How to change a module variable from another module?

Suppose I have a package named bar, and it contains bar.py:

a = None

def foobar():
    print a

and __init__.py:

from bar import a, foobar

Then I execute this script:

import bar

print bar.a
bar.a = 1
print bar.a
bar.foobar()

Here's what I expect:

None
1
1

Here's what I get:

None
1
None

Can anyone explain my misconception?