[Python-3000] Is this a bug with list comprehensions or not?

Carl Johnson carl at carlsensei.com
Thu Jul 10 11:44:01 CEST 2008

Filed http://bugs.python.org/issue3331

I added this to the bottom of the report, but perhaps it should be a  
discussion topic, not a "bug" per se:


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

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

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

     >>> 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? Or is this the hobgoblin of a little mind?

More information about the Python-3000 mailing list