[Python-ideas] Revised**5 PEP on yield-from

Greg Ewing greg.ewing at canterbury.ac.nz
Sat Feb 21 06:39:53 CET 2009

Jacob Holm wrote:

>>             try:
>>                 _v = yield _u
>>             except Exception, _e:
>>                 if hasattr(_i, 'throw'):
>>                     _i.throw(_e)
> Shouldn't this be  "_u = _i.throw(e)" instead?

Yes, I think possibly it should... but I need to wait
until my brain stops hurting from thinking about all
this before I'll know for certain...

I'm no longer sure that the implementation I'm working
on can be exactly described in terms of any Python
expansion, anyway.

One problem is that if the generator gets None from
a yield, it has no way of knowing whether it came from
a next() or a send(None), so it doesn't know which
method to call on the subiterator. The best that it
could do is

                    if _v is None:
                      _u = _i.next()
                      _u = _i.send(_v)

but I would have to modify my implementation as it
currently stands to make it work like that.

This may be the right thing to do, as it would bring
things into line with the advertised equivalence of
next() and send(None), and you would still get an
exception if you tried to send something non-None to
an object without a send() method.

> I strongly suspect that the lower bound for this problem is O(logn/loglogn)
> per operation, and I can see a number of ways to do it in O(logn) time. 

Before you bust too many neurons on O() calculations,
be aware that the constant factors are important here.
I'm relying on traversing a delegation chain using
C calls taking negligible time compared to doing it
in Python, so that it can be treated as a constant-time
operation for all practical purposes regardless of the
length of the chain. Testing will reveal whether this
turns out to be true in real life...


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