human
Solve the following python programming problem given the following code:
def _make_groups(self, trajectory_categories, sort_category) : 
    'Groups the sample ids in `self._metadata_map` by the values in
        `trajectory_categories`

        Creates `self._groups`, a dictionary keyed by category and values are
        dictionaries in which the keys represent the group name within the
        category and values are ordered lists of sample ids

        If `sort_category` is not None, the sample ids are sorted based on the
        values under this category in the metadata map. Otherwise, they are
        sorted using the sample id.

        Parameters
        ----------
        trajectory_categories : list of str
            A list of metadata categories to use to create the groups.
            Default: None, compute all of them
        sort_category : str or None
            The category from self._metadata_map to use to sort groups
        ' 
    if sort_category : 
        def sort_val(sid) : 
            return self._metadata_map [sort_category] [sid] 
else : 
        def sort_val(sid) : 
            return sid 
self._groups = defaultdict (dict) 
    for cat in trajectory_categories : 
        gb = self._metadata_map.groupby (sort_category) 
        for (g, df) in gb : 
            self._groups [cat] [g] = realsorted (df.index, key = sort_val) 

    
    

    Fix the buggy line: gb = self._metadata_map.groupby (sort_category)
assistant
#Here is Python code to fix the buggy line: gb = self.

gb = self._metadata_map.groupby (cat)