8.6. Python Interface

Certain Sherpa classes and methods can be made available to the Python interpreter in the form of an extension module. This module can be loaded in Python and provides access to certain functionalities of the Sherpa event generator in Python. In order to build the module, Sherpa must be configured with the option -DSHERPA_ENABLE_PYTHON=ON. Running make then invokes the automated interface generator SWIG [Bea03] to create the Sherpa module using the Python C/C++ API. SWIG version 1.3.x or later is required for a successful build. Problems might occur if more than one version of Python is present on the system since automake currently doesn’t always handle multiple Python installations properly. If you have multiple Python versions installed on your system, please set the PYTHON environment variable to the Python 2 executable via

$ export PYTHON=<path-to-python2>

before executing cmake script (see. Certain Sherpa classes and methods can be made available to the Python interpreter in the form of an extension module. This module can be loaded in Python and provides access to certain functionalities of the Sherpa event generator in Python. It was designed specifically for the computation of matrix elements in python (Using the Python interface) and its features are currently limited to this purpose. In order to build the module, Sherpa must be configured with the option -DSHERPA_ENABLE_PYTHON=ON. Running make then invokes the automated interface generator SWIG [Bea03] to create the Sherpa module using the Python C/C++ API. SWIG version 1.3.x or later is required for a successful build. Problems might occur if more than one version of Python is present on the system since automake currently doesn’t always handle multiple Python installations properly. A possible workaround is to temporarily uninstall one version of python, configure and build Sherpa, and then reinstall the temporarily uninstalled version of Python.

The following script is a minimal example that shows how to use the Sherpa module in Python. In order to load the Sherpa module, the location where it is installed must be added to the PYTHONPATH. There are several ways to do this, in this example the sys module is used. The sys module also allows it to directly pass the command line arguments used to run the script to the initialization routine of Sherpa. The script can thus be executed using the normal command line options of Sherpa (see Command Line Options). Furthermore it illustrates how exceptions that Sherpa might throw can be taken care of. If a run card is present in the directory where the script is executed, the initialization of the generator causes Sherpa to compute the cross sections for the processes specified in the run card. See Computing matrix elements for individual phase space points using the Python Interface for an example that shows how to use the Python interface to compute matrix elements or Generate events using scripts to see how the interface can be used to generate events in Python.

Note that if you have compiled Sherpa with MPI support, you need to source the mpi4py module using from mpi4py import MPI.

#!/usr/bin/python
import sys
sys.path.append('<sherpa-prefix>/lib/<your-python-version>/site-packages/>')
import Sherpa

# set up the generator
Generator=Sherpa.Sherpa(len(sys.argv),sys.argv)

# initialize the generator, pass command line arguments to initialization routine
try:
  Generator.InitializeTheRun()

 # catch exceptions
 except Sherpa.Exception as exc:
   print exc