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FITS checksum comments set by add_checksum(when=...) are overwritten on flush in update mode #20789
Description
Description
When a FITS file containing existing CHECKSUM and DATASUM keywords is opened with mode="update", calling add_checksum(when=...) correctly updates the checksum comments in memory. However, when the HDUList is flushed or closed, the comments are overwritten with Astropy's default timestamp comments.
This happens because _update_checksum() calls add_checksum() without passing a when argument, even if the user has already explicitly updated the checksums with custom comments.
It seems like it might be preferable for automatic checksum updates to preserve existing comments, particularly when the checksum cards have already been explicitly updated by the user.
Expected behavior
An explicit call to add_checksum(when=...) should preserve the supplied comments when the file is saved. Automatic checksum recalculation should not silently overwrite them.
How to Reproduce
from astropy.io import fits
orig_header = fits.getheader("astropy_demo.fits.gz")
print(orig_header.comments["CHECKSUM"]) # HDU already has a CHECKSUM and DATASUM
with fits.open("astropy_demo.fits.gz", mode="update") as hdul:
hdul[0].data[0, 0] = 5 # change the data
hdul[0].add_checksum(when="This is a custom `when` comment")
print(hdul[0].header.comments["CHECKSUM"]) # confirm that the custom comment did take
new_header = fits.getheader("astropy_demo.fits.gz")
print(new_header.comments["CHECKSUM"]) # overwritten by default `when` comment(also works with a non-gzipped FITS file, but GitHub only allows .gz attachments)
Versions
import astropy
astropy.system_info()platform
--------
platform.platform() = 'Linux-7.0.0-42-generic-x86_64-with-glibc2.39'
platform.version() = '#42~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Mon Oct 19 14:43:01 UTC 2'
platform.python_version() = '3.13.18'
packages
--------
astropy 8.0.2
numpy 2.5.4
scipy 1.18.1
matplotlib 3.11.2
pandas 3.1.0
pyerfa 2.0.1.5
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