8000 Check parameter type for legend(labels) by timhoffm · Pull Request #16864 · matplotlib/matplotlib · GitHub
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Check parameter type for legend(labels) #16864

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Merged
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Apr 16, 2020
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Check parameter type for legend(labels) #16864

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4 changes: 2 additions & 2 deletions examples/text_labels_and_annotations/custom_legends.py
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Expand Up @@ -33,10 +33,10 @@

fig, ax = plt.subplots()
lines = ax.plot(data)
ax.legend(lines)
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@timhoffm timhoffm Mar 21, 2020 •
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The check caught this wrong usage in an example. 🤓

Fixed to do what is claimed to be done above (" If you simply plot the lines
want a legend item to show up for each one. If you simply plot the lines
and call ax.legend(), you will get the following:").

Overall the example is only semi-instructive. I will keep a rewrite for a separate PR. I just made the minimal change here to make it correct and consistent.

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I wouldn't call it wrong per se. If you remove the argument, no legend will be there at all - which is not what the example wants to convey. It should also trigger a warning like "No handles with labels found" or similar?

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Maybe not technically wrong, but carelessly written and misleading. See my comment below.

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:

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4 changes: 4 additions & 0 deletions lib/matplotlib/legend.py
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Expand Up @@ -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)]
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7 changes: 7 additions & 0 deletions lib/matplotlib/tests/test_legend.py
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Expand Up @@ -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:
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