[Python-ideas] Changing semantics of for-loop variable: Alternative version

Greg Ewing greg.ewing at canterbury.ac.nz
Wed Oct 5 01:15:44 CEST 2011

Ronald Oussoren wrote:

> What worries me with this proposal is that it only affects 
> the loop variable, and not other variables. This makes it easy 
> to introduce subtle bugs when you forget this. 
>     for val in sequence:
>         other = lookup(val)
>         result.append(lambda val=val, other=other): doit(val, other))

This is a good point, and I have an alternative version of
the idea that addresses it.

Instead of the cell-replacement behaviour being automatic,
we provide a way to explicity request it, such as

     for new i in stuff:

The advantage is that can be applied to anything that binds
a name, e.g.

     for x in stuff:
         new i = 2 * x
         def f():
             print i

The disadvantage is that you need to be aware of it and
remember to use it when required. However that's no worse
than the status quo, and I think it would provide a nicer
solution than the default argument hack or any of its
proposed variations.


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