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BUG: DataFrame.__setitem__ raising ValueError with string indexer and empty df and df to set #38931

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Merged
merged 3 commits into from
Jan 6, 2021
Merged

BUG: DataFrame.__setitem__ raising ValueError with string indexer and empty df and df to set #38931

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63205ca
Fix 38831 first step
phofl Dec 30, 2020
629cdde
BUG: DataFrame.__setitem__ raising ValueError with string indexer and…
phofl Jan 3, 2021
a3c3ee2
Merge branch 'master' of https://github.com/pandas-dev/pandas into 38831
phofl Jan 5, 2021
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v1.3.0.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -244,6 +244,7 @@ Indexing
- Bug in :meth:`CategoricalIndex.get_indexer` failing to raise ``InvalidIndexError`` when non-unique (:issue:`38372`)
- Bug in inserting many new columns into a :class:`DataFrame` causing incorrect subsequent indexing behavior (:issue:`38380`)
- Bug in :meth:`DataFrame.loc`, :meth:`Series.loc`, :meth:`DataFrame.__getitem__` and :meth:`Series.__getitem__` returning incorrect elements for non-monotonic :class:`DatetimeIndex` for string slices (:issue:`33146`)
- Bug in :meth:`DataFrame.__setitem__` raising ``ValueError`` with empty :class:`DataFrame` and specified columns for string indexer and non empty :class:`DataFrame` to set (:issue:`38831`)
- Bug in :meth:`DataFrame.iloc.__setitem__` and :meth:`DataFrame.loc.__setitem__` with mixed dtypes when setting with a dictionary value (:issue:`38335`)
- Bug in :meth:`DataFrame.loc` dropping levels of :class:`MultiIndex` when :class:`DataFrame` used as input has only one row (:issue:`10521`)
-
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15 changes: 8 additions & 7 deletions pandas/core/frame.py
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Expand Up @@ -3334,13 +3334,14 @@ def _ensure_valid_index(self, value):
"""
# GH5632, make sure that we are a Series convertible
if not len(self.index) and is_list_like(value) and len(value):
try:
value = Series(value)
except (ValueError, NotImplementedError, TypeError) as err:
raise ValueError(
"Cannot set a frame with no defined index "
"and a value that cannot be converted to a Series"
) from err
if not isinstance(value, DataFrame):
try:
value = Series(value)
except (ValueError, NotImplementedError, TypeError) as err:
raise ValueError(
"Cannot set a frame with no defined index "
"and a value that cannot be converted to a Series"
) from err

# GH31368 preserve name of index
index_copy = value.index.copy()
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10 changes: 10 additions & 0 deletions pandas/tests/frame/indexing/test_setitem.py
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Expand Up @@ -338,6 +338,16 @@ def test_setitem_bool_with_numeric_index(self, dtype):

tm.assert_index_equal(df.columns, expected_cols)

@pytest.mark.parametrize("indexer", ["B", ["B"]])
def test_setitem_frame_length_0_str_key(self, indexer):
# GH#38831
df = DataFrame(columns=["A", "B"])
other = DataFrame({"B": [1, 2]})
df[indexer] = other
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do we need/want to check any of the other indexers?

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Don't know if this would be helpful, the string-DataFrame case is a special case while indexer in [list, Index, Series, array] is all the same (which was working previously)

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hmm, can you check df.loc[:, indexer] ? should be the same result i think.

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@phofl phofl Jan 5, 2021 •
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No unfortunately not, in case of indexer=["B"] we align lhs and rhs, which leads to an empty result. This is losely related to our dicussion in #30439 @jbrockmendel

When indexer="B" this works as the case above

expected = DataFrame({"A": [np.nan] * 2, "B": [1, 2]})
expected["A"] = expected["A"].astype("object")
tm.assert_frame_equal(df, expected)


class TestDataFrameSetItemWithExpansion:
def test_setitem_listlike_views(self):
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