On 23 April 2013 17:30, Ian Kelly <ian.g.ke...@gmail.com> wrote: > On Tue, Apr 23, 2013 at 10:21 AM, Chris Angelico <ros...@gmail.com> wrote: >> The definition of the for loop is sufficiently simple that this is >> safe, with the caveat already mentioned (that __iter__ is just >> returning self). And calling next() inside the loop will simply >> terminate the loop if there's nothing there, so I'd not have a problem >> with code like that - for instance, if I wanted to iterate over pairs >> of lines, I'd happily do this: >> >> for line1 in f: >> line2=next(f) >> print(line2) >> print(line1) >> >> That'll happily swap pairs, ignoring any stray line at the end of the >> file. Why bother catching StopIteration just to break? > > The next() there will *not* "simply terminate the loop" if it raises a > StopIteration; for loops do not catch StopIteration exceptions that > are raised from the body of the loop. The StopIteration will continue > to propagate until it is caught or it reaches the sys.excepthook. In > unusual circumstances, it is even possible that it could cause some > *other* loop higher in the stack to break (i.e. if the current code is > being run as a result of the next() method being called by the looping > construct).
I don't find that the circumstances are unusual. Pretty much any time one of the functions in the call stack is a generator this problem will occur if StopIteration propagates. I just thought I'd add that Python 3 has a convenient way to avoid this problem with next() which is to use the starred unpacking syntax: >>> numbers = [1, 2, 3, 4] >>> first, *numbers = numbers >>> first 1 >>> for x in numbers: ... print(x) ... 2 3 4 >>> first, *numbers = [] Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: need more than 0 values to unpack Since we get a ValueError instead of a StopIteration we don't have the described problem. Oscar -- http://mail.python.org/mailman/listinfo/python-list