Pycharm Plugin add DocString to Python Function

Hello,

i want to add a DocString to a python function via plugin. If the function has a DocString I ca do it by

"\"\"\"" + "test123"+ "\"\"\"");

pyFunction.getDocStringExpression();
docStringExpression.replace(docstring);

but I don't know how to add the DocString if

docStringExpression == null

what would be the best way to do it?