[Python-Dev] PEP for Better Control of Nested Lexical Scopes

Jeremy Hylton jeremy at alum.mit.edu
Tue Feb 21 15:32:55 CET 2006


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

> 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 namespace_name.name_in_space notation.
>
>
>     namespace g_alias  # g_alias.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.
>
> 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
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> >>
> >>
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