How to Mock a User-Defined Function (UDF) in Python

from typing import Any, Callable


# Mock a user-defined function (UDF) that was previously complex or external
def mock_udf(name: str, implementation: Callable[..., Any], *, calls: list[Any]) -> Callable[..., Any]:
    """Wrap a real implementation with call logging to simulate a UDF."""
    def wrapper(*args: Any, **kwargs: Any) -> Any:
        calls.append((args, kwargs))
        return implementation(*args, **kwargs)
    return wrapper


# Real implementation of a simple UDF (e.g., a data transformation)
def real_udf(value: int) -> int:
    return value * 2 + 1


if __name__ == "__main__":
    # Track how many times the UDF is called
    call_log: list[Any] = []

    # Create the mocked version
    mocked = mock_udf("transform_element", real_udf, calls=call_log)

    # Simulate a data pipeline calling the UDF
    values = [1, 2, 3, 4]
    results = [mocked(v) for v in values]

    print("Results:", results)
    print("Number of calls:", len(call_log))
    print("Call log:", call_log)