That is not a documentation bug (i.e. a problem with what is written at docs.python.org), this is a feature request to try to improve the exception message. That would require adding a new parameter format (http://docs.python.org/3/c-api/arg.html?highlight=pyarg_parse#other-objects) which allows for a converter function which also takes the positional number and/or keyword argument that the converter function was called for.