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

[perfringo]

Let's have a look at some examples:
seven(times(five())) # must return 35

If this is all what is required then I would approach it in simple way.

I observe that five() is argument of function times() and function seven() argument(s) is/are result of function times.

So - five() should return 5. Simple enough:

def five():
    return 5

Now we have to think what should return function times(). It takes integer as input and this should be returned, otherwise we have nothing to calculate. We also should pass operator, otherwise we have no operation to perform. As this is probably homework I will go for operator module:

import operator

def times(num):
    return operator.mul, num 

Now we have to write function seven() which takes times() as argument. Our function returns operator and integer, we just unpack and calculate:

def seven(func):
    multiple, num = func 
    return multiple(7, num)

Now I can call as in example and get required result:

print(seven(times(five()))) -> 35

As times() expects number one can provide it directly, without calling function:

print(seven(times(5))) -> 35
print(seven(times(7))) -> 49