Compare the following behaviors:

    Python 3.0a5 (r30a5:62856, May 10 2008, 10:34:28)
    [GCC 4.0.1 (Apple Inc. build 5465)] on darwin
    Type "help", "copyright", "credits" or "license" for more 
information.
     >>> def f(x):
    ...  if x > 5: raise StopIteration
    ...
     >>> [x for x in range(100) if not f(x)]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 1, in <listcomp>
      File "<stdin>", line 2, in f
    StopIteration
     >>> list(x for x in range(100) if not f(x))
    [0, 1, 2, 3, 4, 5]

One might object that the behavior of the list comprehension is 
identical to that of a for-loop:

    >>> r = []
    >>> for x in range(100):
    ...  if not f(x):
    ...   r.append(x)
    ... 
    Traceback (most recent call last):
      File "<stdin>", line 2, in <module>
      File "<stdin>", line 2, in f
    StopIteration

However, it can be argued that in Python 3 list comprehensions should be 
thought of as "syntatic sugar" for ``list(generator expression)`` not a 
for-loop with an accumulator. (This seems to be the motivation for no 
longer "leaking" variables from list comprehensions into their enclosing 
namespace.)

One interesting question that this raises (for me at least) is whether 
the for-loop should also behave like a generator expression. Of course, 
the behavior of the generator expression can already be simulated by 
writing:

    >>> r = []
    >>> for x in range(100):
    ...  try:
    ...   if f(x):
    ...    r.append(x)
    ...  except StopIteration:
    ...   break
    ... 
    >>> r
    [0, 1, 2, 3, 4, 5]

This raises the question, do we need both a ``break`` statement and 
``raise StopIteration``? Can the former just be made into syntatic sugar 
for the later?