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

Almann T. Goo almann.goo at gmail.com
Tue Feb 21 05:29:53 CET 2006

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 <http://www.python.org/peps/pep-0227.html>, 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<http://www.python.org/peps/pep-0020.html>). 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.

  1. 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

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