def __setitem__(self, key, value):
        """
        Associate 'value' with 'key'. If 'key' already exists, but
        in different case, it will be replaced.
        """
        k = self._lowerOrReturn(key)
        self.data[k] = (key, value)

        Associate 'value' with 'key'. If 'key' already exists, but
        in different case, it will be replaced.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/util.py