code: def row_key_type(self):
      return self.row_type._select_fields(self.row_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 14 for calling the tmatrix obj's underlying member method code with example usage: obj.row_key_type() and returns: self