pyarrow.compute.register_scalar_function(func, function_name, function_doc, in_types, out_type)

Register a user-defined scalar function.

A scalar function is a function that executes elementwise operations on arrays or scalars, i.e. a scalar function must be computed row-by-row with no state where each output row is computed only from its corresponding input row. In other words, all argument arrays have the same length, and the output array is of the same length as the arguments. Scalar functions are the only functions allowed in query engine expressions.


A callable implementing the user-defined function. The first argument is the context argument of type ScalarUdfContext. Then, it must take arguments equal to the number of in_types defined. It must return an Array or Scalar matching the out_type. It must return a Scalar if all arguments are scalar, else it must return an Array.

To define a varargs function, pass a callable that takes varargs. The last in_type will be the type of all varargs arguments.


Name of the function. This name must be globally unique.


A dictionary object with keys “summary” (str), and “description” (str).

in_typesDict[str, DataType]

A dictionary mapping function argument names to their respective DataType. The argument names will be used to generate documentation for the function. The number of arguments specified here determines the function arity.


Output type of the function.


>>> import pyarrow as pa
>>> import pyarrow.compute as pc
>>> func_doc = {}
>>> func_doc["summary"] = "simple udf"
>>> func_doc["description"] = "add a constant to a scalar"
>>> def add_constant(ctx, array):
...     return pc.add(array, 1, memory_pool=ctx.memory_pool)
>>> func_name = "py_add_func"
>>> in_types = {"array": pa.int64()}
>>> out_type = pa.int64()
>>> pc.register_scalar_function(add_constant, func_name, func_doc,
...                   in_types, out_type)
>>> func = pc.get_function(func_name)
>>> answer = pc.call_function(func_name, [pa.array([20])])
>>> answer
<pyarrow.lib.Int64Array object at ...>