get function from functions_mapping,
        if not found, then try to check if builtin function.

    Args:
        variable_name (str): variable name
        variables_mapping (dict): variables mapping

    Returns:
        mapping function object.

    Raises:
        exceptions.FunctionNotFound: function is neither defined in debugtalk.py nor builtin.

def get_mapping_function(function_name, functions_mapping):
    """ get function from functions_mapping,
        if not found, then try to check if builtin function.

    Args:
        variable_name (str): variable name
        variables_mapping (dict): variables mapping

    Returns:
        mapping function object.

    Raises:
        exceptions.FunctionNotFound: function is neither defined in debugtalk.py nor builtin.

    """
    if function_name in functions_mapping:
        return functions_mapping[function_name]

    elif function_name in ["parameterize", "P"]:
        from httprunner import loader
        return loader.load_csv_file

    elif function_name in ["environ", "ENV"]:
        return utils.get_os_environ

    try:
        # check if HttpRunner builtin functions
        from httprunner import loader
        built_in_functions = loader.load_builtin_functions()
        return built_in_functions[function_name]
    except KeyError:
        pass

    try:
        # check if Python builtin functions
        item_func = eval(function_name)
        if callable(item_func):
            # is builtin function
            return item_func
    except (NameError, TypeError):
        # is not builtin function
        raise exceptions.FunctionNotFound("{} is not found.".format(function_name))