[Python-Dev] 2.5 and beyond (original) (raw)

Tim Peters tim.peters at gmail.com
Sat Jul 1 01:23:56 CEST 2006

[Andrew Koenig]

I saw messages out of sequence and did not realize that this would be a change in behavior from 2.4. Sigh.

[Ka-Ping Yee]

Yes, this is not a good time to change it.

I hope Py3000 has lexical scoping a la Scheme...

Me too -- that would be really nice.

[Guido]

That's not a very constructive proposal (especially since I don't know Scheme). Perhaps you could elaborate on what needs to change?

It's effectively the opposite of Python <0.1 wink>: a name is local to a scope in Scheme if and only if a declaration says it is. For example, the "let" family of forms is often used for this purpose, and

(let ((x 2) (y 3)) # declares x and y as local to this let, and gives initial values (let ((x 7) (z (+ x y))) # x comes from outer let so is 2, and z is 2+3=5 (* z x))) # x comes from inner let, so this is 5*7=35

If you use let* instead of let in the inner one, z picks up the inner x=7, so that z is 7+3=10, and the result is 7*10 = 70 instead. The bindings in a let "happen" in an undefined order. In let*, a binding is visible "to its right" within the let*. Then there's letrec, which allows establishing mutually recursive bindings.

While the let family is entirely about declaration, there are lots of other forms that mix in some declaration as part of their purpose (for example, the names in a lambda's argument list are local to the lambda's body), but they're all explicit in Scheme.

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". In Scheme, the scope a name x belongs to is found by searching enclosing scopes until you hit the first with an explicit "x belongs to me" declaration (OK, there's a hokey fallback to "top level" definitions too). Searching "textually up" always suffices (there's no form of delayed declaration -- a name must be declared before use). Scheme's assignment of assignment:

 (set! variable expression)

has nothing to do with establishing the scope of variable.

More information about the Python-Dev mailing list