BUG: interpolate with fillna methods fail to fill across multiblocks

doc/source/whatsnew/v2.1.0.rst

@@ -434,6 +434,7 @@ Indexing
 
 Missing
 ^^^^^^^
+- Bug in :meth:`DataFrame.interpolate` failing to fill across multiblock data when ``method`` is "pad", "ffill", "bfill", or "backfill" (:issue:`53898`)
 - Bug in :meth:`DataFrame.interpolate` ignoring ``inplace`` when :class:`DataFrame` is empty (:issue:`53199`)
 - Bug in :meth:`Series.interpolate` and :meth:`DataFrame.interpolate` failing to raise on invalid ``downcast`` keyword, which can be only ``None`` or "infer" (:issue:`53103`)
 - Bug in :meth:`Series.interpolate` and :meth:`DataFrame.interpolate` with complex dtype incorrectly failing to fill ``NaN`` entries (:issue:`53635`)

pandas/core/generic.py

@@ -7968,9 +7968,17 @@ def interpolate(
         axis = self._get_axis_number(axis)
 
         fillna_methods = ["ffill", "bfill", "pad", "backfill"]
-        should_transpose = axis == 1 and method not in fillna_methods
 
-        obj = self.T if should_transpose else self
+        if method not in fillna_methods:
+            should_transpose = axis == 1
+        elif not self._mgr.is_single_block and axis == 1:
+            if inplace:
+                raise NotImplementedError()
+            should_transpose = True
+        else:
+            should_transpose = False
+
+        obj, axis = (self.T, 1 - axis) if should_transpose else (self, axis)
 
         if obj.empty:
             if inplace:
@@ -8024,7 +8032,6 @@ def interpolate(
             # TODO(3.0): remove this case
             # TODO: warn/raise on limit_direction or kwargs which are ignored?
             #  as of 2023-06-26 no tests get here with either
-
             new_data = obj._mgr.pad_or_backfill(
                 method=method,
                 axis=axis,

pandas/tests/frame/methods/test_interpolate.py

@@ -451,8 +451,11 @@ def test_interp_string_axis(self, axis_name, axis_number):
         expected = df.interpolate(method="linear", axis=axis_number)
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("multiblock", [True, False])
     @pytest.mark.parametrize("method", ["ffill", "bfill", "pad"])
-    def test_interp_fillna_methods(self, request, axis, method, using_array_manager):
+    def test_interp_fillna_methods(
+        self, request, axis, multiblock, method, using_array_manager
+    ):
         # GH 12918
         if using_array_manager and axis in (1, "columns"):
             # TODO(ArrayManager) support axis=1
@@ -465,6 +468,10 @@ def test_interp_fillna_methods(self, request, axis, method, using_array_manager)
                 "C": [3.0, 6.0, 9.0, np.nan, np.nan, 30.0],
             }
         )
+        if multiblock:
+            df["D"] = np.nan
+            df["E"] = 1.0
+
         method2 = method if method != "pad" else "ffill"
         expected = getattr(df, method2)(axis=axis)
         msg = f"DataFrame.interpolate with method={method} is deprecated"