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Mon May 1 17:34:28 CEST 2006 · https://mail.python.org/pipermail/python-dev/2006-May/064702.html

At 07:32 AM 5/1/2006 -0700, Guido van Rossum wrote:
>On 4/30/06, Ben Wing <ben at 666.com> wrote:
> > [1] ideally, change this behavior, either for 2.6 or 3.0.  maybe have a
> > `local' keyword if you really want a new scope.
> > [2] until this change, python should always print a warning in this
> > situation.
> > [3] the current 'UnboundLocal' exception should probably be more
> > helpful, e.g. suggesting that you might need to use a `global foo'
> > declaration.
>
>You're joking right?

While I agree that item #1 is a non-starter, it seems to me that in the 
case where the compiler statically knows a name is being bound in the 
module's globals, and there is a *non-argument* local variable being bound 
in a function body, the odds are quite high that the programmer forgot to 
use "global".  I could almost see issuing a warning, or having a way to 
enable such a warning.

And for the case where the compiler can tell the variable is accessed 
before it's defined, there's definitely something wrong.  This code, for 
example, is definitely missing a "global" and the compiler could in 
principle tell:

     foo = 1

     def bar():
         foo+=1

So I see no problem (in principle, as opposed to implementation) with 
issuing a warning or even a compilation error for that code.  (And it's 
wrong even if the snippet I showed is in a nested function definition, 
although the error would be different.)

If I recall correctly, the new compiler uses a control-flow graph that 
could possibly be used to determine whether there is a path on which a 
local could be read before it's stored.

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Showing content from https://mail.python.org/pipermail/python-dev/2006-May/064702.html below:

[Python-Dev] global variable modification in functions [Re: elimination of scope bleeding of iteration variables]