[Python-Dev] Don't set local variable in a list comprehension or generator

Victor Stinner victor.stinner at haypocalc.com
Wed May 18 23:34:09 CEST 2011

Le mercredi 18 mai 2011 à 16:19 +0200, Nadeem Vawda a écrit :
> I'm not sure why you would encounter code like that in the first place.

Well, I found the STORE_FAST/LOAD_FAST "issue" while trying to optimize
the this module which reimplements rot13 using a dict in Python 3:

d = {}
for c in (65, 97):
    for i in range(26):
        d[chr(i+c)] = chr((i+13) % 26 + c)

I tried:

d = {chr(i+c): chr((i+13) % 26 + c)
     for i in range(26)
     for c in (65, 97)}

But it is slower whereas I read somewhere than generators are faster
than loops. By the way, (c for c in ...) is slower than [c for c
in ...]. I suppose that a generator is slower because it exits/reenter
into PyEval_EvalFrameEx() at each step, whereas [c for c ...] uses
BUILD_LIST in a dummy (but fast) loop.

(c for c in ...) and [c for c in ...] is stupid, but I used a simplified
example to explain the problem. A more realistic example would be:

   squares = (x*x for x in range(10000))

You don't really need the "x" variable, you just want the square.
Another example is the syntax using a if the filter the data set:

   (x for x in ... if condition(x))

> > I heard about optimization in the AST tree instead of working on the
> > bytecode. What is the status of this project?
> Are you referring to issue11549? There was some related discussion [1] on
> python-dev about six weeks ago, but I haven't seen anything on the topic
> since then.

Ah yes, it looks to be this issue. I didn't know that there was an


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