[AstroPy] Find record(s) with specific key in fits table

[Erik Bray]

On 08/31/2015 09:36 AM, Steffen Brinkmann wrote:
> Hello,
>
> Currently, after a tedious hour of trying different things, I do the
> following to extract records with a specific key from a fits table. In
> this case the key name is 'OBJECT' and, say, I am looking for '18_Sco':
>
> $ from astropy.io import fits
> $ fits_data = fits.getdata("file.fits")
> $ fits_data[list(fits_data['OBJECT']).index('18_Sco')]
>
> The disadvantage is that only the first record with that 'OBJECT' key is
> found.
>
> This:
> $ fits_data[list(fits_data['OBJECT']) == '18_Sco']
>
> does not work. The output is a different record with another 'OBJECT'
> name. No idea why...

Because you're comparing a list to a string.  This is the same as if you tried:

 >>> ['a', 'b', 'c'] == 'd'
False

Python lists compare to other objects to by comparing element-wise.  They'll 
only compare true to something if that something is another list containing the 
same values in the same order.

In Python True and False are equivalent to the integers 1 and 0, so the line of 
code you quoted is always equivalent to:

 >>> fits_data[0]

Since the comparison will always return False.

Phil's response is the right approach.

FITS data returned by astropy.io.fits is in a Numpy array (for the most part, 
with some caveats, though none that apply here).  If you'll be working with 
Python extensively you'll save yourself a lot of trouble in the future by 
getting comfortable with Numpy (much of which is inspired by matlab, if that helps).

The Numpy documentation is found here: 
http://docs.scipy.org/doc/numpy/user/index.html

I have an introductory tutorial to Numpy that I teach, which can be read here:
https://github.com/embray/notebooks/blob/master/numpy.ipynb

You should be able to find other people at MPIA who are familiar with Numpy as well.

Best,
Erik