Warn for every (nested) function with free variables (recursively)

I'd like to do the following:

for every nested function f anywhere in this_py_file:
    if has_free_variables(f):
        print warning

Why? Primarily as insurance against the late-binding closure gotcha as described elsewhere. Namely:

>>> def outer():
...     rr = []
...     for i in range(3):
...         def inner():
...             print i
...         rr.append(inner)
...     return rr
... 
>>> for f in outer(): f()
... 
2
2
2
>>> 

And whenever I get warned about a free variable, I would either add an explicit exception (in the rare case that I would want this behaviour) or fix it like so:

...         def inner(i=i):

Then the behaviour becomes more like nested classes in Java (where any variable to be used in an inner class has to be final).

(As far as I know, besides solving the late-binding issue, this will also promote better use of memory, because if a function "closes over" some variables in an outer scope, then the outer scope cannot be garbage collected for as long as the function is around. Right?)

I can't find any way to get hold of functions nested in other functions. Currently, the best way I can think of is to instrument a parser, which seems like a lot of work.