Behavior of 'fitsopen' for a non-standard FITS file

AstroPy v2.0 Test file used :- io/fits/tests/data/test0.fits (Contains 4 Image extensions)

Background

I was trying to generate a non-standard FITS file to test the compliance fixing fixing of fitscheck script. which is why I tried to delete a required keyword EXTEND from PrimaryHDU. This was not possible because astropy tries hard to not let such a thing happen. This is what I tried :-

>>> import astropy.io.fits as pf
>>> hlist = pf.open('./data/test0.fits', mode='update')
>>> del hlist[0].header['EXTEND']
>>> hlist.writeto('./data/test0.fits', output_verify='ignore', overwrite=True)
>>> hlist.close(output_verify='ignore') # Will call fits/cards.py:image

and

>>> import astropy.io.fits as pf
>>> hlist = pf.open('./data/test0.fits', mode='update')
>>> del hlist[0].header['EXTEND']
>>> hlist.flush(output_verify='ignore') # Will call fits/cards.py:image

As mentioned in the comment fits/cards.py:image has a hardcoded call to _Verify.verify with option to fix+warn. Hence I think it is not possible to generate a non-standard file via astropy.

The Issue

I replaced the EXTEND card by a blank using a text editor and opened it again.

>>> import astropy.io.fits as pf
>>> hlist = pf.open('./data/test0.fits', mode='update')
>>> hlist[0].header['EXTEND']
True

The original file remains the same.

• IMO since it fixes the standard violation while opening but that is not reflected in the actual file, there is no way to know if the original file is actually FITS compliant.
• fitscheck script checks compliance by first opening the file (when the keyword is already added!) and then checking for compliance. (One of the tests include checking for EXTEND keyword which will obviously return True now). Which is why it too can’t detect that EXTEND is missing.

[MSeifert04]

This may seem unexpected, but as usual with io.fits there must be a good reason why it is done.

@saimn optimistic or sarcastic? 😄

[saimn]

@MSeifert04 - Just realistic, from my experience there is always a good reason 😉