>>> def gen_123():
...     yield 1
...     yield 2
...     yield 3
...
>>> gen_123  # doctest: +ELLIPSIS
<function gen_123 at 0x...>
>>> gen_123()   # doctest: +ELLIPSIS
<generator object gen_123 at 0x...>
>>> for i in gen_123():
...     print(i)
1
2
3
>>> g = gen_123()
>>> next(g)
1
>>> next(g)
2
>>> next(g)
3
>>> next(g)
Traceback (most recent call last):
  ...
StopIteration


>>> def gen_AB():
...     print('start')
...     yield 'A'
...     print('continue')
...     yield 'B'
...     print('end.')
...
>>> for c in gen_AB():
...     print('--->', c)
...
start
---> A
continue
---> B
end.