[GitHub] [arrow] wjones127 commented on a diff in pull request #13454: ARROW-13612: [Python] Allow specifying a custom type for converting ExtensionScalar to python object

[GitBox <gi...@apache.org>]

wjones127 commented on code in PR #13454:
URL: https://github.com/apache/arrow/pull/13454#discussion_r914942510


##########
docs/source/python/extending_types.rst:
##########
@@ -286,6 +286,33 @@ are available).
 The same ``__arrow_ext_class__`` specialization can be used with custom types defined
 by subclassing :class:`ExtensionType`.
 
+Custom scalar conversion
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you want scalars of your custom extension type to convert to a custom type when
+:meth:`ExtensionScalar.as_py()` is called, you can override the :meth:`ExtensionType.scalar_as_py()`
+method. For example, if we wanted the above example 3D point type to return a custom
+3D point class instead of a list, we would implement::
+
+    Point3D = namedtuple("Point3D", ["x", "y", "z"])
+
+    class Point3DType(pa.PyExtensionType):
+        def __init__(self):
+            pa.PyExtensionType.__init__(self, pa.list_(pa.float32(), 3))
+
+        def __reduce__(self):
+            return Point3DType, ()
+
+        def scalar_as_py(self, scalar: ListScalar) -> Point3D:
+            return Point3D(*scalar.as_py())
+
+Arrays built using this extension type now provide scalars that convert to our ``Point3D`` class::
+
+    >>> storage = pa.array([[1, 2, 3], [4, 5, 6]], pa.list_(pa.float32(), 3))
+    >>> arr = pa.ExtensionArray.from_storage(Point3DType(), storage)
+    >>> arr[0].as_py()
+    Point3D(x=1.0, y=2.0, z=3.0)
+

Review Comment:
   Maybe it might be compelling to also show that `as_py_list()` returns `List[Point3D]` too.



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