Question: Why is io.fits convenience 'getdata' much faster than fits.open()?

>>> import sys
>>> print(sys.version)
2.7.13 |Continuum Analytics, Inc.| (default, Dec 20 2016, 23:05:08) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)]
>>> import astropy 
>>> astropy.__version__
u'2.0.dev17848'
>>> import timeit
>>> timeit.timeit("h=fits.open('jcdua3f4q_flc.fits'); d=h[3].data; h.close()",
... setup="from astropy.io import fits", number=1000) 
22.351375102996826
>>> timeit.timeit("d=fits.getdata('jcdua3f4q_flc.fits', ext=3)",
... setup="from astropy.io import fits", number=1000)
8.482270956039429

How is this possible and what can be done to make “regular” fits.io operations as fast as “convenience” functions?

[saimn]

The explanation is in a comment in getheader: https://github.com/astropy/astropy/blob/master/astropy/io/fits/convenience.py#L115

    # Use _close instead of close to close without loading any
    # remaining HDUs for pre-lazy-loading backwards compatibility
    # In other words, when the full HDUList is opened by a user they
    # previously expected to be able to look at arbitrary HDUs even
    # after the file was closed, but for getheader and other convenience
    # functions this is irrelevant

[embray]

Due to this weird implementation of “lazy”, now regular functions are slower than “convenience” functions.

In fairness, they’re just as slow as they always were 😉