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Make slowness warning for legend(loc="best") more accurate. #14894

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Merged
merged 1 commit into from
Jul 26, 2019
Merged

Make slowness warning for legend(loc="best") more accurate. #14894

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15 changes: 9 additions & 6 deletions lib/matplotlib/legend.py
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@
"""

import logging
import time

import numpy as np

Expand Down Expand Up @@ -1112,13 +1113,9 @@ def _find_best_position(self, width, height, renderer, consider=None):
# should always hold because function is only called internally
assert self.isaxes

start_time = time.perf_counter()

verts, bboxes, lines, offsets = self._auto_legend_data()
if self._loc_used_default and verts.shape[0] > 200000:
# this size results in a 3+ second render time on a good machine
cbook._warn_external(
'Creating legend with loc="best" can be slow with large '
'amounts of data.'
)

bbox = Bbox.from_bounds(0, 0, width, height)
if consider is None:
Expand All @@ -1145,6 +1142,12 @@ def _find_best_position(self, width, height, renderer, consider=None):
candidates.append((badness, idx, (l, b)))

_, _, (l, b) = min(candidates)

if self._loc_used_default and time.perf_counter() - start_time > 1:
cbook._warn_external(
'Creating legend with loc="best" can be slow with large '
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include the actual run time? Wish we could use the walrus here ;)

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would rather not, as if we do I'm fairly worried the next thing is that someone will ask for the warning threshold to be configurable and whatnot.

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fair enough.

'amounts of data.')

return l, b

def contains(self, event):
Expand Down
22 changes: 13 additions & 9 deletions lib/matplotlib/tests/test_legend.py
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Original file line number Diff line number Diff line change
Expand Up @@ -550,24 +550,28 @@ def test_alpha_handles():

def test_warn_big_data_best_loc():
fig, ax = plt.subplots()
ax.plot(np.arange(200001), label='Is this big data?')
fig.canvas.draw() # So that we can call draw_artist later.
for idx in range(1000):
ax.plot(np.arange(5000), label=idx)
with rc_context({'legend.loc': 'best'}):
legend = ax.legend()
with pytest.warns(UserWarning) as records:
with rc_context({'legend.loc': 'best'}):
ax.legend()
fig.canvas.draw()
fig.draw_artist(legend) # Don't bother drawing the lines -- it's slow.
# The _find_best_position method of Legend is called twice, duplicating
# the warning message.
assert len(records) == 2
for record in records:
assert str(record.message) == (
'Creating legend with loc="best" can be slow with large'
' amounts of data.')
'Creating legend with loc="best" can be slow with large '
'amounts of data.')


def test_no_warn_big_data_when_loc_specified():
fig, ax = plt.subplots()
ax.plot(np.arange(200001), label='Is this big data?')
fig.canvas.draw()
for idx in range(1000):
ax.plot(np.arange(5000), label=idx)
legend = ax.legend('best')
with pytest.warns(None) as records:
ax.legend('best')
fig.canvas.draw()
fig.draw_artist(legend)
assert len(records) == 0