[Data] Allow user to specify schema for write_parquet (ray-project#…

…48631)

Fixes ray-project#48630

Signed-off-by: Balaji Veeramani <bveeramani@berkeley.edu>

python/ray/data/_internal/datasource/parquet_datasink.py

@@ -72,13 +72,16 @@ def write(
         write_kwargs = _resolve_kwargs(
             self.arrow_parquet_args_fn, **self.arrow_parquet_args
         )
+        schema = write_kwargs.pop("schema", None)
+        if schema is None:
+            schema = BlockAccessor.for_block(blocks[0]).to_arrow().schema
 
         def write_blocks_to_path():
             with self.open_output_stream(write_path) as file:
-                schema = BlockAccessor.for_block(blocks[0]).to_arrow().schema
+                tables = [BlockAccessor.for_block(block).to_arrow() for block in blocks]
                 with pq.ParquetWriter(file, schema, **write_kwargs) as writer:
-                    for block in blocks:
-                        table = BlockAccessor.for_block(block).to_arrow()
+                    for table in tables:
+                        table = table.cast(schema)
                         writer.write_table(table)
 
         logger.debug(f"Writing {write_path} file.")

python/ray/data/tests/test_parquet.py

@@ -1316,6 +1316,15 @@ def test_count_with_filter(ray_start_regular_shared):
     assert isinstance(ds.count(), int)
 
 
+def test_write_with_schema(ray_start_regular_shared, tmp_path):
+    ds = ray.data.range(1)
+    schema = pa.schema({"id": pa.float32()})
+
+    ds.write_parquet(tmp_path, schema=schema)
+
+    assert pq.read_table(tmp_path).schema == schema
+
+
 if __name__ == "__main__":
     import sys