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Better error message for unescaped underscores #18357

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Better error message for unescaped underscores #18357

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21 changes: 20 additions & 1 deletion lib/matplotlib/tests/test_texmanager.py
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Original file line number Diff line number Diff line change
@@ -1,7 +1,14 @@
import matplotlib.pyplot as plt
import pytest

from matplotlib import pyplot as plt, checkdep_usetex
from matplotlib.texmanager import TexManager


needs_usetex = pytest.mark.skipif(
not checkdep_usetex(True),
reason="This test needs a TeX installation")


def test_fontconfig_preamble():
"""
Test that the preamble is included in _fontconfig
Expand All @@ -16,3 +23,15 @@ def test_fontconfig_preamble():
font_config2 = tm2.get_font_config()

assert font_config1 != font_config2


@needs_usetex
def test_usetex_missing_underscore():
"""
Test that failed TeX rendering due to an unescaped underscore has a
custom error message.
"""
with pytest.raises(RuntimeError,
match='caused by an unescaped underscore'):
plt.text(0, 0, 'foo_bar', usetex=True)
plt.draw() # TeX rendering is done at draw time
40 changes: 32 additions & 8 deletions lib/matplotlib/texmanager.py
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Original file line number Diff line number Diff line change
Expand Up @@ -281,14 +281,38 @@ def _run_checked_subprocess(self, command, tex, *, cwd=None):
'Failed to process string with tex because {} could not be '
'found'.format(command[0])) from exc
except subprocess.CalledProcessError as exc:
raise RuntimeError(
'{prog} was not able to process the following string:\n'
'{tex!r}\n\n'
'Here is the full report generated by {prog}:\n'
'{exc}\n\n'.format(
prog=command[0],
tex=tex.encode('unicode_escape'),
exc=exc.output.decode('utf-8'))) from exc
tex_log = exc.output.decode('utf-8')

# whether the exception was likely caused by an unescaped
# underscore. This cannot be easily determined unambiguously.
# By requiring `'$' not in tex` we err on the side of preventing
# false positives, because we only want to issue the customized
# error message if we are sure there's an unescaped underscore.
unescaped_underscore = (
'_' in tex and '$' not in tex and
re.match(r'.*Missing \$ inserted.*_\n', tex_log,
flags=re.DOTALL)
)
if unescaped_underscore:
message = (
'{prog} was not able to process the following string:\n'
'{tex!r}\n\n'
'This is likely caused by an unescaped underscore.\n'
'You may escape the underscore or add '
'r"\\usepackage{{underscore}}"\n'
'to the matplotlib rcParam text.latex.preamble'.format(
prog=command[0],
tex=tex.encode('unicode_escape')))
else:
message = (
'{prog} was not able to process the following string:\n'
'{tex!r}\n\n'
'Here is the full report generated by {prog}:\n'
'{exc}\n\n'.format(
prog=command[0],
tex=tex.encode('unicode_escape'),
exc=tex_log))
raise RuntimeError(message) from exc
_log.debug(report)
return report

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