[Paul Rubin]
> It looks to me like you can't have two threads in the same generator:
You can't even have one thread in a generator-iterator get away with
activating the generator-iterator while it's already active. That's
an example in PEP 255:
"""
Restriction: A generator cannot be resumed while it is actively
running:
>>> def g():
... i = me.next()
... yield i
>>> me = g()
>>> me.next()
Traceback (most recent call last):
...
File "<string>", line 2, in g
ValueError: generator already executing
"""
Same thing if more than one thread tries to do that, but perhaps
harder to see then.
To make some intuitive sense of those, note that a generator-iterator
reuses a single stack frame across resumptions. There is only once
such frame per generator-iterator, hence only (among other things)
one "program counter" per generator-iterator. It should be obvious
that multiple threads can't be allowed to muck with _the_ frame's
program counter simultaneously, and the example above is actually
subtler on that count (because nothing in non-resumable functions
prepares you for that generators make it possible for a _single_
thread to _try_ to "be in two places at the same time" inside a single
invocation of a function -- although any attempt to try to do that
with a single thread is necessarily convoluted, like the example
above, the implementation still has to prevent it).
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