[Python-Dev] elimination of scope bleeding of iteration variables

Sun Apr 30 16:23:54 CEST 2006

Ben Wing wrote:
> apologies if this has been brought up on python-dev already.
>> a suggestion i have, perhaps for python 3.0 since it may break some code 
> (but imo it could go into 2.6 or 2.7 because the likely breakage would 
> be very small, see below), is the elimination of the misfeature whereby 
> the iteration variable used in for-loops, list comprehensions, etc. 
> bleeds out into the surrounding scope.
>> [i'm aware that there is a similar proposal for python 3.0 for list 
> comprehensions specifically, but that's not enough.]

List comprehensions will be fixed in Py3k. However, the scoping of for loop 
variables won't change, as the current behaviour is essential for search loops 
that use a break statement to terminate the loop when the item is found. 
Accordingly, there is plenty of code in the wild that *would* break if the for 
loop variables were constrained to the for loop, even if your own code 
wouldn't have such a problem.
Outside pure scripts, significant control flow logic (like for loops) should 
be avoided at module level. You are typically much better off moving the logic 
inside a _main() function and invoking it at the end of the module. This 
avoids the 'accidental global' problem for all of the script-only variables, 
not only the ones that happen to be used as for loop variables.
> # Replace property named PROP with NEW in PROPLIST, a list of tuples.
> def property_name_replace(prop, new, proplist):
> for i in xrange(len(proplist)):
> if x[i][0] == prop:
> x[i] = (new, x[i][1])

This wouldn't have helped with your name-change problem, but you've got a lot 
of unnecessary indexing going on there:
def property_name_replace(prop, new, proplist):
 for i, (name, value) in enumerate(proplist):
 if name == prop:
 proplist[i] = (new, value)
Cheers,
Nick.
-- 
Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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