[Antoine Pitrou <solipsis@pitrou.net>]
This implementation doesn't work with Python 3.7 or 3.8. I've tried it here: https://gist.github.com/pitrou/b3991f638300edb6d06b5be23a4c66d6
and get: Traceback (most recent call last): File "mytee.py", line 14, in gen mylast = last[1] = last = [next(it), None] StopIteration
The above exception was the direct cause of the following exception:
Traceback (most recent call last): File "mytee.py", line 47, in <module> run(mytee1) File "mytee.py", line 36, in run lists[i].append(next(iters[i])) RuntimeError: generator raised StopIteration
(Yuck!)
Thanks for trying! I wonder whether that will break other code. I wrote PEP 255, and this part was intentional at the time: """ If an unhandled exception-- including, but not limited to, StopIteration --is raised by, OR PASSES THROUGH [emphasis added], a generator function, then the exception is passed on to the caller in the usual way, and subsequent attempts to resume the generator function raise StopIteration. """ I've exploited that a number of times.
In short, you want the following instead:
try: mylast = last[1] = last = [next(it), None] except StopIteration: return
No, I don't ;-) If I have to catch StopIteration myself now, then I want the entire "white True:" loop in the "try" block. Setting up try/except machinery anew on each iteration would add significant overhead; doing it just once per derived generator wouldn't.
def mytee(xs, n): last = [None, None]
def gen(it, mylast): nonlocal last while True: mylast = mylast[1] if not mylast: mylast = last[1] = last = [next(it), None]
That's smart and obscure :-o The way it works is that the `last` assignment changes the `last` value seen by all derived generators, while the `last[1]` assignment updates the bindings made in the other generators' `mylast` lists... It's difficult to find the words to explain it.
Which is why I didn't even try - I did warn people that if they thought it "was obvious", they hadn't yet thought hard enough ;-) Good job!
The chained assignment makes it more difficult to parse as well (when I read this I don't know if `last[i]` or `last` gets assigned first; apparently the answer is `last[i]`, otherwise the recipe wouldn't work correctly).
Ya, I had to look it up too :-) Although, like almost everything else in Python, chained assignments proceed "left to right". I was just trying to make it as short as possible, to increase the "huh - can something that tiny really work?!" healthy skepticism factor :-)
Perhaps like this:
while True: mylast = mylast[1] if not mylast: try: # Create new list link mylast = [next(it), None] except StopIteration: return else: # Append to other generators `mylast` linked lists last[1] = mylast # Update shared list link last = last[1] yield mylast[0]
I certainly agree that's easier to follow. But that wasn't really the point ;-)