[AstroPy] Healpy

[Thomas Robitaille]

Hi Nathan and others,

I already tried to contact the Free Software Foundation about this kind
of question (they have a special address for licensing questions), but
got no reply.

In these kinds of discussion, people often claim to know the answer (one
way or another), but none of us are lawyers, and the truth is that the
term 'derivative work' as used in GPL is poorly defined. There is a
whole section in the GPL Wikipedia article regarding statically linking
and dynamically linking against a GPL library:

https://en.wikipedia.org/wiki/GNU_General_Public_License#Linking_and_derived_works

with different point of views rather than a single interpretation. I
think there really is no clear answer and we won't know until it's
tested in court. Since this is not a solved issue for linking, the
situation is even fuzzier for importing Python packages.

As a side note, I did contact the HEALPIX developers, and asked if they
could re-license under LGPL (or other) to avoid these issues, and one of
the developers answered and actually argued that healpy and any package
using it could actually be BSD as long as they don't modify HEALPIX, and
that they wouldn't seriously consider a change of license unless we can
definitely prove that this is an issue when importing Python modules.
All they want to do is prevent linking with proprietary codes (so I very
much doubt they will sue us).

Regarding my email about reproject, I did not say that users should
abide by the GPL only if they use the healpy-based functionality, but if
they *install* healpy. After all, if a user installs the reproject
package into an empty Python environment with just Astropy, and does not
have healpy installed, there is no reason they need to use that code and
obey the GPL just because my code contains the words 'import healpy'. In
a way, I am dual-licensing the software - if healpy is not installed,
people can use it under the BSD license, and if healpy is installed,
they should follow the GPL.

In any case, the issue will be moot soon. WCSLIB includes an
implementation of HEALPIX which I plan to swtich to as soon as possible.

Cheers,
Tom

Nathan Goldbaum wrote:
>
> On Sat, Oct 3, 2015 at 2:46 AM, Thomas Robitaille
> > wrote:
>
>     Hi Emil,
>
>     The 'reproject' Astropy-affiliated package provides a way to easily
>     reproject images using Healpy in addition to astropy.wcs:
>
>     http://reproject.readthedocs.org/en/stable/
>
>     The functionality with Healpy is optional, so the package is normally
>     BSD-licensed, but if you do install Healpy, then as indicated here:
>
>     http://reproject.readthedocs.org/en/stable/healpix.html
>
>     you have to abide by the GPL license instead.
>
>
> Hi Tom,
>
> I don't want to stir up any problems for you, but I don't understand how
> this works legally.
>
> You're distributing code that imports healpy. Even if it's "optional"
> functionality, the code that imports healpy is being distributed under a
> BSD license. As far as I understand it, the intent of the user doesn't
> matter for the licensing, all that matters is the license the code is
> distributed under.
>
> I'd strongly urge you to contact e.g. debian-legal or the software
> freedom conservancy about this to get an opinion from an expert. I
> suspect your only legal recourses here are either to no longer import
> healpy in the reproject package or relicense reproject under a GPLv2
> compatible license.

>  
> -Nathan
>
>
>     Cheers,
>     Tom
>
>     Emil Lenc wrote:
>     > Hi All,
>     >
>     > I was wondering if there were any plans to incorporate healpy (the
>     python interface to the HEALPIX library) into astropy? I often work
>     between HEALPIX and FITS format images and it would be really
>     convenient to have these two common formats available within the
>     same package.
>     >
>     > Cheers,
>     >
>     > Emil.
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