[Python-Dev] Re: closure semantics

Thu Oct 23 13:51:16 EDT 2003

 >> That would be solved if, instead of marking variables in inner scopes
 >> that refer to outer scopes, it were the other way round, and
 >> variables in the outer scope were marked as being rebindable in inner
 >> scopes.
 ...
 Guido> This would only apply to *assignment* from inner scopes, not to
 Guido> *use* from inner scopes, right? (Otherwise it would be seriously
 Guido> backwards incompatible.)
Given that the global keyword or something like it is here to stay (being
preferable over some attribute-style access) and that global variable writes
needs to be known to the compiler for future efficiency reasons, I think we
need to consider modifications of the current global statement. The best
thing I've seen so far (I forget who proposed it) is
 'global' vars [ 'in' named_scope ]
where named_scope can only be the name of a function which encloses the
function containing the declaration. In Greg's example of inc_x_by nested
inside f, he'd have declared:
 global x in f
in inc_x_by. The current global statement (without a scoping clause) would
continue to refer to the outermost scope of the module.
This should be compatible with existing usage. The only problem I see is
whether the named_scope needs to be known at compile time or if it can be
deferred until run time. For example, should this
 import random
 def outer(a):
 x = a
 def inner(a):
 x = 42
 def innermost(r):
 if r < 0.5:
 global x in inner
 else:
 global x in outer
 x = r
 print " inner, x @ start:", x
 innermost(random.random())
 print " inner, x @ end:", x
 print "outer, x @ start:", x
 inner(a)
 print "outer, x @ end:", x
 outer(12.73)
be valid? My thought is that it shouldn't.
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