As part of implementing RFC-239 determine how astropy.io.fits handles FITS header cards with missing (unknown) values. I am virtually certain it cannot write such cards, but what happens when it tries to read them?
This will inform implementation of RFC-239.
PropertyList should support missing values and possibly units
PropertySet does not support values of None
Add support for missing values to PropertyList
Astropy issue discussing Undefined
Astropy Pull Request
Astropy does it by having a astropy.io.fits.card.Undefined special value. That strikes me as really unhelpful since it doesn't behave like any other value that you might expect. It also acts like it's True so you can't say if "FILTER" in header and header["FILTER"]: but have to have special code for dealing with is Undefined.
I have filed a pull request on Astropy to change the behavior such that FITS undefined value maps to None in python: https://github.com/astropy/astropy/pull/8572
Astropy accepted my pull request. Now Astropy and PropertyList translate python None to FITS undefined and vice versa.