[Paul Ganssle]
> "grab a single component" use is overwhelmingly the common case,
> I think it's worth it in the end.

Dong-hee Na, you can now go ahead with the patch.  

The relevant function is date_isocalendar() located in Modules/_datetime.c.  Model the structseq code after the PyFloat_GetInfo() function in Objects/floatobject.h.

When you have a PR with a doc update and tests, please request my review and I help you wrap it up.

Ask questions here if you get stuck.  Good luck!