make interpolate pad method use ffill

bigframes/core/block_transforms.py

@@ -22,6 +22,7 @@
 import bigframes.core.blocks as blocks
 import bigframes.core.ordering as ordering
 import bigframes.core.window_spec as windows
+import bigframes.dtypes as dtypes
 import bigframes.operations as ops
 import bigframes.operations.aggregations as agg_ops
 
@@ -113,7 +114,6 @@ def interpolate(block: blocks.Block, method: str = "linear") -> blocks.Block:
         "nearest",
         "zero",
         "slinear",
-        "pad",
     ]
     if method not in supported_methods:
         raise NotImplementedError(
@@ -131,7 +131,9 @@ def interpolate(block: blocks.Block, method: str = "linear") -> blocks.Block:
         if len(index_columns) != 1:
             raise ValueError("only method 'linear' support multi-index")
         xvalues = block.index_columns[0]
-        # TODO: verify numeric
+        dtypes.NUMERIC_BIGFRAMES_TYPES
+        if block.index_dtypes[0] not in dtypes.NUMERIC_BIGFRAMES_TYPES:
+            raise ValueError("Can only interpolate on numeric index.")
 
     for column in original_columns:
         # null in same places column is null
@@ -145,21 +147,20 @@ def interpolate(block: blocks.Block, method: str = "linear") -> blocks.Block:
                 "values": "linear",
                 "index": "linear",
                 "slinear": "linear",
-                "pad": "ffill",
                 "zero": "ffill",
                 "nearest": "nearest",
             }
             extrapolating_methods = ["linear", "values", "index"]
             interpolate_method = interpolate_method_map[method]
             do_extrapolate = method in extrapolating_methods
-            block, interpolated_and_ffilled = _interpolate_column(
+            block, interpolated = _interpolate_column(
                 block,
                 column,
                 xvalues,
                 interpolate_method=interpolate_method,
                 do_extrapolate=do_extrapolate,
             )
-            output_column_ids.append(interpolated_and_ffilled)
+            output_column_ids.append(interpolated)
         else:
             output_column_ids.append(column)
 

bigframes/dataframe.py

@@ -1435,6 +1435,8 @@ def reindex_like(self, other: DataFrame, *, validate: typing.Optional[bool] = No
         return self.reindex(index=other.index, columns=other.columns, validate=validate)
 
     def interpolate(self, method: str = "linear") -> DataFrame:
+        if method == "pad":
+            return self.ffill()
         result = block_ops.interpolate(self._block, method)
         return DataFrame(result)
 

bigframes/series.py

@@ -469,6 +469,8 @@ def replace(
             return Series(block.select_column(result_col))
 
     def interpolate(self, method: str = "linear") -> Series:
+        if method == "pad":
+            return self.ffill()
         result = block_ops.interpolate(self._block, method)
         return Series(result)
 

tests/system/small/test_series.py

@@ -294,6 +294,11 @@ def test_series_interpolate(method):
     pd_result = pd_series.astype("float64").interpolate(method=method)
     bf_result = bf_series.interpolate(method=method).to_pandas()
 
+    print("pandas")
+    print(pd_result.to_string())
+    print("bigframes")
+    print(bf_result.to_string())
+
     # pd uses non-null types, while bf uses nullable types
     pd.testing.assert_series_equal(
         pd_result,