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

doc/source/whatsnew/v1.4.0.rst

@@ -177,6 +177,7 @@ Other enhancements
 - :meth:`DataFrame.__pos__`, :meth:`DataFrame.__neg__` now retain ``ExtensionDtype`` dtypes (:issue:`43883`)
 - The error raised when an optional dependency can't be imported now includes the original exception, for easier investigation (:issue:`43882`)
 - Added :meth:`.ExponentialMovingWindow.sum` (:issue:`13297`)
+- :meth:`DataFrame.dropna` now accepts a single label as ``subset`` along with array-like (:issue:`41021`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/frame.py

@@ -5928,7 +5928,7 @@ def dropna(
         axis: Axis = 0,
         how: str = "any",
         thresh=None,
-        subset=None,
+        subset: IndexLabel = None,
         inplace: bool = False,
     ):
         """
@@ -5960,7 +5960,7 @@ 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
@@ -6044,11 +6044,14 @@ def dropna(
 
         agg_obj = self
         if subset is not None:
+            # subset needs to be list
+            if not is_list_like(subset):
+                subset = [subset]
             ax = self._get_axis(agg_axis)
             indices = ax.get_indexer_for(subset)
             check = indices == -1
             if check.any():
-                raise KeyError(list(np.compress(check, subset)))
+                raise KeyError(np.array(subset)[check].tolist())
             agg_obj = self.take(indices, axis=agg_axis)
 
         count = agg_obj.count(axis=agg_axis)

pandas/tests/frame/methods/test_dropna.py

@@ -243,3 +243,27 @@ 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)
+
+    def test_subset_is_nparray(self):
+        # GH 41021
+        df = DataFrame({"A": [1, 2, np.NaN], "B": list("abc"), "C": [4, np.NaN, 5]})
+        expected = DataFrame({"A": [1.0], "B": ["a"], "C": [4.0]})
+        result = df.dropna(subset=np.array(["A", "C"]))
+        tm.assert_frame_equal(result, expected)