[Python-3000] Is this a bug with list comprehensions or not? (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Thu Jul 10 15:28:01 CEST 2008

Carl Johnson <carl carlsensei.com> writes:

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.)

I don't know the exact history but I think leaking the iteration variable didn't have any practical use and could be the possible cause of subtle bugs, which is a sufficient reason for changing that behaviour. Consistency is an added bonus :-)

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?

I'd say the latter :-)

Also, I don't see where consistency is improved. IMO, StopIteration is meant to be raised by the code which produces the iterator, not by the code consuming it

That is, in your example:

def f(x): ... if x > 5: raise StopIteration ... list(x for x in range(100) if not f(x)) [0, 1, 2, 3, 4, 5]

f() certainly shouldn't raise a StopIteration. There's no reason for doing that, other than taking dirty implementation shortcuts and ending up with difficult to maintain code.

(if you want to stop an iterator based on some predicate you can use itertools.takewhile())

Regards

Antoine.

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