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7 changes: 7 additions & 0 deletions pandas/tools/tests/test_concat.py
Original file line number Diff line number Diff line change
Expand Up @@ -1006,6 +1006,13 @@ def test_union_categoricals_sort(self):
categories=['a', 'b', 'c'])
tm.assert_categorical_equal(result, expected)

c1 = Categorical(['a', 'b'], categories=['c', 'a', 'b'])
c2 = Categorical(['b', 'c'], categories=['c', 'a', 'b'])
result = union_categoricals([c1, c2], sort_categories=True)
expected = Categorical(['a', 'b', 'b', 'c'],
categories=['a', 'b', 'c'])
tm.assert_categorical_equal(result, expected)

# fastpath - skip resort
c1 = Categorical(['a', 'b'], categories=['a', 'b', 'c'])
c2 = Categorical(['b', 'c'], categories=['a', 'b', 'c'])
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2 changes: 1 addition & 1 deletion pandas/types/concat.py
Original file line number Diff line number Diff line change
Expand Up @@ -263,7 +263,7 @@ def union_categoricals(to_union, sort_categories=False):

if sort_categories and not categories.is_monotonic_increasing:
categories = categories.sort_values()
indexer = first.categories.get_indexer(categories)
indexer = categories.get_indexer(first.categories)
new_codes = take_1d(indexer, new_codes, fill_value=-1)
elif all(not c.ordered for c in to_union):
# different categories - union and recode
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