Are generator functions thread-safe?

[Tim Peters]

[David Eppstein]
> Also, if a call to the generator object is running from one thread, you
> will get an exception if you attempt to simultaneously call it (e.g.
> from another thread).  I don't know how thread-safe is the code that
> prevents the generator from being called twice simultaneously, though.

A generator-iterator has an internal "am I currently running?" flag that's
inspected and mutated under protection of the GIL, so it's thread-safe.  The
flag is exposed as the gi_running attribute of a generator-iterator object,
although I haven't found a use for that yet <wink>:

>>> def g():
...     print i.gi_running
...     yield 42
...
>>> i = g()
>>> i.gi_running
0
>>> x = i.next()
1
>>> x
42
>>> i.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
StopIteration
>>>