["pl1", "pl2", "pl3"]))
+    assert len(legend.legend_handles) == 2
+    assert len(legend.get_texts()) == 2