[Python-ideas] Yield-from: Nonexistent throw() and close()?

Greg Ewing greg.ewing at canterbury.ac.nz
Sat Feb 21 09:03:20 CET 2009

I'm happy to raise exceptions in the face of nonexistent
send() methods, but I'm not so sure about throw() and

The purpose of close() is to make sure the generator is
cleaned up before discarding it. If it's delegating to
something that doesn't have a close(), then presumably
that thing doesn't need to do any cleaning up, in which
case we should just ignore it and close() the generator
as usual.

In the case of throw(), if we raise an AttributeError
when the iterator doesn't have throw(), we're still
going to end up raising an exception in the delegating
generator, just not the one that was thrown in.

One use case I can think of is for killing a generator-
based thread -- you could define a KillThread exception
for this and throw it into the thread you want to kill.
The main loop of your scheduler would then catch
KillThread exceptions and silently remove the thread
from the system.

But if the KillThread gets turned into something else,
it won't get caught and everything will fall over for
no good reason. So at least in that case I think it
makes more sense to ignore the lack of a throw() method
and raise the thrown-in exception in the delegating

Does this reasoning make sense?


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