On Wed, Jan 8, 2014 at 12:51 PM, James Stroud <xtald00d@gmail.com> wrote:

Here, the “-Qnew” flag forces the python interpreter to work with new division (where “/“ does not strictly mean integer division).

This flag basically overrides the expected python behavior to which python programmers have been writing code for over a decade, wrecks some dependencies, and stifles new code that itself depends on cctbx.

It is highly frustrating to work around this highly unnecessary flag!!

Fortunately, python has a better way to handle this problem by just having “from __future__ import division” at the top of .py files. In fact, I have spot-checked several files in the cctbx source tree and noticed that the ALL have this line at the top.

So why does the -Qnew flag still exist in the cctbx.python script? It is entirely unnecessary and requires any and all dependencies to be programmed to a behavior that is not standard for the language.

There are basically two reasons for this:

1) The old behavior is, in our opinion, fundamentally broken (and Python 3 completely discards it). (We didn't realize how many other packages were accidentally dependent on it until much later.)

2) Many of the development choices involving CCTBX are made based on the requirements of Phenix. Unfortunately, whereas Ralf was very militant about enforcing certain development rules for CCTBX code (as we continue to be), it has been nearly impossible to get the other Phenix developers to follow the same (or any!) rules. ("Cat herding" doesn't even begin to describe it.) Adding -Qnew was the easiest way to ensure that all of the code in Phenix had similar behavior with respect to division.

I agree that this creates problems for third-party packages. It would certainly be safer and more intuitive if we could avoid it altogether; I will discuss this with the other developers. Since it is somewhat non-trivial at the moment to fix the rest of Phenix to have the desired behavior, and I'm trying to revise a paper among other things, I'm deferring this until some point when I have time and free brain cells (possibly until our next Phenix meeting in mid-March). For now, I've modified libtbx/env_config.py so you can do this:

/usr/bin/python /path/to/libtbx/configure.py --old_division cctbx

which will give you cleaner dispatchers. I'm pretty sure the rest of CCTBX should still behave sensibly, since we're pretty militant about including the __future__ statement in these files. Is that sufficiently usable for the short term?

-Nat