code: def check_mutate_fields( op, op_data ):
      
  
      mutate_fields = op_get_mutate_fields( op )
      assert mutate_fields is not None, "No mutate fields defined for %s" % op
  
      missing = []
      for field in mutate_fields:
          if not op_data.has_key(field):
              missing.append(field)
  
      assert len(missing) == 0, "Missing mutation fields for %s: %s" % (op, ",".join(missing))
      return True
docs: Verify that all mutate fields are present.
      Return True if so.
      Raise an exception (AssertionError) if not.