Note: I am all for not enforcing anything here -- let's keep duck typing
alive!
If static type checkers want to be more pedantic, they can be -- that's
kinda what they are for :-)
But the OP wrote:
"""
"[i]terators are required to have an __iter__()
https://docs.python.org/3/reference/datamodel.html#object.__iter__ method"
which neither `for` nor `iter()` actually enforce.
"""
I'm confused -- as far as I can tell `for` does enforce this -- well, it
doesn't enforce it, but it does require it, which is the same thing, yes?
But does it need to?
On Wed, Sep 15, 2021 at 4:07 PM Guido van Rossum
Reminder about how for-loops work:
This:
for x in seq: <body>
translates (roughly) to this:
_it = iter(seq) while True: try: x = next(_it) except StopIteration: break <body>
exactly -- that call to iter is always made, yes? The "trick" here is that we want it to be easy to use a for loop with either an iterable or an iterator. Otherwise, we would require people to write: for i in iter(a_sequence): ... which I doubt anyone would want, backward compatibility aside. And since iter() is going to always get called, we need __iter__ methods that return self. However, I suppose one could do a for loop something like this instead. _it = seq while True: try: x = next(_it) except TypeError: _it = iter(_it) x = next(_it) except StopIteration: break <body> That is, instead of making every iterator an iterable, keep the two concepts more distinct: An "Iterator" has a __next__ method that returns an item or raises StopIteration. An "Iterable" has an __iter__ method that returns an iterator. That would mean that one couldn't write a single class that is both an iterable and an iterator, and uses (abuses) __iter__ to reset itself. But would that be a bad thing? Anyway, this is just a mental exercise, I am not suggesting changing anything. -CHB -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker@noaa.gov