I don't accept your conclusion. I understand that making %S consume microseconds or ".%f" be "optional" would be a load. What's the problem with forcing __str__ and isoformat to emit microseconds in all cases though? That would allow you to parse what they produce using existing code. No new constructor needed.

The issue of sometimes emitting microseconds, sometimes not, is annoying, even beyond this issue. I think for consistency's sake it makes sense for the string version of datetime and time objects to always be the same length.