ENH: Support arrow/parquet roundtrip for nullable integer / string extension dtypes

doc/source/development/extending.rst

@@ -251,6 +251,48 @@ To use a test, subclass it:
 See https://github.com/pandas-dev/pandas/blob/master/pandas/tests/extension/base/__init__.py
 for a list of all the tests available.
 
+.. _extending.extension.arrow:
+
+Compatibility with Apache Arrow
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+An ``ExtensionArray`` can support conversion to / from ``pyarrow`` arrays
+(and thus support for example serialization to the Parquet file format)
+by implementing two methods: ``ExtensionArray.__arrow_array__`` and
+``ExtensionDtype.__from_arrow__``.
+
+The ``ExtensionArray.__arrow_array__`` ensures that ``pyarrow`` knowns how
+to convert the specific extension array into a ``pyarrow.Array`` (also when
+included as a column in a pandas DataFrame):
+
+.. code-block:: python
+
+    class MyExtensionArray(ExtensionArray):
+        ...
+
+        def __arrow_array__(self, type=None):
+            # convert the underlying array values to a pyarrow Array
+            import pyarrow
+            return pyarrow.array(..., type=type)
+
+The ``ExtensionDtype.__from_arrow__`` method then controls the conversion
+back from pyarrow to a pandas ExtensionArray. This method receives a pyarrow
+``Array`` or ``ChunkedArray`` as only argument and is expected to return the
+appropriate pandas ``ExtensionArray`` for this dtype and the passed values:
+
+.. code-block:: none
+
+    class ExtensionDtype:
+        ...
+
+        def __from_arrow__(self, array: pyarrow.Array/ChunkedArray) -> ExtensionArray:
+            ...
+
+See more in the `Arrow documentation <https://arrow.apache.org/docs/python/extending_types.html>`__.
+
+Those methods have been implemented for the nullable integer and string extension
+dtypes included in pandas, and ensure roundtrip to pyarrow and the Parquet file format.
+
 .. _extension dtype dtypes: https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/dtypes.py
 .. _extension dtype source: https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/base.py
 .. _extension array source: https://github.com/pandas-dev/pandas/blob/master/pandas/core/arrays/base.py

doc/source/user_guide/io.rst

@@ -4716,6 +4716,9 @@ Several caveats.
 * The ``pyarrow`` engine preserves the ``ordered`` flag of categorical dtypes with string types. ``fastparquet`` does not preserve the ``ordered`` flag.
 * Non supported types include ``Period`` and actual Python object types. These will raise a helpful error message
   on an attempt at serialization.
+* The ``pyarrow`` engine preserves extension data types such as the nullable integer and string data
+  type (requiring pyarrow >= 1.0.0, and requiring the extension type to implement the needed protocols,
+  see the :ref:`extension types documentation <extending.extension.arrow>`).
 
 You can specify an ``engine`` to direct the serialization. This can be one of ``pyarrow``, or ``fastparquet``, or ``auto``.
 If the engine is NOT specified, then the ``pd.options.io.parquet.engine`` option is checked; if this is also ``auto``,

doc/source/whatsnew/v1.0.0.rst

@@ -114,6 +114,9 @@ Other enhancements
 - Added ``encoding`` argument to :meth:`DataFrame.to_string` for non-ascii text (:issue:`28766`)
 - Added ``encoding`` argument to :func:`DataFrame.to_html` for non-ascii text (:issue:`28663`)
 - :meth:`Styler.background_gradient` now accepts ``vmin`` and ``vmax`` arguments (:issue:`12145`)
+- Roundtripping DataFrames with nullable integer or string data types to parquet
+  (:meth:`~DataFrame.to_parquet` / :func:`read_parquet`) using the `'pyarrow'` engine
+  now preserve those data types with pyarrow >= 1.0.0 (:issue:`20612`).
 
