Hello, I want to know if open a transaction inside another is safe and encouraged?

I have a method:

def foo():
    session.begin
    try:
          stuffs
    except Exception, e:
         session.rollback()
         raise e
    session.commit()

and a method that calls the first one, inside a transaction:

def bar():
    stuffs
    try:
         foo()   #<<<< there it is :)
         stuffs
    except Exception, e:
        session.rollback()
        raise e
    session.commit()

if I get and exception on the foo method, all the operations will be rolled back? and everything else will work just fine? thanks!!