[Python-ideas] Revised^4 PEP on yield-from

[Antoine Pitrou]

Greg Ewing <greg.ewing at ...> writes:
> 
> Using a specialised syntax opens up possibilities for optimisation
> when there is a long chain of generators.  Such chains can arise, for
> instance, when recursively traversing a tree structure.  The overhead
> of passing ``next()`` calls and yielded values down and up the chain
> can cause what ought to be an O(n) operation to become O(n\*\*2).

It should be relatively easy to avoid O(n**2) behaviour when traversing a tree,
so I find this argument quite artificial.

> It has been suggested that some mechanism other than ``return`` in
> the subgenerator should be used to establish the value returned by
> the ``yield from`` expression. However, this would interfere with
> the goal of being able to think of the subgenerator as a suspendable
> function, since it would not be able to return values in the same way
> as other functions.

The problem I have with allowing "return" in generators is that it makes things
much more confusing (try explaining a beginner that he has the right to return a
value from a generator but the value can't be retrieved through any conventional
means: a "for" loop, a builtin function or method consuming the iterator, etc.).

I can imagine it being useful in some Twisted-like situation: the inner
generator would first yield a bunch of intermediate Deferreds to wait for the
completion of some asynchronous thing, and then return the final Deferred for
its caller to retrieve. But I think it wouldn't need the "yield from" construct
to function, just the "return" thing.

Regards

Antoine.