[Python-Dev] PEP for Better Control of Nested Lexical Scopes (original) (raw)

Jeremy Hylton jeremy at alum.mit.edu
Tue Feb 21 15:37:06 CET 2006

On 2/21/06, Jeremy Hylton <jeremy at alum.mit.edu> wrote:

I had to lookup top-post :-).

On 2/21/06, Bengt Richter <bokr at oz.net> wrote: > On Tue, 21 Feb 2006 08:02:08 -0500, "Jeremy Hylton" <jeremy at alum.mit.edu> wrote: > >Jeremy > Hey, only Guido is allowed to top-post. He said so ;-) The Gmail UI makes it really easy to forget where the q

Sorry about that. Hit the send key by mistake.

The Gmail UI makes it really easy to forget where the quoted text is in relation to your own text.

> But to the topic, it just occurred to me that any outer scopes could be given names > (including global namespace, but that would have the name global by default, so > global.x would essentially mean what globals()['x'] means now, except it would > be a name error if x didn't pre-exist when accessed via namespacename.nameinspace notation.

Isn't this suggestion that same as Greg Ewing's?

> namespace galias # galias.x becomes alternate spelling of global.x > def outer(): > namespace mezzanine > a = 123 > print a # => 123 > print mezzanine.a # => 123 (the name space name is visible and functional locally) > def inner(): > print mezzanine.a => 123 > mezznine.a =456 > inner() > print a # = 456 > global.x = re-binds global x, name error if not preexisting. > > This would allow creating mezzanine like an attribute view of the slots in that local namespace, > as well as making namespace itself visible there, so the access to mezzanine would look like a read access to > an ordinary object named mezzanine that happened to have attribute slots matching outer's local name space.

I don't think using attribute access is particularly clear here. It introduces an entirely new concept, a first-class namespace, in order to solve a small scoping problem. It looks too much like attribute access and not enough like accessing a variable.

Jeremy

> Efficiency might make it desirable not to extend named namespaces with new names, function locals being > slotted in a fixed space tied into the frame (I think). But there are tricks I guess. > Anyway, I hadn't seen this idea before. Seems > > Regards, > Bengt Richter > > > > >On 2/20/06, Almann T. Goo <almann.goo at gmail.com> wrote: > >> I am considering developing a PEP for enabling a mechanism to assign to free > >> variables in a closure (nested function). My rationale is that with the > >> advent of PEP 227 , Python has proper nested lexical scopes, but can have > >> undesirable behavior (especially with new developers) when a user makes > >> wants to make an assignment to a free variable within a nested function. > >> Furthermore, after seeing numerous kludges to "solve" the problem with a > >> mutable object, like a list, as the free variable do not seem "Pythonic." I > >> have also seen mention that the use of classes can mitigate this, but that > >> seems, IMHO, heavy handed in cases when an elegant solution using a closure > >> would suffice and be more appropriate--especially when Python already has > >> nested lexical scopes. > >> > >> I propose two possible approaches to solve this issue: > >> > >> 1. Adding a keyword such as "use" that would follow similar semantics as > >> "global" does today. A nested scope could declare names with this keyword > >> to enable assignment to such names to change the closest parent's binding. > >> The semantic would be to keep the behavior we experience today but tell the > >> compiler/interpreter that a name declared with the "use" keyword would > >> explicitly use an enclosing scope. I personally like this approach the most > >> since it would seem to be in keeping with the current way the language works > >> and would probably be the most backwards compatible. The semantics for how > >> this interacts with the global scope would also need to be defined (should > >> "use" be equivalent to a global when no name exists all parent scopes, etc.) > >> > >> > >> def incgen( inc = 1 ) : > >> a = 6 > >> def incrementer() : > >> use a > >> #use a, inc <-- list of names okay too_ _> >> a += inc > >> return a > >> return incrementer > >> > >> Of course, this approach suffers from a downside that every nested scope > >> that wanted to assign to a parent scope's name would need to have the "use" > >> keyword for those names--but one could argue that this is in keeping with > >> one of Python's philosophies that "Explicit is better than implicit" (PEP > >> 20). This approach also has to deal with a user declaring a name with " > >> use" that is a named parameter--this would be a semantic error that could be > >> handled like "global " does today with a SyntaxError. > >> > >> 2. Adding a keyword such as "scope" that would behave similarly to > >> JavaScript's " var" keyword. A name could be declared with such a keyword > >> optionally and all nested scopes would use the declaring scope's binding > >> when accessing or assigning to a particular name. This approach has similar > >> benefits to my first approach, but is clearly more top-down than the first > >> approach. Subsequent "scope" declarations would create a new binding at the > >> declaring scope for the declaring and child scopes to use. This could > >> potentially be a gotcha for users expecting the binding semantics in place > >> today. Also the scope keyword would have to be allowed to be used on > >> parameters to allow such parameter names to be used in a similar fashion in > >> a child scope. > >> > >> > >> def incgen( inc = 1 ) : > >> #scope inc <-- allow scope declaration for bound parameters (not a big_ _> >> fan of this) > >> scope a = 6 > >> def incrementer() : > >> a += inc > >> return a > >> return incrementer > >> > >> This approach would be similar to languages like JavaScript that allow for > >> explicit scope binding with the use of "var" or more static languages that > >> allow re-declaring names at lower scopes. I am less in favor of this, > >> because I don't think it feels very "Pythonic". > >> > >> As a point of reference, some languages such as Ruby will only bind a new > >> name to a scope on assignment when an enclosing scope does not have the name > >> bound. I do believe the Python name binding semantics have issues (for > >> which the "global" keyword was born), but I feel that the "fixing" the > >> Python semantic to a more "Ruby-like" one adds as many problems as it solves > >> since the "Ruby-like" one is just as implicit in nature. Not to mention the > >> backwards compatibility impact is probably much larger. > >> > >> I would like the community's opinion if there is enough out there that think > >> this would be a worthwile endevour--or if there is already an initiative > >> that I missed. Please let me know your questions, comments. > >> > >> Best Regards, > >> Almann > >> > >> -- > >> Almann T. Goo > >> almann.goo at gmail.com > >> ________________________ > >> Python-Dev mailing list > >> Python-Dev at python.org > >> http://mail.python.org/mailman/listinfo/python-dev > >> Unsubscribe: > >> http://mail.python.org/mailman/options/python-dev/jeremy%40alum.mit.edu > >> > >> > >> > >______________________ > >Python-Dev mailing list > >Python-Dev at python.org > >http://mail.python.org/mailman/listinfo/python-dev > >Unsubscribe: http://mail.python.org/mailman/options/python-dev/python-python-dev%40m.gmane.org > > > > _> ________________________ > Python-Dev mailing list > Python-Dev at python.org > http://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: http://mail.python.org/mailman/options/python-dev/jeremy%40alum.mit.edu >

More information about the Python-Dev mailing list