Appending @Nikolaj's answer, please make sure that the path is a proper string

In my case, it was erroring out due to the comma because of incorrect commenting

file_name = 'dir/something.fits', # zshift = 1.738
hdus = fits.open(file_name)

Simply making it

file_name = 'dir/something.fits' # zshift = 1.738
hdus = fits.open(file_name)

solved the problem.