User defined lexical scoping... can I do this?

[porkfried]

I want to define a 'with' command that makes entries
in dictionary available within the local scope, and
stores new local variables into that dictionary.  The
original scope should be restored on exit, and called
functions should not see anything special.  Can I do this?

my_dict = dict(a=1, b=2)
with MyScope(my_dict):
    print "A", a, "B", b
    x = 3
print my_dict["x"]
print x # FAIL, unbound