code: PyStringExpr getOutputAsString() {
      Preconditions.checkState(getOutputVarName() != null);
  
      initOutputVarIfNecessary();
      return new PyListExpr(getOutputVarName(), Integer.MAX_VALUE).toPyString();
    }
docs: Provide the output object as a string. Since we store all data in the output variables as a
  list for concatenation performance, this step does the joining to convert the output into a
  String.
  
  @return A PyExpr object of the output joined into a String.