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... #25315

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616e6cb
FIX: apply std to groupby with as_index=False
xieyuheng Feb 14, 2019
dd6137b
remove extra newline
xieyuheng Feb 14, 2019
89d39f1
test_groupby_std_with_as_index_false
xieyuheng Feb 14, 2019
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14 changes: 11 additions & 3 deletions pandas/core/groupby/groupby.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1171,10 +1171,18 @@ def std(self, ddof=1, *args, **kwargs):
ddof : integer, default 1
degrees of freedom
"""

# TODO: implement at Cython level?
nv.validate_groupby_func('std', args, kwargs)
return np.sqrt(self.var(ddof=ddof, **kwargs))
if ddof == 1:
try:
return self._cython_agg_general('std', **kwargs)
except Exception:
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Not entirely clear what this is for but except for legacy code we never catch just base Exception so will need something more explicit

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Copy / paste isn't the right way to go here. Can you still keep the dispatch to self.var and just see what is mangling the expression once passed as an argument to np.sqrt instead?

f = lambda x: x.std(ddof=ddof, **kwargs)
with _group_selection_context(self):
return self._python_agg_general(f)
else:
f = lambda x: x.std(ddof=ddof, **kwargs)
with _group_selection_context(self):
return self._python_agg_general(f)

@Substitution(name='groupby')
@Appender(_common_see_also)
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32 changes: 32 additions & 0 deletions pandas/tests/groupby/test_groupby.py
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Expand Up @@ -1698,3 +1698,35 @@ def test_groupby_agg_ohlc_non_first():
result = df.groupby(pd.Grouper(freq='D')).agg(['sum', 'ohlc'])

tm.assert_frame_equal(result, expected)


def test_groupby_std_with_as_index_false():
# https://github.com/pandas-dev/pandas/issues/16799

df = pd.DataFrame({
"A": ["a", "b", "a", "b"],
"B": ["A", "B", "A", "B"],
"X": [1, 2, 3, 4]
})

group_var = df.groupby(
["A", "B"],
as_index=False,
).var()

# A B X
# 0 a A 2
# 1 b B 2

group_std = df.groupby(
["A", "B"],
as_index=False,
).std()

# A B X
# 0 a A 1.414214
# 1 b B 1.414214

assert_series_equal(
np.sqrt(group_var["X"]),
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Unless there is a reason for using np.sqrt here just explicitly construct the expected values in a variable called expected. Also do result = on the groupby op you are testing and simply make this line assert_series_equal(result, expected) (should see this usage throughout tests if unclear from this comment)

group_std["X"])