vibhatha commented on code in PR #13687:
URL: https://github.com/apache/arrow/pull/13687#discussion_r979134031
##########
docs/source/python/compute.rst:
##########
@@ -370,3 +370,133 @@ our ``even_filter`` with a ``pc.field("nums") > 5``
filter:
:class:`.Dataset` currently can be filtered using :meth:`.Dataset.to_table`
method
passing a ``filter`` argument. See :ref:`py-filter-dataset` in Dataset
documentation.
+
+
+User-Defined Functions
+======================
+
+.. warning::
+ This API is **experimental**.
+
+PyArrow allows defining and registering custom compute functions.
+These functions can then be called from Python as well as C++ (and potentially
+any other implementation wrapping Arrow C++, such as the R ``arrow`` package)
+using their registered function name.
+
+To register a UDF, a function name, function docs, input types and
+output type need to be defined. Using
:func:`pyarrow.compute.register_scalar_function`,
+
+.. code-block:: python
+
+ import numpy as np
+
+ import pyarrow as pa
+ import pyarrow.compute as pc
+
+ function_name = "numpy_gcd"
+ function_docs = {
+ "summary": "Calculates the greatest common divisor",
+ "description":
+ "Given 'x' and 'y' find the greatest number that divides\n"
+ "evenly into both x and y."
+ }
+
+ input_types = {
+ "x" : pa.int64(),
+ "y" : pa.int64()
+ }
+
+ output_type = pa.int64()
+
+ def to_np(val):
+ if isinstance(val, pa.Scalar):
+ return val.as_py()
+ else:
+ return np.array(val)
+
+ def gcd_numpy(ctx, x, y):
+ np_x = to_np(x)
+ np_y = to_np(y)
+ return pa.array(np.gcd(np_x, np_y))
+
+ pc.register_scalar_function(gcd_numpy,
+ function_name,
+ function_docs,
+ input_types,
+ output_type)
+
+
+The implementation of a user-defined function always takes first *context*
+parameter (named ``ctx`` in the example above) which is an instance of
+:class:`pyarrow.compute.ScalarUdfContext`.
+This context exposes several useful attributes, particularly a
+:attr:`~pyarrow.compute.ScalarUdfContext.memory_pool` to be used for
+allocations in the context of the user-defined function.
+
+PyArrow UDFs accept input types of both :class:`~pyarrow.Scalar` and
:class:`~pyarrow.Array`,
+and there will always be at least one input of type :class:`~pyarrow.Array`.
+The output should always be a :class:`~pyarrow.Array`.
Review Comment:
Ah nice catch on the description. Removing it.
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