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

[Nick Coghlan]

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
---------------------------------------------------------------
             http://www.boredomandlaziness.org