def get_body(self, key):
return self._bodies.get(key, key)<EOL>
Get a body by key.

        Parameters
        ----------
        key : str, None, or :class:`Body`
            The key for looking up a body. If this is None or a :class:`Body`
            instance, the key itself will be returned.

        Returns
        -------
        body : :class:`Body`
            The body in the world with the given key.