astropy.io.fits: How to append new cards to a header of a fits file?

Question

I'm trying to insert/append new cards to an existing header (the PRIMARY header) of a FITS file. With the code I have below, I can see on the terminal that I am 'successful' in performing this action. But when I open the FITS file in DS9 and check the header info, my new card is not present. So my action is not being saved. What am I missing here?

from astropy.io import fits

with fits.open('my.fits') as hdul:
    hdr = hdul[0].header
    hdr.append(('NEWCARD', 'value', 'A comment.'), end=True)
    
    hdul.info()
    print(repr(hdr))

Thanks in advance!

Also, would it be better to create a new header and append my new cards there? There's no need to answer this, I'm just curious. Since I'm new to handling fits files, I wonder if that's a better approach.

Answer 1

Indeed with your code you're not saving the updated file. You must use the update mode and call .flush():

from astropy.io import fits

with fits.open('my.fits', mode='update') as hdul:
    hdr = hdul[0].header
    hdr.append(('NEWCARD', 'value', 'A comment.'), end=True)
    hdul.flush()  # changes are written back to original.fits

https://docs.astropy.org/en/latest/io/fits/#save-file-changes