 Build Changes
 ^^^^^^^^^^^^^

pandas/core/arrays/integer.py

@@ -85,6 +85,35 @@ def construct_array_type(cls):
         """
         return IntegerArray
 
+    def __from_arrow__(self, array):
+        """Construct IntegerArray from passed pyarrow Array/ChunkedArray"""
+        import pyarrow
+
+        if isinstance(array, pyarrow.Array):
+            chunks = [array]
+        else:
+            # pyarrow.ChunkedArray
+            chunks = array.chunks
+
+        results = []
+        for arr in chunks:
+            buflist = arr.buffers()
+            data = np.frombuffer(buflist[1], dtype=self.type)[
+                arr.offset : arr.offset + len(arr)
+            ]
+            bitmask = buflist[0]
+            if bitmask is not None:
+                mask = pyarrow.BooleanArray.from_buffers(
+                    pyarrow.bool_(), len(arr), [None, bitmask]
+                )
+                mask = np.asarray(mask)
+            else:
+                mask = np.ones(len(arr), dtype=bool)
+            int_arr = IntegerArray(data.copy(), ~mask, copy=False)
+            results.append(int_arr)
+
+        return IntegerArray._concat_same_type(results)
+
 
 def integer_array(values, dtype=None, copy=False):
     """

pandas/core/arrays/string_.py

@@ -85,6 +85,24 @@ def construct_array_type(cls) -> "Type[StringArray]":
     def __repr__(self) -> str:
         return "StringDtype"
 
+    def __from_arrow__(self, array):
+        """Construct StringArray from passed pyarrow Array/ChunkedArray"""
+        import pyarrow
+
+        if isinstance(array, pyarrow.Array):
+            chunks = [array]
+        else:
+            # pyarrow.ChunkedArray
+            chunks = array.chunks
+
+        results = []
+        for arr in chunks:
+            # using _from_sequence to ensure None is convered to np.nan
+            str_arr = StringArray._from_sequence(np.array(arr))
+            results.append(str_arr)
+
+        return StringArray._concat_same_type(results)
+
 
 class StringArray(PandasArray):
     """

pandas/tests/arrays/string_/test_string.py

@@ -171,3 +171,19 @@ def test_arrow_array():
     arr = pa.array(data)
     expected = pa.array(list(data), type=pa.string(), from_pandas=True)
     assert arr.equals(expected)
+
+
+@td.skip_if_no("pyarrow", min_version="0.15.1.dev")
+def test_arrow_roundtrip():
+    # roundtrip possible from arrow 1.0.0
+    import pyarrow as pa
+
+    data = pd.array(["a", "b", None], dtype="string")
+    df = pd.DataFrame({"a": data})
+    table = pa.table(df)
+    assert table.field("a").type == "string"
+    result = table.to_pandas()
+    assert isinstance(result["a"].dtype, pd.StringDtype)
+    tm.assert_frame_equal(result, df)
+    # ensure the missing value is represented by NaN and not None
+    assert np.isnan(result.loc[2, "a"])

pandas/tests/arrays/test_integer.py

@@ -829,6 +829,18 @@ def test_arrow_array(data):
     assert arr.equals(expected)
 
 
+@td.skip_if_no("pyarrow", min_version="0.15.1.dev")
+def test_arrow_roundtrip(data):
+    # roundtrip possible from arrow 1.0.0
+    import pyarrow as pa
+
+    df = pd.DataFrame({"a": data})
+    table = pa.table(df)
+    assert table.field("a").type == str(data.dtype.numpy_dtype)
+    result = table.to_pandas()
+    tm.assert_frame_equal(result, df)
+
+
 @pytest.mark.parametrize(
     "pandasmethname, kwargs",
     [

pandas/tests/io/test_parquet.py

@@ -514,13 +514,19 @@ def test_additional_extension_arrays(self, pa):
                 "b": pd.Series(["a", None, "c"], dtype="string"),
             }
         )
-        # currently de-serialized as plain int / object
-        expected = df.assign(a=df.a.astype("int64"), b=df.b.astype("object"))
+        if LooseVersion(pyarrow.__version__) >= LooseVersion("0.15.1.dev"):
+            expected = df
+        else:
+            # de-serialized as plain int / object
+            expected = df.assign(a=df.a.astype("int64"), b=df.b.astype("object"))
         check_round_trip(df, pa, expected=expected)
 
         df = pd.DataFrame({"a": pd.Series([1, 2, 3, None], dtype="Int64")})
-        # if missing values in integer, currently de-serialized as float
-        expected = df.assign(a=df.a.astype("float64"))
+        if LooseVersion(pyarrow.__version__) >= LooseVersion("0.15.1.dev"):
+            expected = df
+        else:
+            # if missing values in integer, currently de-serialized as float
+            expected = df.assign(a=df.a.astype("float64"))
         check_round_trip(df, pa, expected=expected)