[Python-ideas] while conditional in list comprehension ??

[Shane Green]

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
> >
> > results_gen = (func1(x) for x in iterable if func2(x))
> > results = list(results_gen)
> >
> 
> That makes a lot of sense. Thank you, Oscar and Joao, for the explanations. I wasn't thinking in enough scopes :)
> 
> Regards,
> 
> Zach Ware
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> http://mail.python.org/mailman/listinfo/python-ideas

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