BUG: Fix MultiIndex DataFrame to_csv() segfault #26355
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pandas/tests/frame/test_to_csv.py
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| @@ -874,6 +874,13 @@ def test_to_csv_index_no_leading_comma(self): | |||
| expected = tm.convert_rows_list_to_csv_str(expected_rows) | |||
| assert buf.getvalue() == expected | |||
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| def test_to_csv_multi_index(self): | |||
| # see gh-26303 | |||
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don’t actually write a file as that leaves an artifact
either use a context manager (see other routines) or write to StringIO
in either case compare the result
Codecov Report
@@ Coverage Diff @@
## master #26355 +/- ##
==========================================
- Coverage 92.04% 92.03% -0.01%
==========================================
Files 175 175
Lines 52292 52292
==========================================
- Hits 48132 48128 -4
- Misses 4160 4164 +4
Continue to review full report at Codecov.
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Codecov Report
@@ Coverage Diff @@
## master #26355 +/- ##
==========================================
- Coverage 91.68% 91.67% -0.01%
==========================================
Files 174 174
Lines 50703 50703
==========================================
- Hits 46488 46484 -4
- Misses 4215 4219 +4
Continue to review full report at Codecov.
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doc/source/whatsnew/v0.25.0.rst
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| @@ -254,6 +254,7 @@ Performance Improvements | |||
| - Improved performance of :meth:`Series.map` for dictionary mappers on categorical series by mapping the categories instead of mapping all values (:issue:`23785`) | |||
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| .. _whatsnew_0250.bug_fixes: | |||
| - Fixed MultiIndex DataFrame to_csv segfault (:issue:`26303`) | |||
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move to Indexing, use :class:`MultiIdex` and :meth:DataFrame.to_csv ``; specify that this is a single-level multi-index
| @@ -946,7 +946,7 @@ def _format_native_types(self, na_rep='nan', **kwargs): | |||
| new_codes.append(level_codes) | |||
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| if len(new_levels) == 1: | |||
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add a comment here that this is a single-level MI
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what do new_levels and new_codes look like here (from your test)
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The value of new_levels is [array(['1', '2', '3'], dtype='<U21')].
The value of new_codes is [FrozenNDArray([0, 2], dtype='int8')]
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Added a comment in d47089e
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ok use levels[0].take(codes[0])
pandas/tests/frame/test_to_csv.py
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| @@ -874,6 +874,18 @@ def test_to_csv_index_no_leading_comma(self): | |||
| expected = tm.convert_rows_list_to_csv_str(expected_rows) | |||
| assert buf.getvalue() == expected | |||
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| def test_to_csv_multi_index(self): | |||
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we already have a multiindex test, can you move this near; change the test to be test_to_csv_single_level_multiindex
pandas/tests/frame/test_to_csv.py
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| index = pd.Index([(1,), (2,), (3,)]) | ||
| df = pd.DataFrame([[1, 2, 3]], columns=index) | ||
| with ensure_clean() as path: | ||
| df = df.reindex(columns=[(1,), (3,)]) |
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why is this reindex needed?
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The reindex is what seems to cause the segfault in DataFrame.to_csv as demonstrated in #26303. It seems that it has something to do with a multi-index having a larger number of levels than there are columns in a data frame.
mahepe
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pandas/core/indexes/multi.py
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| @@ -946,7 +946,9 @@ def _format_native_types(self, na_rep='nan', **kwargs): | |||
| new_codes.append(level_codes) | |||
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| if len(new_levels) == 1: | |||
| return Index(new_levels[0])._format_native_types() | |||
| # a single-level multi-index | |||
| return Index(new_levels[0].take(new_codes[0])).\ | |||
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Shouldn't need the explicit line break here
pandas/tests/frame/test_to_csv.py
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| df.to_csv(path, line_terminator='\n') | ||
| expected = b",1,3\n0,1,3\n" | ||
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| with open(path, mode='rb') as f: |
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Do you need to physically write this to a file to reproduce the segfault or can you just inspect the result returned as a string?
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Turns out that no: Tried the example provided in the issue. The segfault occured even if the csv was never written to a file.
Updated the test so that it doesn't write to a file.
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thanks @mahepe |
git diff upstream/master -u -- "*.py" | flake8 --diffThis fix for #26303 would avoid an indexing issue caused by using MultiIndexes which results to a segfault in the example provided in #26303.