I added some comments on Rietveld. I guess the documentation should get notices and What’s New entries saying it is new in 3.6. Test cases would be good, including negative ones to check error handling is sensible if the Z is missing. Also shanmbic, perhaps look at signing the contributor agreement <https://www.python.org/psf/contrib/> if you haven’t already.

Adding to the list of format codes means that the statement at the top of the list about them all being required by C89 will need fixing.

For parsing, perhaps the existing %z code could be extended to accept colons, without needing to specify a new %:z code. Although if %:z is added for formatting, it should also be supported for parsing.

I am not convinced that it is worth adding %::z or %:::z to Python. The documentation of tzinfo.utcoffset() says it returns the offset “in minutes”, hinting that sub-minute offsets are not supported. RFC 3339 acknowledges that sub-minute offsets exist in history, but AFAIK neither RFC 3339 nor ISO 8601 support them. So I think it is too specialized to build %::z into Python. And I don’t imagine the %:::z necessary precision version would be used much either.