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

doc/source/whatsnew/v1.3.4.rst

@@ -42,7 +42,7 @@ Bug fixes
 
 Other
 ~~~~~
--
+- :meth:`DataFrame.dropna` now accepts a single label as ``subset`` along with array-like. (:issue:`41021`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/frame.py

@@ -5874,7 +5874,7 @@ def dropna(
         axis: Axis = 0,
         how: str = "any",
         thresh=None,
-        subset=None,
+        subset: IndexLabel = None,
         inplace: bool = False,
     ):
         """
@@ -5906,7 +5906,7 @@ 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
@@ -5990,6 +5990,8 @@ def dropna(
 
         agg_obj = self
         if subset is not None:
+            # subset needs to be list
+            subset = com.maybe_make_list(subset)
             ax = self._get_axis(agg_axis)
             indices = ax.get_indexer_for(subset)
             check = indices == -1

pandas/tests/frame/methods/test_dropna.py

@@ -243,3 +243,20 @@ def test_dropna_pos_args_deprecation(self):
             result = df.dropna(1)
         expected = DataFrame({"a": [1, 2, 3]})
         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
+        df = DataFrame({"A": [1, 2, 3]})
+
+        # Column not present
+        with pytest.raises(KeyError, match="['D']"):
+            df.dropna(subset="D", axis=0)