[SciPy-user] HDULists sharing HDU entries

C. Allen callen@gemini....
Fri Jul 11 20:53:05 CDT 2008


thanks chris

On Fri, 2008-07-11 at 21:36 -0400, Christopher Hanley wrote:

> C. Allen wrote:
> > I have been making a system for handling astronomical data, the actual 
> > fits manipulation using pyfits. Please correct me if there is another 
> > list this question would be more appropriate on, I did not see a pyfits 
> > list at the project page.
> > 
> > When I started this system I was new to python (but experienced in many 
> > other languages) and in the process I have done some things I am not 
> > sure about. I of course test my ideas to make sure they work, but of 
> > course, such tests can miss some condition in which the idiom ultimately 
> > is bound to fail horribly.
> > 
> > So... I'm curious if something I'm doing with HDULists is ok.
> > 
> > Basically, we have an abstraction for our data, lets call it AstroData 
> > because that's what we call it.  It contains a member which is the 
> > HDUList as returned by pyfits when opening a MEF.
> > 
> > I also have some syntax that allows one to get an AstroData instance of 
> > just sub-selections of extensions in the MEF, e.g.:
> > 
> > if ad = AstroData(fitsfilename) then (e.g.) ad["SCI"] returns a separate 
> > AstroData instance from 'ad'. In this case I build the HDUList out of 
> > the HDU entries in the original instance.  I make a python list with the 
> > PHU and whatever extensions are being selected (in the sample case, 
> > those where EXTNAME="SCI"), and initialize a new HDUList instance with 
> > this list.  Then this unique HDUList is stored in the new AstroData 
> > instance.  So each AstroData instance has it's own HDUList, but the HDU 
> > entries in it are shared.
> > 
> > Everything works fine, to write the constructed HDUList is to save just 
> > those extensions listed, as desired, and I can pass the HDUList around 
> > as normal to anything that wants HDUList instances. Of course, if I 
> > change any of the shared extensions, it will be seen in all HDULists 
> > that refer to it, which is confusing to some but is exactly what we want 
> > in this case. 
> > 
> > All this runs perfectly fine in practice. Does anyone know any reason 
> > this might get me into trouble? It seems I have nothing to worry about, 
> > but I have a nagging idea that perhaps the HDU is more tightly linked to 
> > the list containing it in some way that I do not yet know.
> > 
> > thanks,
> > craig
> > 
> > 
> > ------------------------------------------------------------------------
> > 
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> > SciPy-user@scipy.org
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> 
> Hi Craig,
> 
> The more appropriate list is probably astropy@scipy.org.
> 
> As to your question.  Assuming that I understand what you are 
> attempting, I don't think you should run into any problems.
> 
> Chris
> 
> 
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