On Sun, Jul 31, 2016 at 4:40 PM, Steven D'Aprano <st...@pearwood.info> wrote: >> 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
Fortunately, a linter can pick this up (maybe even right in your editor, before you move off the line of code). Particularly if type hints are available, a linter can say "Hey, this function always returns None, and you're assigning it to something - that looks wrong". ChrisA -- https://mail.python.org/mailman/listinfo/python-list
