8000 Groupby transform preserves output dtype by jcrist · Pull Request #9873 · pandas-dev/pandas · GitHub
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Groupby transform preserves output dtype #9873

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b97fd3a
Groupby transform preserves output dtype
jcrist Apr 13, 2015
0afd1a6
Updated release notes [ci skip]
jcrist Apr 13, 2015
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Groupby transform preserves output dtype 
Previously `transform` output was always the same dtype as the groupby
object. This allows the output dtype to differ from the input. Fixes #9807.
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@jcrist
jcrist committed Apr 13, 2015
commit b97fd3a8c556a738ab7f2fcc46a884a3cbc8663c
2 changes: 1 addition & 1 deletion pandas/core/groupby.py
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Expand Up @@ -3006,7 +3006,7 @@ def transform(self, func, *args, **kwargs):
if ((not isinstance(obj.index,MultiIndex) and
type(result.index) != type(obj.index)) or
len(result.index) != len(obj.index)):
results = obj.values.copy()
results = np.empty_like(obj.values, result.values.dtype)
indices = self.indices
for (name, group), (i, row) in zip(self, result.iterrows()):
indexer = indices[name]
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8 changes: 8 additions & 0 deletions pandas/tests/test_groupby.py
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Expand Up @@ -1003,6 +1003,14 @@ def test_transform_broadcast(self):
for idx in gp.index:
assert_fp_equal(res.xs(idx), agged[idx])

def test_transform_dtype(self):
# GH 9807
# Check transform dtype output is preserved
df = DataFrame([[1, 3], [2, 3]])
result = df.groupby(1).transform('mean')
expected = DataFrame([[1.5], [1.5]])
assert_frame_equal(result, expected)

def test_transform_bug(self):
# GH 5712
# transforming on a datetime column
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0