ENH: check for string and convert to list in DataFrame.dropna subset argument

doc/source/whatsnew/v1.2.5.rst

@@ -35,7 +35,7 @@ Bug fixes
 Other
 ~~~~~
 
--
+- :meth:`DataFrame.sum` now accepts a single column / string as ``subset`` instead of array-like
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/frame.py

@@ -27,7 +27,9 @@
     Hashable,
     Iterable,
     Iterator,
+    Optional,
     Sequence,
+    Union,
     cast,
     overload,
 )
@@ -5803,7 +5805,7 @@ def dropna(
         axis: Axis = 0,
         how: str = "any",
         thresh=None,
-        subset=None,
+        subset: Optional[Union[Hashable, Sequence[Hashable]]] = None,
         inplace: bool = False,
     ):
         """
@@ -5835,7 +5837,7 @@ def dropna(
 
         thresh : int, optional
             Require that many non-NA values.
-        subset : array-like, optional
+        subset : column label or sequence of labels, optional
             Labels along other axis to consider, e.g. if you are dropping rows
             these would be a list of columns to include.
         inplace : bool, default False
@@ -5919,6 +5921,14 @@ def dropna(
 
         agg_obj = self
         if subset is not None:
+            if (
+                not np.iterable(subset)
+                or isinstance(subset, str)
+                or isinstance(subset, tuple)
+                and subset in self.columns
+            ):
+                # subset needs to be list like
+                subset = (subset,)
             ax = self._get_axis(agg_axis)
             indices = ax.get_indexer_for(subset)
             check = indices == -1

pandas/tests/frame/methods/test_dropna.py

@@ -231,3 +231,23 @@ def test_dropna_with_duplicate_columns(self):
 
         result = df.dropna(subset=["A", "C"], how="all")
         tm.assert_frame_equal(result, expected)
+
+    def test_set_single_column_subset(self):
+        # GH 41021
+        df = DataFrame({"A": [1, 2, 3], "B": list("abc"), "C": [4, np.NaN, 5]})
+        expected = DataFrame(
+            {"A": [1, 3], "B": list("ac"), "C": [4.0, 5.0]}, index=[0, 2]
+        )
+        result = df.dropna(subset="C")
+        tm.assert_frame_equal(result, expected)
+
+    def test_single_column_not_present_in_axis(self):
+        # GH 41021
+        """
+        Test if KeyError is raised when subset is not present in axis
+        """
+        df = DataFrame({"A": [1, 2, 3], "B": list("abc"), "C": [4, np.NaN, 5]})
+
+        # Column not present
+        with pytest.raises(KeyError, match="['D']"):
+            df.dropna(subset="D", axis=0)