On 1 June 2011 09:26, Carl M. Johnson <cmjohnson.mailinglist@gmail.com> wrote:
I don't think that's a counterexample to the point I'm trying to make. We all agree that if there's an x= somewhere in the function body, then we have to treat the variable as a local. The only possible way around that would be to solve the halting problem in order to figure out if a particular line of code will be reached or not. Agreed, sure, we have to treat the LHS of = as a local. But += is fundamentally different. You cannot have a += statement unless somewhere out there there is a matching = statement. It cannot exist independently. It never works on its own. So, if there is a += statement in the function body and there isn't an = statement in the function body it cannot work. Ever. All function bodies that have a += but no corresponding = or nonlocal are, as of today, broken code. So, if we were to change Python to make += not cause a variable to become a local, it wouldn't change how any (working) Python code today functions (it might causes some tests to change if they were counting on the error). This would be a completely backwards compatible change. Or am I missing something? Is there any scenario where you can get away with using += without = or nonlocal? I guess you could do something with locals().update or the stackframe, but my understanding is that those hacks don't count for language purposes.
The place to start here is section 4.1 of the language reference (Naming and Binding). Specifically, "A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block." Your modification of augmented assignment implies that a block can contain 2 different scopes - consider x = 1 def f(): # The next statement uses the global x x += 1 x = 2 # From here, you have a local x That fundamentally changes the language semantics. If you want to push this change, I'd suggest you start by proposing a change to the language reference section I mentioned above to define your proposed new scoping rules. In my view, that would be sufficiently hard that it'd kill this proposal, but if you can manage to do it, then you may have a chance to get your change accepted. Paul.