Access CASA in Python without casapy

2014 February 20

Update (2017 July): These instructions might still be helpful in some cases, but I now provide compiled packages of the CASA Python libraries based on the conda-forge project, which achieve the same effect in a much more reliable manner. Here are the installation instructions for that effort.

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Like several large applications, CASA bundles its own Python interpreter. I can totally understand the decision, but sometimes it’s really annoying when you want to combine CASA’s Python modules with personal ones or those from another large package.

Fortunately, it’s not actually that hard to clone the CASA modules so that they can be accessed by your system’s Python interpreter — with the major caveat that the procedure might totally fail if the two different interpreters aren’t binary-compatible. I’ve had success in the two attempts I’ve made so far, though.

Really all you do is copy the key files. There’s a wrinkle, however, in that you need to set up the dynamically-linked libraries so that they can all find each other. This can all work automatically with the right RPATH/RUNPATH settings in the binary files, but empirically 99% of programmers are too clueless to use them correctly. Grrr. Fortunately, a tool called patchelf helps us fix things up.

Anyway, here’s how to equip an arbitrary Python interpreter with the key casac module — subject to binary compatibility of the Python module systems. I’m assuming Linux and 64-bit architecture; changes will be needed for other kinds of systems.

1. Download and install patchelf. It’s painless.

2. Download and unpack a CASA binary package. We’ll call the CASA directory {CASA}.

3. Identify a directory that your Python interpreter will search for modules, that you can write to. The global directory is something like /usr/lib64/python2.7/site-packages/, but if you have a directory for personal python modules listed in your $PYTHONPATH environment variable, that’s better. We’ll call this directory {python}.

4. Customize the following short script to your settings, and run it:

   #! /bin/sh
   
   casa={CASA} # customize this!
   python={python} # customize this!
   
   cd $casa/lib64
   
   # copy basic Python files
   cp -a python2.7/casac.py python2.7/__casac__ $python
   
   # copy dependent libraries, with moderate sophistication
   for f in lib*.so* ; do
     if [ -h $f ] ; then
       cp -a $f $python/__casac__ # copy symlinks as links
     else
       case $f in
         *_debug.so) ;; # skip -- actually text files
         libgomp.*)
           # somehow patchelf fries this particular file
           cp -a $f $python/__casac__ ;;
         *)
           cp -a $f $python/__casac__
           patchelf --set-rpath '$ORIGIN' $python/__casac__/$f ;;
       esac
     fi
   done
   
   # patch rpaths of Python module binary files
   cd $python/__casac__
   for f in _*.so ; do
     patchelf --set-rpath '$ORIGIN' $f
   done
   

5. At this point you can blow away your unpacked CASA tree, though certain functionality will require files in its data/ directory.

All this does is copy the files (casac.py, __casac__/, and dependent shared libraries) and then run patchelf on the shared libraries as appropriate. For some reason patchelf fries the libgomp library, but that one doesn’t actually need patching anyway.

After doing this, you should be able to fire up your Python interpreter and execute

import casac

successfully, showing that you’ve got access to the CASA Python infrastructure. You can then use the standard CASA “tools” like this (assuming you’re using CASA version > 4.0; things were different before):

import casac
tb = casac.casac.table()
ms = casac.casac.ms()
ms.open('vis.ms')
print(ms.nrow())
ms.close()

I’ve written some modules that provide higher-level access to functionality relying only on the casac module: casautil.py for low-level setup (in particular, controlling logging without leaving turds all over your filesystem), and tasklib.py for a scripting-friendly library of basic CASA tasks, with a small shim called casatask to provide quick command-line access to them. With these, you can start processing data using CASA without suffering the huge, irritating overhead of the casapy environment.

Note: for Macs, I believe that instead of patchelf, the command to run is something like install_name_tool -add_rpath @loader_path libfoo.dylib — but I haven’t tested this.