Create a FunctionDescriptor from a class.

        Args:
            cls: Current class which is required argument for classmethod.
            target_class: the python class used to create the function
                descriptor.

        Returns:
            The FunctionDescriptor instance created according to the class.

def from_class(cls, target_class):
        """Create a FunctionDescriptor from a class.

        Args:
            cls: Current class which is required argument for classmethod.
            target_class: the python class used to create the function
                descriptor.

        Returns:
            The FunctionDescriptor instance created according to the class.
        """
        module_name = target_class.__module__
        class_name = target_class.__name__
        return cls(module_name, "__init__", class_name)