Looking at PEP 380 (http://www.python.org/dev/peps/pep-0380/), the need for yield forwarding syntax comes from the inability to delegate yielding functionality in the usual way. For example, if you have a recurring pattern including yields, like this (this is a toy example, please don't take it for more than that):
if a: yield a if b: yield b
you cannot do the extract function refactoring in the usual way:
def yield_if_true(x): if x: yield x yield_if_true(a) yield_if_true(b)
because yield_if_true would become a generator.
PEP 380 addresses this by making the workaround - "for x in yield_if_true(a): yield x" - easier to write.
But suppose you could address the source instead? Suppose you could write yield_if_true in such a way that it did not become a generator despite yielding?
Syntactically this could be done with a yielding *function* in addition to the yield statement/expression, either as a builtin or a function in the sys module. Let's call it 'yield_' , for lack of a better name. The function would yield the nearest generator on the call stack.
Now the example would work with a slight modifiction:
def yield_if_true(x): if x: yield_(x) yield_if_true(a) yield_if_true(b)
The real benefits from a yield_ function come with recursive functions. A recursive tree traversal that yields from the leaf nodes currently suffers from a performance penalty: Every yield is repeated as many times as the depth of the tree, turning a O(n) traversal algorithm into an O(n lg(n)) algorithm. PEP 380 does not change that.
But a yield_ function could be O(1), regardless of the forwarding depth.
To be fair, a clever implementation might be able to short-circuit a 'yield from' chain and achieve the same thing.
Two main drawbacks of yield_: - Difficulty of implementation. Generators would need to keep an entire stack branch alive, instead of merely a single frame, and if that somehow affects the efficiency of simple generators, that would be bad. - 'the nearest generator on the call stack' is sort of a dynamic scoping thing, which has its problems. For example, if you forget to make the relevant function a generator (the "if 0: yield None" trick might have been needed but was forgotten), then the yield would trickle up to some random generator higher up, with confusing results. And if you use yield_ in a callback, well, let's just say that an interesting case too.
All the same, if a yield_ function is practical, I think it is a better way to address the problems that motivate PEP 380. I'm guessing you could implement 'yield from' as a pure-Python function using yield_, making yield_ strictly more powerful, although I couldn't say for sure as I haven't studied the enhanced generator protocol.