A property returning the url of the recipe with ``{version}``
        replaced by the :attr:`url`. If accessing the url, you should use this
        property, *not* access the url directly.

def versioned_url(self):
        '''A property returning the url of the recipe with ``{version}``
        replaced by the :attr:`url`. If accessing the url, you should use this
        property, *not* access the url directly.'''
        if self.url is None:
            return None
        return self.url.format(version=self.version)