On Sunday, July 31, 2016 at 12:10:33 PM UTC+5:30, Steven D'Aprano wrote: > On Sun, 31 Jul 2016 02:01 pm, D'Arcy J.M. Cain wrote: > > > On Sun, 31 Jul 2016 13:32:16 +1000 > > Steven D'Aprano wrote: > >> Many beginners make the mistake of writing: > >> > >> mylist = mylist.sort() > >> > >> or try to write: > >> > >> mylist.sort().reverse() > >> > >> If we had procedures, that would be an obvious error (possibly even a > >> compile-time syntax error) instead of a perplexing source of mystery > >> bugs. > > > > While neither is a syntax error, the latter is definitely a run-time > > error: > > > >>>> mylist.sort().reverse() > > Traceback (most recent call last): > > File "<stdin>", line 1, in <module> > > AttributeError: 'NoneType' object has no attribute 'reverse' > > Hence my description of it as a perplexing source of mystery bugs. How did > mylist suddenly turn into None? > > Well, it didn't, but it sure looks like it did, for those who didn't realise > that sort() is intended to be used *as if* it were a procedure with no > return value. If it *actually* had no return value, Python could give a > more useful error message: > > ProcedureError: list.sort() has no return value > > as soon as the interpreter tries to push the non-existent return value onto > the stack. Python can't do that, because None is an ordinary object line > any other. There's no practical way for Python to distinguish (whether at > compile-time or run-time) None as an ordinary object from None as a > stand-in for "no object at all". > > In practice, for moderately experienced programmers, the difference is minor > and most people quickly learn how to debug "None object has no attribute" > exceptions. But for a teaching language where we want to be stricter about > the dichotomy of: > > - subroutines that transform values (functions); and > - subroutines with side-effects (procedures) > > its a little sub-optimal.
That’s a nice summary of my position — tnx. Not that python as a production language with a fundamentally dynamic DNA needs to change. Rather that beginners not having procedure←→function as abstractions of effect←→value are subjected to a more than necessarily arduous learning curve -- https://mail.python.org/mailman/listinfo/python-list
