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DEPR: Enforce DataFrameGroupBy.__iter__ returning tuples of length 1 #50064

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Dec 5, 2022
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DEPR: Enforce DataFrameGroupBy.__iter__ returning tuples of length 1 #50064

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2 changes: 1 addition & 1 deletion doc/source/whatsnew/v2.0.0.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -596,7 +596,7 @@ Removal of prior version deprecations/changes
- Enforced deprecation of silently dropping nuisance columns in groupby and resample operations when ``numeric_only=False`` (:issue:`41475`)
- Changed default of ``numeric_only`` in various :class:`.DataFrameGroupBy` methods; all methods now default to ``numeric_only=False`` (:issue:`46072`)
- Changed default of ``numeric_only`` to ``False`` in :class:`.Resampler` methods (:issue:`47177`)
-
- When providing a list of columns of length one to :meth:`DataFrame.groupby`, the keys that are returned by iterating over the resulting :class:`DataFrameGroupBy` object will now be tuples of length one (:issue:`47761`)

.. ---------------------------------------------------------------------------
.. _whatsnew_200.performance:
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17 changes: 4 additions & 13 deletions pandas/core/groupby/groupby.py
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Expand Up @@ -70,7 +70,6 @@ class providing the base-class of operations.
cache_readonly,
doc,
)
from pandas.util._exceptions import find_stack_level

from pandas.core.dtypes.cast import ensure_dtype_can_hold_na
from pandas.core.dtypes.common import (
Expand Down Expand Up @@ -832,19 +831,11 @@ def __iter__(self) -> Iterator[tuple[Hashable, NDFrameT]]:
for each group
"""
keys = self.keys
result = self.grouper.get_iterator(self._selected_obj, axis=self.axis)
if isinstance(keys, list) and len(keys) == 1:
warnings.warn(
(
"In a future version of pandas, a length 1 "
"tuple will be returned when iterating over a "
"groupby with a grouper equal to a list of "
"length 1. Don't supply a list with a single grouper "
"to avoid this warning."
),
FutureWarning,
stacklevel=find_stack_level(),
)
return self.grouper.get_iterator(self._selected_obj, axis=self.axis)
# GH#42795 - when keys is a list, return tuples even when length is 1
result = (((key,), group) for key, group in result)
return result


# To track operations that expand dimensions, like ohlc
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16 changes: 4 additions & 12 deletions pandas/tests/groupby/test_groupby.py
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Expand Up @@ -2743,18 +2743,10 @@ def test_groupby_none_column_name():

def test_single_element_list_grouping():
# GH 42795
df = DataFrame(
{"a": [np.nan, 1], "b": [np.nan, 5], "c": [np.nan, 2]}, index=["x", "y"]
)
msg = (
"In a future version of pandas, a length 1 "
"tuple will be returned when iterating over "
"a groupby with a grouper equal to a list of "
"length 1. Don't supply a list with a single grouper "
"to avoid this warning."
)
with tm.assert_produces_warning(FutureWarning, match=msg):
values, _ = next(iter(df.groupby(["a"])))
df = DataFrame({"a": [1, 2], "b": [np.nan, 5], "c": [np.nan, 2]}, index=["x", "y"])
result = [key for key, _ in df.groupby(["a"])]
expected = [(1,), (2,)]
assert result == expected


@pytest.mark.parametrize("func", ["sum", "cumsum", "cumprod", "prod"])
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