/tmp ξ cat a.py
a = None

def get_a():
    print(a)
def set_a(val):
    global a
    a = val


/tmp ξ cat b.py
a = __import__('a')
a.get_a()
a.a = 5
a.get_a()
a.set_a(6)
print(a.a)

/tmp ξ python b.py
None
5
6