Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
Makoto Kuwata wrote: > Question about 'yield from'. > > I understand that:: > > yield from xs > > is syntax suger of:: > > for x in xs: > yield x Not quite syntactic sugar. For simple cases, it does exactly the same thing. For more complicated cases, no. Suppose you have a generator: def spam(): yield "Spam!" yield "More spam!" yield "Delicious spam!!!" and you have another generator which delegates to the spam generator: def lunch1(): for food in spam(): yield food yield "plus a fried egg" We can now re-write the generator using "yield from": def lunch2(): yield from spam() yield "plus a fried egg" That saves one line of code. Big deal. Here, it is pure syntactic sugar. There are more interesting cases, where "yield from" is more powerful than the for-loop version. Here is an example with throw: py> it = lunch1() # Create an iterator. py> next(it) 'Spam!' py> it.throw(ValueError) Traceback (most recent call last): File "", line 1, in File "", line 3, in lunch1 ValueError Notice that the ValueError is raised inside lunch1(). There is no easy way to push the exception back inside spam(). But with "yield from", it works: py> it = lunch2() # New "yield from" generator. py> next(it) 'Spam!' py> it.throw(ValueError) Traceback (most recent call last): File "", line 1, in File "", line 2, in lunch2 File "", line 2, in spam ValueError Of course, spam() is free to catch the exception and perform processing. > And:: > > val = yield from xs > > is same as:: > > for x in xs: > ret = yield x > val = ret No. This is closer to what happens: # Not exactly this. # This is a simplified version. try: for x in xs: yield x except StopIteration as err: val = err.value The real version is much more complicated, 39 lines of code, and deals with generator .close() and .throw() methods, error checking, and various other issues. That is why "yield from" was added to Python. The simple case is too simple to care about, the complicated cases are too complicated to expect people to write their own solutions, so it was added to the language. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
On Thu, Aug 14, 2014 at 7:02 PM, Chris Angelico wrote: > On Thu, Aug 14, 2014 at 7:59 PM, Makoto Kuwata wrote: > > I understand that 'val = yield from xs' is completely different from:: > > > >for x in xs: > > ret = yield x > >val = x > > > > Return value is propagated by StopIteration, like: > > > >it = iter(xs) > >try: > > while 1: > >yield next(it) > >except StopIteration as ex: > > val = ex.value > > It's even more complicated than that. The PEP specifies the exact > semantics. > > ChrisA > Well, I wrote the above code in order to describe `value is propagated by StopIteration' because I misunderstood that it is propagated by return value of yield statement (see my first post). I have known that `yield from` is very complicated (thanks to your reply). -- regards, kwatch -- https://mail.python.org/mailman/listinfo/python-list
Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
Makoto Kuwata : > Thank you. It seems too complicated... I recommend you stop trying to associate the "old" yield with the "new" yield. Asyncio coroutines "abuse" "yield from" for a specific effect. The classic purpose of "yield" was to spoonfeed a sequence of return values to the caller. The coroutine meaning of "yield from" has nothing whatsoever to do about delivering computation results; instead, it denotes a state where a blocking operation is waited for and the control is handed off to other activities. Marko -- https://mail.python.org/mailman/listinfo/python-list
Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
On Thu, Aug 14, 2014 at 7:59 PM, Makoto Kuwata wrote: > I understand that 'val = yield from xs' is completely different from:: > >for x in xs: > ret = yield x >val = x > > Return value is propagated by StopIteration, like: > >it = iter(xs) >try: > while 1: >yield next(it) >except StopIteration as ex: > val = ex.value It's even more complicated than that. The PEP specifies the exact semantics. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
Makoto Kuwata : > val = yield from xs > > is same as:: > > for x in xs: > ret = yield x > val = ret > > Is it true? Do I understand correctly? The return value is not one of the yielded values. Instead, it's the value returned by the generator/coroutine. Marko -- https://mail.python.org/mailman/listinfo/python-list
Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
On Thu, Aug 14, 2014 at 6:38 PM, Chris Angelico wrote: > On Thu, Aug 14, 2014 at 7:35 PM, Makoto Kuwata wrote: > > I understand that:: > > > > yield from xs > > > > is syntax suger of:: > > > > for x in xs: > > yield x > > Not just. It's like that for simple cases, but there are edge cases > that are much more complicated to do manually, and are simply taken > care of. Best would be to read the PEP itself: > > http://www.python.org/dev/peps/pep-0380/ > > ChrisA > Thank you. It seems too complicated... I understand that 'val = yield from xs' is completely different from:: for x in xs: ret = yield x val = x Return value is propagated by StopIteration, like: it = iter(xs) try: while 1: yield next(it) except StopIteration as ex: val = ex.value Thanks. -- regards, kwatch -- https://mail.python.org/mailman/listinfo/python-list
Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
On Thu, Aug 14, 2014 at 7:35 PM, Makoto Kuwata wrote: > I understand that:: > > yield from xs > > is syntax suger of:: > > for x in xs: > yield x Not just. It's like that for simple cases, but there are edge cases that are much more complicated to do manually, and are simply taken care of. Best would be to read the PEP itself: http://www.python.org/dev/peps/pep-0380/ ChrisA -- https://mail.python.org/mailman/listinfo/python-list
