yield from

[snippsat]

def my_join(*iterables, my_joiner=None):
    if not iterables:
        return
    #yield from iterables[0]
    # The same as the loop under
    for item in iterables[0]:
         yield item
    for iterable in iterables[1:]:
        yield my_joiner
        yield from iterable

list1 = list(my_join([1, 2, 3], [4, 5], [6, 7], my_joiner=9999))
print(list1)
list2 = list(my_join([1, 2, 3], [4, 5], [6, 7], my_joiner='***'))
print(list2)
# Now i have made a default argument <None> so now dos this work
list3 = list(my_join([1, 2, 3], [4, 5], [6, 7]))
print(list3)

Output:
[1, 2, 3, 9999, 4, 5, 9999, 6, 7]
[1, 2, 3, '***', 4, 5, '***', 6, 7]
[1, 2, 3, None, 4, 5, None, 6, 7

yield from was new in Python 3.3.
As see in code i have comment it out,and write same code as a loop