'Return a :term:`URL` to the cover image for this issue or `None`.
By default it returns the value of the member `self.cover_url` which
is normally `None`. If you want your recipe to download a cover for the e-book
override this method in your subclass, or set the member variable `self.cover_url`
before this method is called.'

def get_cover_url(self):

    return getattr(self, 'cover_url', None)
