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BUG: fix sorting with observed=True in groupby #25173

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BUG: fix sorting with observed=True in groupby #25173

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2 changes: 1 addition & 1 deletion doc/source/whatsnew/v0.24.2.rst
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Expand Up @@ -73,7 +73,7 @@ Bug Fixes

**Reshaping**

-
- Fixed bug in :meth:`DataFrame.groupby` where using ``observed=True`` didn't sort properly (:issue:`25167`)
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move to 0.25.0

-
-

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3 changes: 1 addition & 2 deletions pandas/core/groupby/categorical.py
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Expand Up @@ -44,8 +44,7 @@ def recode_for_groupby(c, sort, observed):
unique_codes = unique1d(c.codes)

take_codes = unique_codes[unique_codes != -1]
if c.ordered:
take_codes = np.sort(take_codes)
take_codes = np.sort(take_codes)

# we recode according to the uniques
categories = c.categories.take(take_codes)
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2 changes: 2 additions & 0 deletions pandas/core/groupby/grouper.py
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Expand Up @@ -300,6 +300,8 @@ def __init__(self, index, grouper=None, obj=None, name=None, level=None,
if observed:
codes = algorithms.unique1d(self.grouper.codes)
codes = codes[codes != -1]
if sort:
codes = np.sort(codes)
else:
codes = np.arange(len(categories))

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8 changes: 8 additions & 0 deletions pandas/tests/groupby/test_categorical.py
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Expand Up @@ -360,6 +360,14 @@ def test_observed(observed):
expected = groups2.agg('mean').reset_index()
tm.assert_frame_equal(result, expected)

# GH 25167
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make this a separate test, parameterize by sort as well

df = pd.DataFrame({'A': pd.Categorical(['b', 'a']), 'B': [1, 2]})
expected = pd.DataFrame([2, 1],
index=pd.CategoricalIndex(['a', 'b'], name='A'),
columns=['B'])
result = df.groupby('A', observed=observed).sum()
tm.assert_frame_equal(result, expected)


def test_observed_codes_remap(observed):
d = {'C1': [3, 3, 4, 5], 'C2': [1, 2, 3, 4], 'C3': [10, 100, 200, 34]}
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2 changes: 1 addition & 1 deletion pandas/tests/groupby/test_grouping.py
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Expand Up @@ -269,7 +269,7 @@ def test_groupby_categorical_index_and_columns(self, observed):
# if we are not-observed we undergo a reindex
# so need to adjust the output as our expected sets us up
# to be non-observed
expected_columns = CategoricalIndex(['A', 'B'],
expected_columns = CategoricalIndex(['B', 'A'],
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expected_columns should have ['B', 'A'], because ordered=True.

categories=categories,
ordered=True)
else:
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