def copy_doc(source):
    """Copy the docstring from another function (decorator).

    The docstring of the source function is prepepended to the docstring of the
    function wrapped by this decorator.

    This is useful when inheriting from a class and overloading a method. This
    decorator can be used to copy the docstring of the original method.

    Parameters
    ----------
    source : function
        Function to copy the docstring from

    Returns
    -------
    wrapper : function
        The decorated function

    Examples
    --------
    >>> class A:
    ...     def m1():
    ...         '''Docstring for m1'''
    ...         pass
    >>> class B (A):
    ...     @copy_doc(A.m1)
    ...     def m1():
    ...         ''' this gets appended'''
    ...         pass
    >>> print(B.m1.__doc__)
    Docstring for m1 this gets appended
    """
    def wrapper(func):
        if source.__doc__ is None or len(source.__doc__) == 0:
            raise ValueError('Cannot copy docstring: docstring was empty.')
        doc = source.__doc__
        if func.__doc__ is not None:
            doc += func.__doc__
        func.__doc__ = doc
        return func
    return wrapper