Question

Does any language or debug tool have a way to print out the scope chain for examination, so as to look at the different situations of what a scope chain contains?

Answer 1

Firebug does for JavaScript. On the ‘Watch’ tab of the ‘Script’ debugger you can open up the scope chain list a look at each parent scope.

Python can read locals from a parent scope in the language itself if you grab a code object, but the way it handles nested scopes means that only the scoped variables that are actually used are bound:

>>> def a():
...     def b():
...         print v1
...     v1= 1
...     v2= 2
...     return b

>>> f= a()
>>> f.func_code.co_freevars
('v1',)
>>> f.func_closure
(<cell at 0x7fb601274da8: int object at ...>,)
>>> f.func_closure[0].cell_contents
1

Though both v1 and v2 are defined in the parent scope, only v1 is actually closed over.