[Python-Dev] Possible resolution of generator expression variable capture dilemma (original) (raw)

Phillip J. Eby pje at telecommunity.com
Wed Mar 24 12:46:10 EST 2004

At 08:24 AM 3/24/04 -0800, Guido van Rossum wrote:

Phillip shows some examples involving inner functions defined inside the for loop. But in current Python these examples work just as well if the function is defined outside the loop (as long as no default argument values are needed):

> for x in 1,2,3: > def y(): > print z > z = x * 2 > y() is the same as def y(): print z for x in 1, 2, 3: z = x*2 y() That would change under Phillip's proposal.

Hm. I was viewing it a bit differently. In effect, I was treating the loop as a new "nested scope", so a function defined within a loop would have different variable capture than one defined outside a loop.

There are similar examples that would break under Greg's original proposal: just use the loop variable in the function, e.g.:

Right, I mentioned at the end that the issue with my proposal was also applicable to Greg's.

All this is just more reason to put this one off until 3.0.

I agree, and withdraw my support for early binding in generator expressions, despite Tim's example. Instead, we should have a consistent failure under "practicality beats purity". That is, a straightforward translation of:

 pipe = source
 for p in predicates:
     # add a filter over the current pipe, and call that the new pipe
     pipe = e for e in pipe if p(e)

to:

 pipe = source
 for p in predicates:
     def __():
         for e in pipe:
            if p(e):
                yield e
     # add a filter over the current pipe, and call that the new pipe
     pipe = __()

produces exactly the same broken behavior in today's Python. The solution, of course is:

 def ifilter(pred,pipe):
     for e in pipe:
         if pred(e):
             yield e

 pipe = source
 for p in predicates:
     # add a filter over the current pipe, and call that the new pipe
     pipe = ifilter(p,pipe)

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