WCS to pixel values and vice versa in Python

It's often necessary to convert pixel coordinates within a binary image to physical(ra, dec) sky coordinates. For instance, one might need to find brightness at the central pixel of the galaxy, calculate flux, any photometry-related task would do, actually. I used astLib.astWCS for this purpose, though I think pywcs might work just as well.

 
from astLib import astWCS

WCS=astWCS.WCS("Galaxy.fits")

center = WCS.wcs2pix(ra, dec)
print center
print 'brightness at the galaxy center', img[center[1], center[0]] 

#thanks, numpy -- python indexes arrays ass-backward (or, rather, column-backward) for some reason. One have to be careful not to swap them.