BUG: unexpected assign by a single-element list (GH19474) (#20732)

Author: kittoku

doc/source/whatsnew/v0.23.0.txt

@@ -1113,6 +1113,7 @@ Indexing
 - Bug in :meth:`DataFrame.first_valid_index` and :meth:`DataFrame.last_valid_index` in presence of entire rows of NaNs in the middle of values (:issue:`20499`).
 - Bug in :class:`IntervalIndex` where some indexing operations were not supported for overlapping or non-monotonic ``uint64`` data (:issue:`20636`)
 - Bug in ``Series.is_unique`` where extraneous output in stderr is shown if Series contains objects with ``__ne__`` defined (:issue:`20661`)
+- Bug in ``.loc`` assignment with a single-element list-like incorrectly assigns as a list (:issue:`19474`)
 - Bug in partial string indexing on a ``Series/DataFrame`` with a monotonic decreasing ``DatetimeIndex`` (:issue:`19362`)
 
 MultiIndex

pandas/core/indexing.py

@@ -532,7 +532,8 @@ def setter(item, v):
 
             def can_do_equal_len():
                 """ return True if we have an equal len settable """
-                if not len(labels) == 1 or not np.iterable(value):
+                if (not len(labels) == 1 or not np.iterable(value) or
+                        is_scalar(plane_indexer[0])):
                     return False
 
                 l = len(value)

pandas/tests/indexing/test_iloc.py

@@ -10,6 +10,7 @@
 from pandas import Series, DataFrame, date_range, concat, isna
 from pandas.util import testing as tm
 from pandas.tests.indexing.common import Base
+from pandas.api.types import is_scalar
 
 
 class TestiLoc(Base):
@@ -526,6 +527,21 @@ def test_iloc_setitem_list_of_lists(self):
                                   B=[5, 6, 11, 13, 9]))
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize(
+        'indexer', [[0], slice(None, 1, None), np.array([0])])
+    @pytest.mark.parametrize(
+        'value', [['Z'], np.array(['Z'])])
+    def test_iloc_setitem_with_scalar_index(self, indexer, value):
+        # GH #19474
+        # assigning like "df.iloc[0, [0]] = ['Z']" should be evaluated
+        # elementwisely, not using "setter('A', ['Z'])".
+
+        df = pd.DataFrame([[1, 2], [3, 4]], columns=['A', 'B'])
+        df.iloc[0, indexer] = value
+        result = df.iloc[0, 0]
+
+        assert is_scalar(result) and result == 'Z'
+
     def test_iloc_mask(self):
 
         # GH 3631, iloc with a mask (of a series) should raise

pandas/tests/indexing/test_loc.py

@@ -11,6 +11,7 @@
 from pandas import Series, DataFrame, Timestamp, date_range, MultiIndex, Index
 from pandas.util import testing as tm
 from pandas.tests.indexing.common import Base
+from pandas.api.types import is_scalar
 
 
 class TestLoc(Base):
@@ -555,6 +556,21 @@ def test_loc_setitem_frame_multiples(self):
         df.loc[2:4] = rhs
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize(
+        'indexer', [['A'], slice(None, 'A', None), np.array(['A'])])
+    @pytest.mark.parametrize(
+        'value', [['Z'], np.array(['Z'])])
+    def test_loc_setitem_with_scalar_index(self, indexer, value):
+        # GH #19474
+        # assigning like "df.loc[0, ['A']] = ['Z']" should be evaluated
+        # elementwisely, not using "setter('A', ['Z'])".
+
+        df = pd.DataFrame([[1, 2], [3, 4]], columns=['A', 'B'])
+        df.loc[0, indexer] = value
+        result = df.loc[0, 'A']
+
+        assert is_scalar(result) and result == 'Z'
+
     def test_loc_coerceion(self):
 
         # 12411