    def __getitem__(self, index):
        """
        Retrieve the child tree one level down at the given ``index``.

        If the index is not in the instance that this tree corresponds
        to and is not known by this tree, whatever error would be raised
        by ``instance.__getitem__`` will be propagated (usually this is
        some subclass of `LookupError`.
        """
        if self._instance is not _unset and index not in self:
            self._instance[index]
        return self._contents[index]
Retrieve the child tree one level down at the given ``index``.

If the index is not in the instance that this tree corresponds
to and is not known by this tree, whatever error would be raised
by ``instance.__getitem__`` will be propagated (usually this is
some subclass of `LookupError`.
jsonschema/exceptions.py