def group_keys(self):
        """Return an iterator over member names for groups only.

        Examples
        --------
        >>> import zarr
        >>> g1 = zarr.group()
        >>> g2 = g1.create_group('foo')
        >>> g3 = g1.create_group('bar')
        >>> d1 = g1.create_dataset('baz', shape=100, chunks=10)
        >>> d2 = g1.create_dataset('quux', shape=200, chunks=20)
        >>> sorted(g1.group_keys())
        ['bar', 'foo']

        """
        for key in sorted(listdir(self._store, self._path)):
            path = self._key_prefix + key
            if contains_group(self._store, path):
                yield key
Return an iterator over member names for groups only.

        Examples
        --------
        >>> import zarr
        >>> g1 = zarr.group()
        >>> g2 = g1.create_group('foo')
        >>> g3 = g1.create_group('bar')
        >>> d1 = g1.create_dataset('baz', shape=100, chunks=10)
        >>> d2 = g1.create_dataset('quux', shape=200, chunks=20)
        >>> sorted(g1.group_keys())
        ['bar', 'foo']