CODEWITHSUNDEEP
pythonheaderastropyfits
Maria Stone
Maria Stone

Reputation: 21

Printing the Header of a FITS file with astropy: NameError: name 'header' is not defined

I am trying to learn to use FITS operations through astropy via http://docs.astropy.org/en/stable/io/fits/

I am following the directions on this website. They are:

"To see the entire header as it appears in the FITS file (with the END card and padding stripped), simply enter the header object by itself, or print(repr(header))"

But when I type header, I get the following error : 

NameError: name 'header' is not defined

I get the same error when I put print(header) or print(repr(header)) commands.

My question is why is it that the "header" command did not work?

Am I supposed to define it somehow?

My code:

from astropy.io import fits
hdulist = fits.open('test1.fits')
hdulist.info()
header

I am using jupyter notebook via Canopy.

Upvotes: 1

Views: 5183

Answers (1)

MSeifert
MSeifert

Reputation: 152597

My question is why is it that the "header" command did not work?

Am I supposed to define it somehow?

In short: It's not a command and you don't need to define it. It's actually an attribute, so you have to look it up on the hdulist.

The hdulist contains hdus and each hdu contains a data and a header, so to access the header of the first hdu you use:

print(repr(hdulist[0].header))

The [0] is because I wanted the first HDU (python uses zero-based indexing) and the .header accesses the header attribute of this HDU.

Even though I said you don't need to define it, but you can define a variable called header:

header = hdulist[0].header   # define a variable named "header" storing the header of the first HDU
print(repr(header))  # now it's defined and you can print it.

How many HDUs are present should be shown by the hdulist.info(), so you can decide which one you want to print or store.

Note that you should always use open as a context manager so it closes the file automatically (even in case of Errors):

from astropy.io import fits

with fits.open('test1.fits') as hdulist:  # this is like the "hdulist = fits.open('test1.fits')"
     hdulist.info()
     for hdu in hdulist:
         print(repr(hdu.header))
# After leaving the "with" the file closes.

This example also shows how you can use a for loop to go over all HDUs in a HDUList.

Upvotes: 3

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