[Python-Dev] PEP for Better Control of Nested Lexical Scopes (original) (raw)

Jeremy Hylton jeremy at alum.mit.edu
Fri Feb 24 23:38:26 CET 2006

On 2/24/06, James Y Knight <foom at fuhm.net> wrote:

On Feb 24, 2006, at 1:54 AM, Greg Ewing wrote: > Thomas Wouters wrote: >> On Thu, Feb 23, 2006 at 05:25:30PM +1300, Greg Ewing wrote: >> >>> As an aside, is there any chance that this could be >>> changed in 3.0? I.e. have the for-loop create a new >>> binding for the loop variable on each iteration. >> >> You can't do that without introducing a whole new scope >> for the body of the 'for' loop, > > There's no need for that. The new scope need only > include the loop variable -- everything else could > still refer to the function's main scope.

No, that would be insane. You get the exact same problem, now even more confusing: l=[] for x in range(10): y = x l.append(lambda: (x, y)) print l0 With your suggestion, that would print (0, 9). Unless python grows a distinction between creating a binding and assigning to one as most other languages have, this problem is here to stay.

The more practical complaint is that list comprehensions use the same namespace as the block that contains them. It's much easier to miss an assignment to, say, i in a list comprehension than it is in a separate statement in the body of a for loop. Since list comps are expressions, the only variable at issue is the index variable. It would be simple to fix by renaming, but I suspect we're stuck with the current behavior for backwards compatibility reasons.

Jeremy

More information about the Python-Dev mailing list