Victor is talking about inspect.signature(). In Python 3.5+ the result is "(fd, /)". In older versions it raises a ValueError. The docstring in 3.4 is 'dup(fd) -> fd2\n\nReturn a duplicate of a file descriptor.' It doesn't contain information that dup() returns int. It contains implicit information that dup() perhaps returns other file descriptor, but this is already explicitly documented by words. I don't see a loss of information here.