Object code parentheses (beginner)

Question

Why do I have to add the () at the end of f in this example?

def do_twice(f):
    f()
    f()

def print_spam():
    print 'spam'

do_twice(print_spam)
spam
spam

Is it because function objects require a specified argument for functions?

Answer 1

Why do I have to add the () at the end of f in this example?

Because f is the function, and f() actually calls the function. In your example do_twice(print_spam) sends the do_twice function the actual print_spam function, not its result. If you would have used do_twice(print_spam()), the do_twice function would have received print_spam's return value, which is None

Answer 2

This is a language decision, python does require all function calls to have () to make things unambiguous. Take these few examples,

 a = b

If b was a function, it would be ambiguous what a would gets (the b function object or the result of calling b?).

 a( b )

what would a receive? (the b function or the result of calling b?). By requiring (), these problems do not arise