This may be a known consequence of python 3.0, but I couldn't find any 
reference to it, nor a test case that covers it. Here's a valid use of yield 
in 2.5.1:

>>> def foo():
...   x=[(yield x) for x in 1,2,3]
...   yield 5
...   yield x
>>> x=foo()
>>> x.next()
1
>>> x.send(6)
2
>>> x.send(7)
3
>>> x.send(8)
5
>>> x.send(9)
[6, 7, 8]
>>> x.send(10)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration


But in python 3.0, this code results in:

>>> def foo():
...   x=[(yield x) for x in (1,2,3)]
...   yield 5
...   yield x
>>> x=foo()
>>> next(x)
5
>>> x.send(6)
<generator object <listcomp> at 0x3678f0>
>>> x.send(7)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration


Looking further, it seems that this is a comprehension:

>>> def foo(): [(yield 5)]
>>> type(foo())
<class 'generator'>

Whereas this is not:

>>> def foo(): [(yield 5) for x in range(3)]
>>> type(foo())
<class 'NoneType'>


Is this expected behavior?