mattia <ger...@gmail.com> wrote: > Il Sun, 22 Mar 2009 16:52:02 +0000, R. David Murray ha scritto: > > > mattia <ger...@gmail.com> wrote: > >> Can you explain me this behaviour: > >> > >> >>> s = [1,2,3,4,5] > >> >>> g = (x for x in s) > >> >>> next(g) > >> 1 > >> >>> s > >> [1, 2, 3, 4, 5] > >> >>> del s[0] > >> >>> s > >> [2, 3, 4, 5] > >> >>> next(g) > >> 3 > >> >>> > >> >>> > >> Why next(g) doesn't give me 2? > > > > Think of it this way: the generator is exactly equivalent to the > > following generator function: > > > > def g(s): > > for x in s: > > yield x > > > > Now, if you look at the documentation for the 'for' statement, there is > > a big "warning" box that talks about what happens when you mutate an > > object that is being looped over: > > > > There is a subtlety when the sequence is being modified by the loop > > (this can only occur for mutable sequences, i.e. lists). An > > internal counter is used to keep track of which item is used next, > > and this is incremented on each iteration. When this counter has > > reached the length of the sequence the loop terminates. This means > > that if the suite deletes the current (or a previous) item from the > > sequence, the next item will be skipped (since it gets the index of > > the current item which has already been treated). Likewise, if the > > suite inserts an item in the sequence before the current item, the > > current item will be treated again the next time through the loop. > > > > As you can see, your case is covered explicitly there. > > > > If you want next(g) to yield 3, you'd have to do something like: > > > > g = (x for x in s[:]) > > > > where s[:] makes a copy of s that is then iterated over. > > Ok, thanks. Yes, I had the idea that a counter was used in order to > explain my problem. Now I know that my intuition was correct. Thanks.
By the way, it's not the 'for' loop that maintains the counter. It's the code that implements the iteration protocol for the thing being looped over. So theoretically you could make a subclass of list that would somehow handle items being deleted or added in a sensible fashion. But I doubt it is worth doing that, since the implementation would be would be pretty idiosyncratic to the use-case. -- R. David Murray http://www.bitdance.com -- http://mail.python.org/mailman/listinfo/python-list
