[Python-ideas] while conditional in list comprehension ?? (original) (raw)

Shane Green shane at umbrellacode.com
Wed Jan 30 01:17:51 CET 2013

Haven't read back far enough to know whether this is as interesting as it looks to me, but..

def until(items): ... stop = None ... counter = 0 ... items = iter(items) ... while not stop: ... stop = yield next(items) ... if stop: ... yield ... counter += 1 ... print(counter) ... gen = until(range(15)) stop = lambda: gen.send(True) [x for x in gen if x < 3 or stop()] 1 2 3 4 [0, 1, 2]

Shane Green www.umbrellacode.com 408-692-4666 | shane at umbrellacode.com

On Jan 29, 2013, at 8:23 AM, Zachary Ware <zachary.ware+pyideas at gmail.com> wrote:

On Jan 29, 2013 10:02 AM, "Oscar Benjamin" <oscar.j.benjamin at gmail.com> wrote: > > On 29 January 2013 15:34, Zachary Ware <zachary.ware+pyideas at gmail.com> wrote: > > > > On Jan 29, 2013 9:26 AM, "Oscar Benjamin" <oscar.j.benjamin at gmail.com> > > wrote: > >> > >> On 29 January 2013 11:51, yoav glazner <yoavglazner at gmail.com> wrote: > >> > Here is very similar version that works (tested on python27) > >> >>>> def stop(): > >> > next(iter([])) > >> > > >> >>>> list((i if i<50 else stop()) for i in range(100))_ _> >> > [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, > >> > 20, > >> > 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, > >> > 39, > >> > 40, 41, 42, 43, 44, 45, 46, 47, 48, 49] > >> > >> That's a great idea. You could also do: > >> >>> list(i for i in range(100) if i<50 or stop())_ _> >> > >> It's a shame it doesn't work for list/set/dict comprehensions, though. > >> > > > > I know I'm showing my ignorance here, but how are list/dict/set > > comprehensions and generator expressions implemented differently that one's > > for loop will catch a StopIteration and the others won't? Would it make > > sense to reimplement list/dict/set comprehensions as an equivalent generator > > expression passed to the appropriate constructor, and thereby allow the > > StopIteration trick to work for each of them as well? > > A for loop is like a while loop with a try/except handler for > StopIteration. So the following are roughly equivalent: > > # For loop > for x in iterable: > func1(x) > else: > func2() > > # Equivalent loop > it = iter(iterable) > while True: > try: > x = next(it) > except StopIteration: > func2() > break > func1(x) > > A list comprehension is just like an implicit for loop with limited > functionality so it looks like: > > # List comp > results = [func1(x) for x in iterable if func2(x)] > > # Equivalent loop > results = [] > it = iter(iterable) > while True: > try: > x = next(it) > except StopIteration: > break > # This part is outside the try/except > if func2(x): > results.append(func1(x)) > > The problem in the above is that we only catch StopIteration around > the call to next(). So if either of func1 or func2 raises > StopIteration the exception will propagate rather than terminate the > loop. (This may mean that it terminates a for loop higher in the call > stack - which can lead to confusing bugs - so it's important to always > catch StopIteration anywhere it might get raised.) > > The difference with the list(generator) version is that func1() and > func2() are both called inside the call to next() from the perspective > of the list() function. This means that if they raise StopIteration > then the try/except handler in the enclosing list function will catch > it and terminate its loop. > > # list(generator) > results = list(func1(x) for x in iterable if func2(c)) > > # Equivalent loop: > def list(iterable): > it = iter(iterable) > results = [] > while True: > try: > # Now func1 and func2 are both called in next() here > x = next(it) > except StopIteration: > break > results.append(x) > return results > > resultsgen = (func1(x) for x in iterable if func2(x)) > results = list(resultsgen) > That makes a lot of sense. Thank you, Oscar and Joao, for the explanations. I wasn't thinking in enough scopes :) Regards, Zach Ware

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