@eteq @crawfordsm -- I ended up implementing this as written, i.e. ``section`` is a string like ``"[:, 100:110]"``. This can, of course be changed, but there were a few reasons:

+ It should be clear to both new users of python and users of IRAF what ``"[:, 100:110]"`` means...something like a dict requires explanation and takes more typing.
+ To the extent that keywords like ``BIASSEC`` are used in FITS files they seem to have a format like this, and this approach something like ``section=ccddata.meta['BIASSEC']`` to be used.
+ It turned out to be easy to implement a function to translate these strings into slice objects.