code: def _col_key_str(self):
      return ", ".join([escape_parsable(k) for k in self.col_key])
  
imports: import pprint
  from hail.typecheck import typecheck_method, sequenceof
  from hail.utils.java import escape_parsable
  from hail.expr.types import dtype, tstruct
  from hail.utils.java import jiterable_to_list
label_desc: Use image node_id 9 for calling the tmatrix obj's underlying member method code with example usage: obj._col_key_str() and returns: str