pymol crashing under Qt on Catalina

[Fred Wright]

On Sun, 13 Oct 2019, Jack Howarth wrote:

> It seems one of the issues that have stymied progress there is that scons
> isn't python 3 compatible.
>
> https://github.com/SCons/scons/issues/3299

That issue seems to be about Python 3.8, not 3.x.  SCons has been 
compatible with Python 3 since v3, though 3.0.0 had some rough edges.

That being said, in general I'm not in favor of abandoning 2.7, since 
there's still lots of Python code in the world that doesn't work with 
Python 3.

>> On Sun, Oct 13, 2019 at 9:31 AM Chris Jones <jonesc at hep.phy.cam.ac.uk>
>> wrote:
[...]
>>>> On 13 Oct 2019, at 1:52 pm, Jack Howarth <howarth.at.macports at gmail.com>
>>> wrote:
>>>>
>>>> 
>>>>       The +python37 variant of pymol runs fine under its Qt interface
>>> on Catalina, however the stock +python27 variant crashes Qt as follows...

Does pymol use any compiled Python extensions?  If so, there are a couple 
of things to be aware of:

1) Python extensions need to be built against the same version of Python 
that they'll run with.  I've seen symptoms ranging from ImportError to 
segfaults when one gets this wrong.  This isn't just about the Python 
version.  For example, I've seen segfaults resulting from switching 
between the MacPorts Python 2.7 and the Apple Python 2.7.

2) Python extensions need source-level differences between Python 2 and 
Python 3, though most likely getting this wrong would result in a build 
failure, not a runtime failure.

Fred Wright