io.fits add ability to efficiently load single extension header

Currently io.fits/pyfits always loads all extension headers regardless whether one wants them all or not.
At least the obvious api function getheader has this behaviour.

This slows down simple tasks like just checking the primary extension headers keys considerably when the number of extensions is large as it needs to seek through the full file.
Obviously seeking the file is required to load the later extensions but it should only try to read as much as it must to satisfy the users request.

What version of Astropy/PyFITS are you using? When I use getheader() (Astropy 1.1.1), I don't see the behavior you reported. It loads only the extension header that I asked for. See below.

First, I create a simple FITS file for testing:

from astropy.io import fits
hdulist = fits.HDUList(fits.PrimaryHDU())
hdulist.append(fits.ImageHDU([[0, 0, 0], [0, 1, 0], [0, 0, 0]]))
hdulist[0].header['FOO_0'] = 'foo_0'
hdulist[1].header['FOO_1'] = 'foo_1'
hdulist.writeto('zfoo.fits')

Now, I try to read each extension header back in:

>>> hdr0 = fits.getheader('zfoo.fits')  # Should be only PRIMARY header
>>> hdr0
SIMPLE  =                    T / conforms to FITS standard                      
BITPIX  =                    8 / array data type                                
NAXIS   =                    0 / number of array dimensions                     
EXTEND  =                    T                                                  
FOO_0   = 'foo_0   '                                                            
>>> hdr1 = fits.getheader('zfoo.fits', ext=1)  # Only EXT=1 header
>>> hdr1
XTENSION= 'IMAGE   '           / Image extension                                
BITPIX  =                   64 / array data type                                
NAXIS   =                    2 / number of array dimensions                     
NAXIS1  =                    3                                                  
NAXIS2  =                    3                                                  
PCOUNT  =                    0 / number of parameters                           
GCOUNT  =                    1 / number of groups           
FOO_1   = 'foo_1   '

I'm using git head, check the seeks astropy actually performs, it reads every header in the file, regardless of what is asked.

@embray

Maybe @embray can clarify when he gets the chance...

see https://github.com/astropy/astropy/blob/master/astropy/io/fits/convenience.py#L102
the getheader function it simply loads the full hdulist via the same code as open uses and throws everything else away.
Maybe I'm using the wrong function, but getheader is probably the first thing you will find when only interested in the header.

io.fits: allow limiting the number of read extensions

@juliantaylor

@juliantaylor - I'm going to close this because I think #5065 has acheived the same affect. But feel free to re-open if you don't agree!

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io.fits add ability to efficiently load single extension header #4589

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io.fits add ability to efficiently load single extension header #4589

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pllim commented on Feb 9, 2016

juliantaylor commented on Feb 9, 2016

pllim commented on Feb 9, 2016

juliantaylor commented on Feb 9, 2016

eteq commented on Dec 9, 2016

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