See, I’d be extremely reluctant to assume the opposite - that he can, even though he doesn’t remember “len”. Given this one bit of information, all I know is that he has exactly one thing in common with everybody who doesn’t know how to program in Python: he doesn’t know the function for determining the length of a string. Now, he may (somehow?) know everything else about Python except for that one thing: I’m assuming the interviewer was a bit surprised (as I would be) that somebody presenting themselves as a Python programmer didn’t know len, but went ahead and asked him a few more questions which he may well have nailed. If the answer to every question was “I don’t know, I have to look it up”, you’d pass on him, too. I just can’t picture how anybody who didn’t remember that could remember much else, but I guess that’s why job interviews last an hour or so.