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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def copy_docstring(source_class):
 """Decorator that copies a method's docstring from another class. Args: source_class (type):
 The class that has the documented method. Returns: Callable: A decorator that will copy the docstring of the same named method in the source class to the decorated method. """

    def decorator(method):
        """Decorator implementation.

        Args:
            method (Callable): The method to copy the docstring to.

        Returns:
            Callable: the same method passed in with an updated docstring.

        Raises:
            ValueError: if the method already has a docstring.
        """
        if method.__doc__:
            raise ValueError('Method already has a docstring.')

        source_method = getattr(source_class, method.__name__)
        method.__doc__ = source_method.__doc__

        return method
    return decorator