FITS header verification change from astropy version 4.2 to 4.3

Description

I am using astropy.io.fits to work with fits files from GOES/SUVI. Those fits files have an issue with the CONTINUE convention, which is implemented incorrectly. Until astropy version 4.2, I was able to use the .tostring() method to convert the corrupt fits header to a string, fix it, and convert it back to a corrected fits header. Starting with astropy version 4.2.1, this is no longer possible: the .tostring() method now seems to do a fits header validity check and will not allow me to access the header. Setting verify to “ignore” does not help, but maybe I am using it wrong.

Is this the desired behaviour? For me this means I am no longer able to fix the corrupt fits header. I believe that if it is known that a fits header is not compliant with the fits standard, there should at least be an option to ignore the validity check so it is still accessible and can be fixed.

Expected behavior

Behaviour with Astropy 4.2 in a one-liner on the command line (test files can be found here):

python -c "import astropy; from astropy.io import fits; hdu = fits.open('OR_SUVI-L1b-Fe171_G16_s20193490001210_e20193490001220_c20193490001462.fits'); print('Astropy version:', astropy.__version__); hdu.verify('ignore'); hdu[0].header.tostring()"
Astropy version: 4.2

Actual behavior

Behaviour with Astropy 4.2.1 in a one-liner on the command line:

python -c "import astropy; from astropy.io import fits; hdu = fits.open('OR_SUVI-L1b-Fe171_G16_s20193490001210_e20193490001220_c20193490001462.fits'); print('Astropy version:', astropy.__version__); hdu.verify('ignore'); hdu[0].header.tostring()"
Astropy version: 4.2.1
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/header.py", line 689, in tostring
    s = str(card)
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/card.py", line 206, in __str__
    return self.image
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/card.py", line 501, in image
    self.verify('fix+warn')
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/verify.py", line 73, in verify
    errs = self._verify(opt)
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/card.py", line 1100, in _verify
    keyword = self._split()[0]
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/card.py", line 815, in _split
    for card in self._itersubcards():
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/card.py", line 1174, in _itersubcards
    if not isinstance(card.value, str):
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/card.py", line 286, in value
    value = self._value = self._parse_value()
  File "/Users/chris/opt/anaconda3/envs/astropy_test/lib/python3.8/site-packages/astropy/io/fits/card.py", line 733, in _parse_value
    raise VerifyError("Unparsable card ({}), fix it first with "
astropy.io.fits.verify.VerifyError: Unparsable card (CONTINUE), fix it first with .verify('fix').

Steps to Reproduce

See above.

System Details

Darwin-20.5.0-x86_64-i386-64bit Python 3.7.10 (default, Feb 26 2021, 10:16:00) [Clang 10.0.0 ] Numpy 1.19.2 astropy 4.2 and astropy 4.2.1 Scipy 1.6.0 Matplotlib 3.3.4

[saimn]

I should have read again the original issue 😬. So this must be a side effect of #11108, maybe @embray will have an idea to fix it.

[nabobalis]

Hi everyone!

What would be a good outcome for this issue?

Allowing a person to get a non-standard fits header through .getheader() and allowing that to be exported to a string via .tostring() to be fixed later on? Fixing the lack of spaces after CONTINUE in the fits header through verify?