Experimental Python interpreter snap

[James Henstridge]

To learn a bit more about I put together a snap for Python 3.6.0,
which can be installed with:

    snap install --edge python36-jamesh

You can then run "python36-jamesh.python3", which will give you the a
Python shell running with strict confinement, with the full standard
library available.

Now I know Snapcraft already has support for packaging Python
applications, so what benefits does this package add?  There were a
few extra points in how I built the package:

1. the interpreter binary and extension modules all have appropriate
$ORIGIN relative rpath set.

2. a sitecustomize.py is provided that will add
$SNAP/lib/python3.6/site-packages to sys.path. (more on why this is
useful later)

This makes the interpreter fully relocatable in the file system while
still being able to find the bundled libraries.  In turn, this means
the interpreter is functional when exported to another snap via the
content interface.

To demonstrate this, I put together a trivial "hello world" snapcraft
project here:

https://github.com/jhenstridge/python-snap-pkg/tree/master/examples/hello-world

After building this package, it can be run after installing and
connecting the interface:

    $ snap install --dangerous hello-world_0.1_amd64.snap
    hello-world 0.1 installed
    $ snap connect hello-world:python3 python36-jamesh:python3
    $ hello-world
    Hello world!

Since the hello-world snap doesn't actually include Python, it is
quite light weight (4 kB, which I think is as small as a squashfs
gets).  The space savings may not be that great with a single snap
(the interpreter snap is almost 20MB), but the space savings increase
as you install more snaps using the interface.  It also means that we
could upgrade to Python 3.6.1 (when it comes out) without rebuilding
this snap.

And since the interpreter is being run under the hello-world snap's
confinement policy, it can do potentially do things the main
"python36-jamesh.python3" binary can't.  For example, if you add the
"network" plug, you'll be able to access the network.

And the sitecustomize script will also mean the interpreter can locate
packages shipped in the plug snap.

I'd be interested in any suggestions or feedback about the snap.

Thanks,

James.