[Python-Dev] variable name resolution in exec is incorrect

[Colin H]

Perhaps the next step is to re-open the issue? If it is seen as a bug,
it would be great to see a fix in 2.6+ - a number of options which
will not break backward compatibility have been put forward - cheers,

Colin

On Thu, May 27, 2010 at 9:05 PM, Reid Kleckner <rnk at mit.edu> wrote:
> On Thu, May 27, 2010 at 11:42 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> However, attaining the (sensible) behaviour Colin is requesting when such
>> top level variable references exist would actually be somewhat tricky.
>> Considering Guido's suggestion to treat two argument exec like a function
>> rather than a class and generate a closure with full lexical scoping a
>> little further, I don't believe this could be done in exec itself without
>> breaking code that expects the current behaviour.
>
> Just to give a concrete example, here is code that would break if exec
> were to execute code in a function scope instead of a class scope:
>
> exec """
> def len(xs):
>    return -1
> def foo():
>    return len([])
> print foo()
> """ in globals(), {}
>
> Currently, the call to 'len' inside 'foo' skips the outer scope
> (because it's a class scope) and goes straight to globals and
> builtins.  If it were switched to a local scope, a cell would be
> created for the broken definition of 'len', and the call would resolve
> to it.
>
> Honestly, to me, the fact that the above code ever worked (ie prints
> "0", not "-1") seems like a bug, so I wouldn't worry about backwards
> compatibility.
>
> Reid
>