return vs print list

Your first example iterates through each item in x, printing each item to the screen. Your second example begins iterating through each item in x, but then it returns the first one, which ends the execution of the function at that point.

Let's take a closer look at the first example:

def list_function(x):
    for y in x:
        print(y)  # Prints y to the screen, then continues on

n = [4, 5, 7]
list_function(n)

Inside the function, the for loop will begin iterating over x. First y is set to 4, which is printed. Then it's set to 5 and printed, then 7 and printed.

Now take a look at the second example:

def list_function(x):
    for y in x:
        return y  # Returns y, ending the execution of the function

n = [4, 5, 7]
print(list_function(n))

Inside the function, the for loop will begin iterating over x. First y is set to 4, which is then returned. At this point, execution of the function is halted and the value is returned to the caller. y is never set to 5 or 7. The only reason this code still does print something to the screen is because it's called on the line print list_function(n), so the return value will be printed. If you just called it with list_function(n) as in the first example, nothing would be printed to the screen.