fix: use fastpath for PyCapsule export when starting from pyarrow-backed Series, respect requested_schema

Author: MarcoGorelli

pandas/core/series.py

@@ -77,6 +77,7 @@
     validate_all_hashable,
 )
 from pandas.core.dtypes.dtypes import (
+    ArrowDtype,
     CategoricalDtype,
     ExtensionDtype,
     SparseDtype,
@@ -580,8 +581,19 @@ def __arrow_c_stream__(self, requested_schema=None):
         PyCapsule
         """
         pa = import_optional_dependency("pyarrow", min_version="16.0.0")
-        ca = pa.chunked_array([pa.Array.from_pandas(self, type=requested_schema)])
-        return ca.__arrow_c_stream__(requested_schema)
+        type = (
+            pa.DataType._import_from_c_capsule(requested_schema)
+            if requested_schema is not None
+            else None
+        )
+        if isinstance(self.dtype, ArrowDtype):
+            # fastpath!
+            ca = self.values._pa_array
+            if type is not None:
+                ca = ca.cast(type)
+        else:
+            ca = pa.chunked_array([pa.Array.from_pandas(self, type=type)])
+        return ca.__arrow_c_stream__()
 
     # ----------------------------------------------------------------------
 

pandas/tests/series/test_arrow_interface.py

@@ -21,3 +21,25 @@ def test_series_arrow_interface():
     ca = pa.chunked_array(s)
     expected = pa.chunked_array([[1, 4, 2]])
     assert ca.equals(expected)
+    ca = pa.chunked_array(s, type=pa.int32())
+    expected = pa.chunked_array([[1, 4, 2]], type=pa.int32())
+    assert ca.equals(expected)
+
+
+def test_series_arrow_interface_arrow_dtypes():
+    s = pd.Series([1, 4, 2], dtype="Int64[pyarrow]")
+
+    capsule = s.__arrow_c_stream__()
+    assert (
+        ctypes.pythonapi.PyCapsule_IsValid(
+            ctypes.py_object(capsule), b"arrow_array_stream"
+        )
+        == 1
+    )
+
+    ca = pa.chunked_array(s)
+    expected = pa.chunked_array([[1, 4, 2]])
+    assert ca.equals(expected)
+    ca = pa.chunked_array(s, type=pa.int32())
+    expected = pa.chunked_array([[1, 4, 2]], type=pa.int32())
+    assert ca.equals(expected)