Nested Scopes unintended behaviour ?

Terry Reedy tjreedy at udel.edu
Thu Mar 18 22:13:18 CET 2010


On 3/18/2010 6:21 AM, Michael Sparks wrote:

> After hearing it's expected behaviour in 2.6 it's clear that assigning
> a name to a value declares the variable to be local,

unless there is a global/nonlocal declaration

  and that unlike
> much of python (but like yield) this appears based on static analysis
> of the function declaration, rather than dynamic.

The language definition requires two passes after parsing.
One collects names and determines their scope (and looks for yield). The 
second generates code.
This allows the local namespace to be implemented as an array rather 
than a dict, so that local name lookup is an array index operation 
rather than a dict lookup operation. This is somewhat made visible by 
the dis module

 >>> from dis import dis
 >>> a = 1
 >>> def f():
	b = 2
	return a,b

 >>> dis(f)
   2           0 LOAD_CONST               1 (2)
               3 STORE_FAST               0 (b)

   3           6 LOAD_GLOBAL              0 (a)
               9 LOAD_FAST                0 (b)
              12 BUILD_TUPLE              2
              15 RETURN_VALUE

STORE/LOAD_FAST means store/load_local. Constants (2 in this case) are 
stored in the same array. There is apparently a separate array of global 
names, as opposed to local values.

Terry Jan Reedy




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