Confusion about global variables in python

one.py:
-------

def my_function():
  if _debug:
    print "debugging!"


two.py:
-------

from one import *
_debug = False

my_function()

NameError: global name '_debug' is not defined

constants.py
------------
debug = True

one.py
------
from constants import debug
#...

two.py
------
from constants import debug
#...

from one import *