def get_function_source(self, function_name: str) -> str:
        """
        Returns the Python callable definition as a string for an EXTERNAL Python function that
        is stored within Unity Catalog. This function can only parse and extract the full callable
        definition for Python functions and cannot be used on SQL or TABLE functions.

        Args:
            function_name: The name of the function to retrieve the Python callable definition for.

        Returns:
            str: The Python callable definition as a string.
        """

        function_info = self.get_function(function_name)
        if function_info.routine_body.value != "EXTERNAL":
            raise ValueError(
                f"Function {function_name} is not an EXTERNAL Python function and cannot be retrieved."
            )
        return dynamically_construct_python_function(function_info)

        Returns the Python callable definition as a string for an EXTERNAL Python function that
        is stored within Unity Catalog. This function can only parse and extract the full callable
        definition for Python functions and cannot be used on SQL or TABLE functions.

        Args:
            function_name: The name of the function to retrieve the Python callable definition for.

        Returns:
            str: The Python callable definition as a string.
        
ai/core/src/unitycatalog/ai/core/databricks.py