Python: how to dynamically set function closure environment

Question

I want to declare a function dynamically and I want to wrap any access to global variables OR alternatively define which variables are free and wrap any access to free variables.

I'm playing around with code like this:

class D:
    def __init__(self):
        self.d = {}     
    def __getitem__(self, k):
        print "D get", k
        return self.d[k]
    def __setitem__(self, k, v):
        print "D set", k, v
        self.d[k] = v
    def __getattr__(self, k):
        print "D attr", k
        raise AttributeError

globalsDict = D()

src = "def foo(): print x"

compiled = compile(src, "<foo>", "exec")
exec compiled in {}, globalsDict

f = globalsDict["foo"]
print(f)

f()

This produces the output:

D set foo <function foo at 0x10f47b758>
D get foo
<function foo at 0x10f47b758>
Traceback (most recent call last):
  File "test_eval.py", line 40, in <module>
    f()
  File "<foo>", line 1, in foo
NameError: global name 'x' is not defined

What I want is somehow catch the access to x with my dict-like wrapper D. How can I do that?

I don't want to predefine all global variables (in this case x) because I want to be able to load them lazily.

Answer 1

What you're looking for is object proxying.

Here is a recipe for an object proxy which supports pre- and post- call hooks:

http://code.activestate.com/recipes/366254-generic-proxy-object-with-beforeafter-method-hooks/

Create a subclass that doesn't actually load the object until the first time the _pre hook is called. Anything accessing the object will cause the real object to be loaded, and all calls will appear to be handled directly by the real object.