send() to a generator in a "for" loop with continue(val)??

Peter Otten __peter__ at
Sat Apr 18 11:44:27 CEST 2009

Dale Roberts wrote:

> I've started using generators for some "real" work (love them!), and I
> need to use send() to send values back into the yield inside the
> generator. When I want to use the generator, though, I have to
> essentially duplicate the machinery of a "for" loop, because the "for"
> loop does not have a mechanism to send into the generator. Here is a
> toy example:
> def TestGen1():
>     for i in xrange(3):
>         sendval = yield i
>         print "   got %s in TestGen()" % sendval
> g = TestGen1()
> sendval = None
> try:
>     while True:
>         val = g.send(sendval)
>         print 'val in "while" loop %d' % val
>         sendval = val * 10
> except StopIteration: pass
> I have to explicitly create the generator with an assignment, send an
> initial None to the generator on the first go, then have to catch the
> StopIteration exception. In other words, replicate the "for"
> mechanism, but use send() instead of next().
> It would be nice if I could just do this instead:
> for val in TestGen1():
>     print 'val in "for" loop %d' % val
>     continue(val*10)
> ...or something similar. Is this an old idea? Has it been shot down in
> the past already? Or is it worth pursuing? I Googled around and saw
> one hit here:
> but not much follow-up.
> I wonder if people want to keep the idea of an "iterator" style
> generator (where send() is not used) separate from the idea of a "co-
> routine" style generator (where send() is used). Maybe combining the
> two idioms in this way would cause confusion?
> What do folks think?

If it were up to me I'd rip out send() immediatly. At first I thought I
would see a compelling use case and be enlightened, but it never happened.

I just grepped the python 3 source for '= yield' and the only matches were
in the unit tests for generators and parser.

Let it die while it hasn't polluted the wider code base.


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