[Python-Dev] 2.5 and beyond

[Guido van Rossum]

On 6/30/06, Andrew Koenig <ark at acm.org> wrote:
> > I read "a la Scheme" here as "actually nothing like Scheme, except I
> > want a non-tricky way to rebind a name from an enclosing scope within
> > an enclosed scope".
>
> Almost.  What I really want is for it to be possible to determine the
> binding of every name by inspecting the source text of the program.  Right
> now, it is often possible to do so, but sometimes it isn't.

Then your example

def f(): return x
x = 42
print f()

is entirely well-defined -- x is a global and the compiler in fact
generates code that benefits from knowing that it's not a local.

Python knows which locals there are; also which locals there are in
surrounding function scopes.

It *could* also know which globals and builtins there are, except the
language currently allows dynamic rebinding of module-level variables
so that they replace builtins.

E.g.

def f(): return len([])
print f()  # prints 0
def len(x): return "booh"
print f()  # prints "booh"
del len
print f()  # prints 0 again

Worse, instead if explicitly overriding len in the module, it could
have been an assignment to __main__.len in some other module.

We've been thinking on how to deal with this for years, since nobody
really likes it in all its freedom.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)