a "closure" over a global variable is commonly known as a function using a global variable ...

IMHO that's far too trivial to be called a closure. :-p

Not sure why you are doing this

    arg = arg # make arg a lexical variable

arg should be a lexical var right away.

you might want to compare this, a is a lexical and clos a generated closure

>>> def test(a):
...   def clos():
...     return a
...   return clos
... 
>>> x=test(1)
>>> y=test(22)
>>> x()
1
>>> y()
22
[download]

This is pretty much a demonstration of lexical scope, you will only deal with dynamic scope when using global vars or accessing class attributes.*

Update

*) Well if functions call each other and close over the same lexical one might call this a dynamic scope too.

For instance a recursive closure.