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BUG: Break reference from grouping level to MI #31133

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Merged
merged 4 commits into from
Jan 20, 2020
Merged

BUG: Break reference from grouping level to MI #31133

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0419f74
BUG: Break reference from grouping level to MI
TomAugspurger Jan 19, 2020
ff8ea04
Merge remote-tracking branch 'upstream/master' into 31068-groupby-mi-…
TomAugspurger Jan 20, 2020
8792b68
Limit copies
TomAugspurger Jan 20, 2020
6bb3c82
avoid unneeded copies
TomAugspurger Jan 20, 2020
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3 changes: 3 additions & 0 deletions pandas/core/indexes/multi.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1235,6 +1235,9 @@ def _set_names(self, names, level=None, validate=True):
def _get_grouper_for_level(self, mapper, level):
indexer = self.codes[level]
level_index = self.levels[level]
# break references back to ourself so that setting the name
# on the output of a groupby doesn't reflect back here.
level_index = level_index.copy()
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you don't need to do this here, rather do it as an else on line 1261 (add it); as take already copies, its just when that condition is not satisfied does it need a copy.

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Fixed, I think. Added a case for Categorical grouper with unobserved categories, so I think all the cases are covered.


if mapper is not None:
# Handle group mapping function and return
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18 changes: 18 additions & 0 deletions pandas/tests/groupby/test_apply.py
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Expand Up @@ -752,3 +752,21 @@ def most_common_values(df):
["17661101"], index=pd.DatetimeIndex(["2015-02-24"], name="day"), name="userId"
)
tm.assert_series_equal(result, expected)


def test_apply_multi_level_name():
# https://github.com/pandas-dev/pandas/issues/31068
df = pd.DataFrame(
{
"A": np.arange(10),
"B": [1, 2] * 5,
"C": list(range(10)),
"D": list(range(10)),
}
).set_index(["A", "B"])
result = df.groupby("B").apply(lambda x: x.sum())
expected = pd.DataFrame(
{"C": [20, 25], "D": [20, 25]}, index=pd.Index([1, 2], name="B")
)
tm.assert_frame_equal(result, expected)
assert df.index.names == ["A", "B"]