Why is the function returning None for a * b instead of number?

[omm]

Problem: Write calculations using functions and get the results. Let's have a look at some examples:
seven(times(five())) # must return 35


This small function is working as expected and returning 7 * 5 as 35 but

a = 0
b = 0
sign = ''

def op(a, sign ,b):
   if sign == '*':
      return a * b 

print(op(5, '*', 7)) # Working fine with result 35

When I am implementing the similar concept in a slightly bigger program, why is it returning None instead of 35?

a = 0
b = 0
sign = ''


def op(a, sign, b):
    print(a, sign, b)

    if sign == '*':
        print(a * b)
        return a * b


def seven(fun):
    global a
    a = 7
    op(a, sign, b)


def times(fun):
    global sign
    sign = '*'

def five():
    global b
    b = 5


print(seven(times(five()))) # expecting  35 but getting None

Also, am I not using the global keyword properly?