A=[1,2,3]
def f(x):
    A+=x,

This throws an error. The solution: state "global a". I find it odd that augmented assignment should be viewed the same as assignment in descerning local variables. This patch repairs such to maintain a as a variable of the global namespace

Some might find the following an issue

def f(x):
    if x:
        A+=4,
    else:
        A=[3]
    print("f",x,A)
def g(x):
    if not x:
        A=[3]
    else:
        A+=4,
    print("g",x,A)

In f, A is a global variable. In g, A is a local variable. Thus g(1) throws UnboundLocalError while f(1) appends 4 to A

A note on the patch, ste->ste_tmpname... lines, along with changes to Lambda_kind, were not added by me. The additional newlines prior to symtable_visit_stmt's declaration are accidental, apologies. I'll avoid patching a snapshot and then pull the old version from hg after realizing I need the old version to run diff on next time

This seems evil to me, when you consider the effect of this patch on immutable types:

>>> A = 3
>>> def f():
...     A += 5
... 
>>> f()
>>> A
8

I find the possibility that a function can implicitly (i.e., without any 'global' declarations) mutate my global module constants... disturbing.

Anyway, such a fundamental change would need proper discussion;  the right place for that is the python-ideas mailing list rather than the tracker:

http://mail.python.org/mailman/listinfo/python-ideas

Note also that there's a moratorium on core language changes in effect at the moment, so the earliest this could change is Python 3.3.

I'm going to close this issue for now;  if the idea gets a good reception on python-ideas it can be reopened.

It's not that much more evil than this:

A = []

def f(x):
  A.append(x)

print(A)  # []
f(4)
print(A)  # [4]

I've always thought this is a borderline case.

True.  I guess there's a mismatch either way around:  currently,
"A += [4]" and "A.append(4)" behave differently for (e.g.,) a list A.  With the proposed change, "n += 3" and "n = n + 3" behave differently for a integer n.  I'm not sure why I find the latter idea more disturbing than the former.

Because the latter (n += 1) is more fundamental, since it uses integers (arguably the most fundamental type).

This is why we've never done it before.

I've modified the patch to be less aggressive, as suggested by Nick at http://mail.python.org/pipermail/python-ideas/2010-June/007428.html

As an aside, runtests.sh should have executable permissions

> As an aside, runtests.sh should have executable permissions.

Doesn't it already?  On my system (without having ever messed with any permissions as far as I can recall), I have:

newton:py3k dickinsm$ ls -l runtests.sh 
-rwxr-xr-x  1 dickinsm  staff  2168 28 Aug  2009 runtests.sh
newton:py3k dickinsm$ svn pl runtests.sh
Properties on 'runtests.sh':
  svn:executable
  svn:eol-style

And I get the same with a fresh hg checkout from http://code.python.org/hg/branches/py3k