I expected the above generator to produce at least 16 values but for reasons beyond me a StopIterator is raised at the end of the innermost for loop.
What am I doing wrong?
>>> def generator():
... counter = 0
... upper_index = [0, 1, 2, 3, 7, 6, 5 , 4]
... lower_index = reversed(upper_index )
... for i in range(4):
... for _ in range(2):
... for up, down in zip(upper_index , lower_index):
... yield (up, down, counter)
... counter += 1
...
>>>
>>>
>>> gen = generator()
>>> for _ in range(16):
... print(next(gen))
...
(0, 4, 0)
(1, 5, 1)
(2, 6, 2)
(3, 7, 3)
(7, 3, 4)
(6, 2, 5)
(5, 1, 6)
(4, 0, 7)
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
StopIterator
reversed returns an iterator, not a list, so once that iterator has been iterated once, it's exhausted (empty). That means that that zip iterator will also be empty as a result.
To allow lower_index to be iterated multiple times, I would instead make it a list using either of these ways:
lower_index = list(reversed(upper_index))
# or
lower_index = upper_index[::-1]