[Python-3000] Sky pie: a "var" keyword

[Talin]

You might want to check out this thread in which I proposed something 
similar (and got similar objections):

http://mail.python.org/pipermail/python-dev/2006-July/066978.html

Neil Toronto wrote:
> As long as patches are flying willy-nilly and we're just kicking around 
> ideas like puppies, I thought I'd offer one of my own.
> 
> No, not a puppy. An idea. Of course, if it ends up beaten to death, I 
> might rather it were a puppy. My apologies if it's been discussed in 
> some other form.
> 
> It occurs to me that some of Python's gotchas are due to the compiler 
> not being able to determine whether the user intends to *create* a 
> variable, or *modify* one.
> 
> The "global" gotcha:
> 
> x = 0
> def f():
>     print x   # 'local variable 'x' referenced before assignment'
>     x = 3
> 
> 
> The "read-only lexical scoping" gotcha:
> 
> def f():
>     x = 0
>     def g():
>         x = 3   # doesn't alter outer 'x'
> 
> 
> And the "changing a writable property name is a very bad plan" gotcha. 
> This bit me severely when I was first learning Python. I changed an 
> attribute name from "x" to "value" in some Bayesian network code, and 
> spent hours trying to determine why it wasn't working anymore. Turns out 
> I missed a "node.x = " somewhere. Nowadays, I'd write a dummy property 
> named "x" that throws an exception, but that seems an extraordinarily 
> hackish thing to do to support an operation that happens a lot.
> 
> A "var" keyword fixes them all. The "global" gotcha:
> 
> x = 0
> def f():
>     print x   # no exception - prints "0"
>     x = 3
> 
> def g():
>     print x   # exception
>     var x = 3
> 
> 
> The read-only lexical scoping gotcha:
> 
> def f():
>     var x = 0
>     def g():
>         x = 3   # alters outer 'x'
> 
> 
> And when the runtime encountered "node.x = " it would have complained 
> that the variable didn't exist. The __init__ method would look like this:
> 
> def __init__(self, stuff):
>    var self.stuff = stuff
>    var self.x = 0
>    # ...
> 
> 
> If you don't like how it looks, we could call it "eric" instead:
> 
>    eric self.x = 0
> 
> 
> Of course, "var" isn't a swear word in any language (as far as I know), 
> but "eric" is heinously rude in Swahili.
> 
> Essentially, "var" means "make a new entry in the dictionary."
> 
> Some Zen arguments:
> 
> Explicit is better than implicit (are you creating? are you modifying?)
> Complex is worse than simple (it's slightly more complex), but complex 
> is better than complicated (which the scoping semantics are now)
> 
> It also has nice duality with the "del" keyword. Same number of letters 
> and everything:
> 
> var x = 0
> del x
> 
> 
> If type annotations ever got into Py3k, they'd go in very naturally. 
> This might also enable some compiler optimizations.
> 
> The biggest downside I can see is that it would break every scrap of 
> Python code ever written.
> 
> Another puppy to kick around: If that's totally unacceptable, how about 
> making the default for assignment act on a variable in the outer scope 
> if one exists, and have "var" override? This would allow read/write 
> lexical scoping and put the "this is meant to be new in this scope" 
> decoration at the point of declaration. (I also think "this is meant to 
> be new" is much cleaner than "this is meant to be old," but that's just me.)
> 
> Grovelingly,
> Neil
> 
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