On Thu, Aug 10, 2017 at 1:49 PM, Terry Reedy <tjre...@udel.edu> wrote: > On 8/10/2017 10:28 AM, Steve D'Aprano wrote: >> >> Every few years, the following syntax comes up for discussion, with some >> people >> saying it isn't obvious what it would do, and others disagreeing and >> saying >> that it is obvious. So I thought I'd do an informal survey. >> >> What would you expect this syntax to return? >> >> [x + 1 for x in (0, 1, 2, 999, 3, 4) while x < 5] > > > I expect it to continue to raise SyntaxError as I would be flabbergasted if > something so awful were to be accepted into Python. > > If it were were to mean what it 'obviously' should, by analogy with the > current meaning and translation > > val = [] > for x in (0, 1, 2, 999, 3, 4): > while x < 5: > val.append(x) > > then it would 'mean' an infinite list of, in this case, 1's, and raise > OutOfMemoryError. This is, of course, useless. Better to leave it as a > SyntaxError. > > If 'while cond' were to not mean 'conditionally loop', but, as some want, > 'conditionally break an existing loop', something like 'if not cond: break', > it would mean 'Python is really confusing and inconsistent'. It would be > terrible to have a keyword with two such related but opposite meanings. > > Better to allow people to say what they mean, something like 'if cond > break'. A 'semi-obvious' translations might then be > > val = [] > it = iter((0, 1, 2, 999, 3, 4)) > for x in it: > if x < 5: > val.append(x+1) > else: > break > > val = [1,2,3]
So, perhaps a better syntax could be: [x + 1 for x in (0, 1, 2, 999, 3, 4) if x < 5 else break] -- https://mail.python.org/mailman/listinfo/python-list
