all tests working

Author: CloseChoice

pandas/core/generic.py

@@ -6881,30 +6881,34 @@ def interpolate(
         inplace = validate_bool_kwarg(inplace, "inplace")
 
         axis = self._get_axis_number(axis)
+        index = self._get_axis(axis)
 
-        if axis == 0:
-            df = self
-        else:
-            df = self.T
-
-        if isinstance(df.index, MultiIndex) and method != "linear":
+        if isinstance(self.index, MultiIndex) and method != "linear":
             raise ValueError(
                 "Only `method=linear` interpolation is supported on MultiIndexes."
             )
 
-        if df.ndim == 2 and np.all(df.dtypes == np.dtype(object)):
+        if self.ndim == 2 and np.all(self.dtypes == np.dtype(object)):
             raise TypeError(
                 "Cannot interpolate with all object-dtype columns "
                 "in the DataFrame. Try setting at least one "
                 "column to a numeric dtype."
             )
 
+
+        if method in ['backfill', 'bfill', 'pad', 'ffill']:
+            return self.fillna(method=method, axis=axis, inplace=inplace, limit=limit, downcast=downcast)
         # create/use the index
+
+        if axis==0:
+            df = self
+        else:
+            df = self.T
+
         if method == "linear":
             # prior default
             index = np.arange(len(df.index))
         else:
-            index = df.index
             methods = {"index", "values", "nearest", "time"}
             is_numeric_or_datetime = (
                 is_numeric_dtype(index)

pandas/tests/frame/methods/test_interpolate.py

@@ -202,7 +202,8 @@ def test_interp_leading_nans(self, check_scipy):
             result = df.interpolate(method="polynomial", order=1)
             tm.assert_frame_equal(result, expected)
 
-    def test_interp_raise_on_only_mixed(self):
+    @pytest.mark.parametrize('axis', [0, 1])
+    def test_interp_raise_on_only_mixed(self, axis):
         df = DataFrame(
             {
                 "A": [1, 2, np.nan, 4],
@@ -213,7 +214,7 @@ def test_interp_raise_on_only_mixed(self):
             }
         )
         with pytest.raises(TypeError):
-            df.interpolate(axis=1)
+            df.astype('object').interpolate(axis=axis)
 
     def test_interp_raise_on_all_object_dtype(self):
         # GH 22985
@@ -284,3 +285,4 @@ def test_interp_time_inplace_axis(self, axis):
         result = expected.interpolate(axis=0, method="time")
         expected.interpolate(axis=0, method="time", inplace=True)
         tm.assert_frame_equal(result, expected)
+