Python: How to make a cross-module variable?

print foo

import __builtin__
__builtin__.foo = 1
import a

var = 1

import a
print a.var
import c
print a.var

import a
a.var = 2

$ python b.py
# -> 1 2

var_name = "my_useful_string"

__builtins__.foo = 'something'
print foo

# my_globals.py
foo = 'something'

import my_globals
print my_globals.foo

import __builtin__
__builtin__.foo = 'some-value'

import module_b
my_var=2
module_b.do_something_with_my_globals(globals())
print my_var

def do_something_with_my_globals(glob): # glob is simply a dict.
    glob["my_var"]=3