From a1a99cdbffffc1a66de122affd7186a4c41d61a7 Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Sat, 21 Mar 2020 17:21:37 +0100
Subject: [PATCH] Check parameter type for legend(labels)

---
 examples/text_labels_and_annotations/custom_legends.py | 4 ++--
 lib/matplotlib/legend.py                               | 4 ++++
 lib/matplotlib/tests/test_legend.py                    | 7 +++++++
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/examples/text_labels_and_annotations/custom_legends.py b/examples/text_labels_and_annotations/custom_legends.py
index 28b73fe65da2..e9f840fe768f 100644
--- a/examples/text_labels_and_annotations/custom_legends.py
+++ b/examples/text_labels_and_annotations/custom_legends.py
@@ -33,10 +33,10 @@
 
 fig, ax = plt.subplots()
 lines = ax.plot(data)
-ax.legend(lines)
+ax.legend()
 
 ##############################################################################
-# Note that one legend item per line was created.
+# Note that no legend entries were created.
 # In this case, we can compose a legend using Matplotlib objects that aren't
 # explicitly tied to the data that was plotted. For example:
 
diff --git a/lib/matplotlib/legend.py b/lib/matplotlib/legend.py
index fb47147d152f..611167b075e7 100644
--- a/lib/matplotlib/legend.py
+++ b/lib/matplotlib/legend.py
@@ -1162,6 +1162,10 @@ def _parse_legend_args(axs, *args, handles=None, labels=None, **kwargs):
     # One argument. User defined labels - automatic handle detection.
     elif len(args) == 1:
         labels, = args
+        if any(isinstance(l, Artist) for l in labels):
+            raise TypeError("A single argument passed to legend() must be a "
+                            "list of labels, but found an Artist in there.")
+
         # Get as many handles as there are labels.
         handles = [handle for handle, label
                    in zip(_get_legend_handles(axs, handlers), labels)]
diff --git a/lib/matplotlib/tests/test_legend.py b/lib/matplotlib/tests/test_legend.py
index 0e340e7f1e82..a2656a85528a 100644
--- a/lib/matplotlib/tests/test_legend.py
+++ b/lib/matplotlib/tests/test_legend.py
@@ -234,6 +234,13 @@ 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.raises(TypeError, match='but found an Artist'):
+            # 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: