Here's a simple coroutine that works perfectly in Python 2.6 but seems
to let Py3.1 enter an infinite loop that ends up eating all memory.

-----------------
def printing_sink():
    "A simple sink that prints the received values."
    while True:
        print( (yield) )

def chunker(chunk_size, target):
    """Receives single items and forwards chunks of a fixed size.

    Usage example:
    >>> sink = printing_sink()
    >>> next(sink)
    >>> cr = chunker(4, sink)
    >>> next(cr)

    >>> for i in range(8):
    ...    cr.send(i)
    [0, 1, 2, 3]
    [4, 5, 6, 7]
    >>> cr.close()
    """
    while True:
        target.send([ (yield) for i in range(chunk_size) ])

if __name__ == '__main__':
    import doctest
    doctest.testmod()
-----------------

Fails on:
Python 3.1 (r31:73572, Jun 28 2009, 21:07:35)
[GCC 4.3.2] on linux2

Works on:
Python 2.6.2 (r262:71600, Apr 17 2009, 11:29:30)
[GCC 4.3.2] on linux2

The problem seems to be the list comprehension. When I replace it with a
normal for-loop, it works perfectly.