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BUG: Fix groupby nunique with NaT #17624

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Merged
merged 5 commits into from
Sep 22, 2017
Merged

BUG: Fix groupby nunique with NaT #17624

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691b384
BUG: Fix groupby nunique with NaT
Licht-T Sep 22, 2017
800d1ad
TST: Add the nunique with NaT test
Licht-T Sep 22, 2017
5e0d924
TST: Move the groupby nunique with NaT test to test_timegrouper and a…
Licht-T Sep 22, 2017
fabc371
DOC: Add the description for the fix groupby nunique with NaT in what…
Licht-T Sep 22, 2017
29d4cf6
DOC: Add the comment for the fix groupby nunique with NaT
Licht-T Sep 22, 2017
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v0.21.0.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -543,6 +543,7 @@ Groupby/Resample/Rolling
- Bug in ``Series.resample(...).apply()`` where an empty ``Series`` modified the source index and did not return the name of a ``Series`` (:issue:`14313`)
- Bug in ``.rolling(...).apply(...)`` with a ``DataFrame`` with a ``DatetimeIndex``, a ``window`` of a timedelta-convertible and ``min_periods >= 1` (:issue:`15305`)
- Bug in ``DataFrame.groupby`` where index and column keys were not recognized correctly when the number of keys equaled the number of elements on the groupby axis (:issue:`16859`)
- Bug in ``groupby.nunique()`` with ``TimeGrouper`` which cannot handle ``NaT`` correctly (:issue:`17575`)

Sparse
^^^^^^
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8 changes: 7 additions & 1 deletion pandas/core/groupby.py
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Expand Up @@ -3177,7 +3177,13 @@ def nunique(self, dropna=True):

out = np.add.reduceat(inc, idx).astype('int64', copy=False)
if len(ids):
res = out if ids[0] != -1 else out[1:]
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can you add a helpful comment here on what is happening

# NaN/NaT group exists if the head of ids is -1,
# so remove it from res and exclude its index from idx
if ids[0] == -1:
res = out[1:]
idx = idx[np.flatnonzero(idx)]
else:
res = out
else:
res = out[1:]
ri = self.grouper.result_index
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13 changes: 13 additions & 0 deletions pandas/tests/groupby/test_timegrouper.py
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Expand Up @@ -608,3 +608,16 @@ def test_first_last_max_min_on_time_data(self):
assert_frame_equal(grouped_ref.min(), grouped_test.min())
assert_frame_equal(grouped_ref.first(), grouped_test.first())
assert_frame_equal(grouped_ref.last(), grouped_test.last())

def test_nunique_with_timegrouper_and_nat(self):
# GH 17575
test = pd.DataFrame({
'time': [Timestamp('2016-06-28 09:35:35'),
pd.NaT,
Timestamp('2016-06-28 16:46:28')],
'data': ['1', '2', '3']})

grouper = pd.TimeGrouper(key='time', freq='h')
result = test.groupby(grouper)['data'].nunique()
expected = test[test.time.notnull()].groupby(grouper)['data'].nunique()
tm.assert_series_equal(result, expected)