Review of return

To review, here is what return does.

def f(n):
    n = n * 2
    return n + 5

if __name__ == '__main__':
    print(f(50))

105

What is happenning here is that first, n gets assigned the value 50. Then, the computation happens inside the function f(). Finally, the value of f(50) becomes (2*50+5), i.e., 105. That's what gets printed.

return stops the execution of a function

Here is a simple function that returns the first even number in the list L. For example, if L = [3, 5, 7, 1, 10, 3, 2, 8], we want to return 10.

def first_even(L):
    for e in L:
        if e % 2 == 0:
            return e
        
if __name__ == '__main__'        :
    print(first_even([3, 5, 7, 1, 10, 3, 2, 8]))

10

Why is 10 returned, and not 2 or 8? Because once e becomes 10, we execute the statement return e, which returns 10, and no further computation is done in first_even().

What if we want the first two even numbers? What we can do is return a list that contains the first two even numbers.

def first_two_evens(L):
    res = []
    for e in L:
        if e % 2 == 0:
            res.append(e)
    
    return res[:2]  #just return [res[0], res[1]]

We can use return here, just to save some computation (but not alter the result.)

def first_two_evens_B(L):
    res = []
    for e in L:
        if e % 2 == 0:
            res.append(r)
    
        if len(res) == 2:
            return res #we built up res, no point going further
                
    
    return res