* iterating on the string the string, stopping when something is wrong
(might process almost all of the string and finally give up because
last part is wrong, EG incorrect microseconds or time zone. Penalize
invalid strings, best case when most of the strings to process are
valid)
* first checking the string is correct, then iterating over it and
handling each part. Early detection of incorrect strings, useless
overhead for valid string. Penalize valid strings, best case when most
of the strings to process are invalid).

I have a preference for solution #1.

As I mentioned at the bug tracker, I would prefer to start with the C implementation falling back to Python. This is what we do for strptime and I don't see why fromisoformat should be different. Let's focus of finalizing the desired behavior and getting the Python implementation checked in. We don't want to maintain two implementations while the features are still subject to revision. Once Python code is mature enough, we can implement the C acceleration.