On Thu, Jan 10, 2019 at 4:47 AM Nathan Dunfield <nat...@dunfield.info> wrote:
>
> I am a developer of the Python package "snappy" which acquires extra features
> when imported inside of Sage. Some of our Linux users have recently reported
> difficulties with SnapPy on machines where Sage was installed by the standard
> package manger making use of the system libraries and in particular the
> system python. I have reproduced the problem on Debian Sid but it also
> manifests itself on Fedora 28 [1]. The root of our problem is that in this
> situation:
>
> python -c "import sage.all"
>
> results the error:
>
> Traceback (most recent call last):
> File "<string>", line 1, in <module>
> File "/usr/lib/python2.7/dist-packages/sage/all.py", line 71, in <module>
> from sage.env import SAGE_ROOT, SAGE_SRC, SAGE_DOC_SRC, SAGE_LOCAL,
> DOT_SAGE, SAGE_ENV
> File "/usr/lib/python2.7/dist-packages/sage/env.py", line 123, in <module>
> _add_variable_or_fallback('SAGE_ETC', opj('$SAGE_LOCAL', 'etc'))
> File "/usr/lib/python2.7/dist-packages/sage/env.py", line 103, in
> _add_variable_or_fallback
> value = sep.join(components)
> TypeError: sequence item 0: expected string, NoneType found
>
> whereas
>
> sage -python -c "import sage.all"
>
> works as expected. Currently, our Python package tests for whether it is
> running in SageMath by:
>
> _within_sage = False
> try:
> import sage.all
> _within_sage = True
> except ImportError:
> pass
>
> One solution for us (I think) would be to simply catch and ignore all types
> of exceptions in the above block, but I was wondering if the current behavior
> of 'python -c "import sage.all"' was expected or is this possibly a bug? Or
> perhaps there is a work-around of some kind?
It's expected (see my previous reply) but obviously not great. For
now I would just rewrite your above test to catch Exception, instead
of just ImportError. This is a nitpick but instead of _within_sage I
would name that variable something like "_have_sage" instead: It's
just a test whether or not sage can be imported.
If you want to look-before-you-leap you could also check if
`SAGE_ROOT` and `SAGE_LOCAL` are set to something reasonable in
`os.environ`. Those are the two environment variables that currently
need to be set at a minimum for that module to work properly. But
even with that it would still be wise to catch exceptions anyways.
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