Fix for issue #5874 for fitscheck script not deleting CHECKSUM and DA… - #5875

That doesn't seem right, why check the checksum if the user doesn't want that?

@MSeifert04 Yes yes, I'll add a null check. Although all I want is _verify_checksum_datasum to be called even if checksum is False because tha is the only function that sets self._checksum (set self._checksum to None) And if that's not set one will get AttributeError every time -k none option is used. Well may be I should modify that to make it little more logical!

(https://github.com/astropy/astropy/blob/master/astropy/io/fits/scripts/fitscheck.py#L143)
The script is going to access hdu._checksum anyways.

But shouldn't it be fixed in the script then?

Not really. IMO When we use fits.open with checksum=False it basically doesn't touch whatever CHECKSUM/DATASUM there is. So if one has used False, he doesn't care about CHECKSUM keys but that doesn't mean that self._checksum shouldn't be set. It should be set to whatever value of CHECKSUM there is (None otherwise).

I'm confused, why does it need to be set if one doesn't want to compare it to the computed checksum? It's a private attribute so anything except the class or subclasses shouldn't really be using it. I mean the script is basically just asking if there's a CHECKSUM in the header, right?

@MSeifert04 Yes. Exactly that's what I thought, self._checksum should really be used as such. Defeats the purpose. But I said all that considering that I don't have to make any changes to the script itself.
This is what a note in the code says and the reason I didn't modify the script :

# NOTE:  private data members _checksum and _datasum are
# used by the utility script "fitscheck" to detect missing
# checksums.

All I was saying is:
Consider a FITS file in which hdu's have a checksum. In such a case if I open it with

• checksum=True it will proceed with verification and correction if possible,
• if checksum='remove' it will remove the keywords when writing the changes (via writeto)
• if checksum=False I understand that it should not proceed with verification, but does that mean _checksum property should remain unset (even if keywords were present).

that test doesn't make sense (yet) 😄

Note that python files need an empty newline at the end of each file