How two modules call functions defined in each other?

[Tian]

I am python beginner, I have a question about the interdependence of
modules.

For example, when I have two modules:

module1.py
-------------
def plus(x):
  return add(x,1)


module2.py
-------------
def add(x,y):
  return x+y

def plus2(x):
  return plus(x)+1


How should I write "import" in both files?
What about the global varibals? is there anything like "extern" keyword
in C?
or python has some other solutions?