ENH: DataFrame.to_parquet() returns bytes if path_or_buf not provided

doc/source/whatsnew/v1.2.0.rst

@@ -221,6 +221,7 @@ Other enhancements
 - :meth:`Rolling.var()` and :meth:`Rolling.std()` use Kahan summation and Welfords Method to avoid numerical issues (:issue:`37051`)
 - :meth:`DataFrame.plot` now recognizes ``xlabel`` and ``ylabel`` arguments for plots of type ``scatter`` and ``hexbin`` (:issue:`37001`)
 - :class:`DataFrame` now supports ``divmod`` operation (:issue:`37165`)
+- :meth:`DataFrame.to_parquet` now returns a ``bytes`` object when no ``path`` argument is passed (:issue:`37105`)
 
 .. _whatsnew_120.api_breaking.python:
 

pandas/core/frame.py

@@ -2289,14 +2289,14 @@ def to_markdown(
     @deprecate_kwarg(old_arg_name="fname", new_arg_name="path")
     def to_parquet(
         self,
-        path: FilePathOrBuffer[AnyStr],
+        path: Optional[FilePathOrBuffer] = None,
         engine: str = "auto",
         compression: Optional[str] = "snappy",
         index: Optional[bool] = None,
         partition_cols: Optional[List[str]] = None,
         storage_options: StorageOptions = None,
         **kwargs,
-    ) -> None:
+    ) -> Optional[bytes]:
         """
         Write a DataFrame to the binary parquet format.
 
@@ -2307,14 +2307,15 @@ def to_parquet(
 
         Parameters
         ----------
-        path : str or file-like object
+        path : str or file-like object, default None
             If a string, it will be used as Root Directory path
             when writing a partitioned dataset. By file-like object,
             we refer to objects with a write() method, such as a file handle
             (e.g. via builtin open function) or io.BytesIO. The engine
-            fastparquet does not accept file-like objects.
+            fastparquet does not accept file-like objects. If path is None,
+            a bytes object is returned.
 
-            .. versionchanged:: 1.0.0
+            .. versionchanged:: 1.2.0
 
             Previously this was "fname"
 
@@ -2357,6 +2358,10 @@ def to_parquet(
             Additional arguments passed to the parquet library. See
             :ref:`pandas io <io.parquet>` for more details.
 
+        Returns
+        -------
+        bytes if no path argument is provided else None
+
         See Also
         --------
         read_parquet : Read a parquet file.
@@ -2392,7 +2397,7 @@ def to_parquet(
         """
         from pandas.io.parquet import to_parquet
 
-        to_parquet(
+        return to_parquet(
             self,
             path,
             engine,

pandas/io/parquet.py

@@ -1,5 +1,6 @@
 """ parquet compat """
 
+import io
 from typing import Any, AnyStr, Dict, List, Optional
 from warnings import catch_warnings
 
@@ -238,28 +239,29 @@ def read(
 
 def to_parquet(
     df: DataFrame,
-    path: FilePathOrBuffer[AnyStr],
+    path: Optional[FilePathOrBuffer] = None,
     engine: str = "auto",
     compression: Optional[str] = "snappy",
     index: Optional[bool] = None,
     storage_options: StorageOptions = None,
     partition_cols: Optional[List[str]] = None,
     **kwargs,
-):
+) -> Optional[bytes]:
     """
     Write a DataFrame to the parquet format.
 
     Parameters
     ----------
     df : DataFrame
-    path : str or file-like object
+    path : str or file-like object, default None
         If a string, it will be used as Root Directory path
         when writing a partitioned dataset. By file-like object,
         we refer to objects with a write() method, such as a file handle
         (e.g. via builtin open function) or io.BytesIO. The engine
-        fastparquet does not accept file-like objects.
+        fastparquet does not accept file-like objects. If path is None,
+        a bytes object is returned.
 
-        .. versionchanged:: 0.24.0
+        .. versionchanged:: 1.2.0
 
     engine : {'auto', 'pyarrow', 'fastparquet'}, default 'auto'
         Parquet library to use. If 'auto', then the option
@@ -298,20 +300,33 @@ def to_parquet(
 
     kwargs
         Additional keyword arguments passed to the engine
+
+    Returns
+    -------
+    bytes if no path argument is provided else None
     """
     if isinstance(partition_cols, str):
         partition_cols = [partition_cols]
     impl = get_engine(engine)
-    return impl.write(
+
+    path_or_buf: FilePathOrBuffer = io.BytesIO() if path is None else path
+
+    impl.write(
         df,
-        path,
+        path_or_buf,
         compression=compression,
         index=index,
         partition_cols=partition_cols,
         storage_options=storage_options,
         **kwargs,
     )
 
+    if path is None:
+        assert isinstance(path_or_buf, io.BytesIO)
+        return path_or_buf.getvalue()
+    else:
+        return None
+
 
 def read_parquet(path, engine: str = "auto", columns=None, **kwargs):
     """

pandas/tests/io/test_parquet.py

@@ -512,6 +512,17 @@ def test_basic_subset_columns(self, pa, df_full):
             read_kwargs={"columns": ["string", "int"]},
         )
 
+    def test_to_bytes_without_path_or_buf_provided(self, pa, df_full):
+        # GH 37105
+
+        buf_bytes = df_full.to_parquet(engine=pa)
+        assert isinstance(buf_bytes, bytes)
+
+        buf_stream = BytesIO(buf_bytes)
+        res = pd.read_parquet(buf_stream)
+
+        tm.assert_frame_equal(df_full, res)
+
     def test_duplicate_columns(self, pa):
         # not currently able to handle duplicate columns
         df = pd.DataFrame(np.arange(12).reshape(4, 3), columns=list("aaa")).copy()