update for changes in pandas-dev#39953

Author: simonjayhawkins

pandas/_libs_numba/algos.py

@@ -583,28 +583,29 @@ def _pad_inplace(
 @numba.jit
 def _pad_inplace_no_limit(values: np.ndarray, mask: np.ndarray) -> None:
     N = len(values)
-    val = values[0]
+    val, prev_mask = values[0], mask[0]
     for i in range(N):
         if mask[i]:
-            values[i] = val
+            values[i], mask[i] = val, prev_mask
         else:
-            val = values[i]
+            val, prev_mask = values[i], mask[i]
 
 
 @numba.jit
 def _pad_inplace_with_limit(values: np.ndarray, mask: np.ndarray, limit: int) -> None:
     N = len(values)
     fill_count = 0
-    val = values[0]
+    val, prev_mask = values[0], mask[0]
     for i in range(N):
         if mask[i]:
             if fill_count >= limit:
                 continue
             fill_count += 1
-            values[i] = val
+            values[i], mask[i] = val, prev_mask
+
         else:
             fill_count = 0
-            val = values[i]
+            val, prev_mask = values[i], mask[i]
 
 
 def pad_2d_inplace(

pandas/core/missing.py

@@ -676,13 +676,12 @@ def _pad_1d(values, limit=None, mask=None):
     if mask is None:
         mask = isna(values)
     algos_numba.pad_inplace(values, mask, limit=limit)
-    return values
+    return values, mask
 
 
 def _backfill_1d(values, limit=None, mask=None):
-    if mask is not None:
-        mask = mask[::-1]
-    return _pad_1d(values[::-1], limit, mask)[::-1]
+    _, new_mask = _pad_1d(values[::-1], limit, mask[::-1] if mask is not None else None)
+    return values, (mask if mask is not None else new_mask)
 
 
 def _pad_2d(values, limit=None, mask=None):
@@ -698,9 +697,10 @@ def _pad_2d(values, limit=None, mask=None):
 
 
 def _backfill_2d(values, limit=None, mask=None):
-    if mask is not None:
-        mask = mask[:, ::-1]
-    return _pad_2d(values[:, ::-1], limit, mask)[:, ::-1]
+    _, new_mask = _pad_2d(
+        values[:, ::-1], limit, mask[:, ::-1] if mask is not None else None
+    )
+    return values, (mask if mask is not None else new_mask)
 
 
 _fill_methods = {"pad": _pad_1d, "backfill": _backfill_1d}