[Q] is 'yield from' syntax sugar for 'for'+'yield'?
Question about 'yield from'. I understand that:: yield from xs is syntax suger of:: for x in xs: yield x And:: val = yield from xs is same as:: for x in xs: ret = yield x val = ret Is it true? Do I understand correctly? quote from https://docs.python.org/3/whatsnew/3.3.html#pep-380-syntax-for-delegating-to-a-subgenerator For simple iterators, yield from iterable is essentially just a shortened form of for item in iterable: yield item: -- 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 kwa...@gmail.com 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
Re: [Q] is 'yield from' syntax sugar for 'for'+'yield'?
On Thu, Aug 14, 2014 at 6:38 PM, Chris Angelico ros...@gmail.com wrote: On Thu, Aug 14, 2014 at 7:35 PM, Makoto Kuwata kwa...@gmail.com 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'?
Makoto Kuwata kwa...@gmail.com: 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 7:59 PM, Makoto Kuwata kwa...@gmail.com 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 kwa...@gmail.com: 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 7:02 PM, Chris Angelico ros...@gmail.com wrote: On Thu, Aug 14, 2014 at 7:59 PM, Makoto Kuwata kwa...@gmail.com 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 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 stdin, line 1, in module File stdin, 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 stdin, line 1, in module File stdin, line 2, in lunch2 File stdin, 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
