PyWrap

Introduction

In some cases, it would be helpful to use a piece of c++ Offline code in a python script. A good example is importing the enums from Offline into a python analysis routine, so numbers recorded in an ntuple could be interpreted.

Method

The wrappers are implemented using swig. This freeware package can interpret a c++ class header file and produce a piece of c++ code with hooks into the class, and piece of python code which can use the hooks to present the c++ class to the user as a python class.

To create a wrapper, there must be a file, pywrap.i in the src (not inc) directory where the source class is created. For example

Offline/MCDataProducts/src/pywrap.i

This file is swig interface file, and specifies the class to be wrapped. The content of this file is a bit abstruse, and in order to try to reproduce the features of c++ in python, it can get complicated. The swig documentation is complete and well-written, and interface content can be developed as needed. It is not unusual that one or two methods of the class will work, but maybe some don't and might require more work.

The creation of the wrapper is triggered by building with the switch

muse build -j 20 --mu2ePyWrap

The wrapper c code will land

build/.../Offline/tmp/MCDataProducts/src/pywrap/MCDataProducts_wrap.cpp

and the wrapper will land

build/.../Offline/pywrap/MCDataProducts.py
build/.../Offline/pywrap/_MCDataProducts.so

and Muse will add this directory to the PYTHONPATH

To use the wrapper, after "muse setup" of a build with the wrappers,

>>> import MCDataProducts
>>> gid=MCDataProducts.GenId()
>>> gid.name(1)
'particleGun'
>>> gid.findByName('particleGun').id()
1
>>> gid.particleGun
1
>>> 
>>> pc=MCDataProducts.ProcessCode()
>>> pc.name(14)
'Decay'
>>> pc.findByName('Decay').id()
14
>>> 
>>> import DbService
>>> dd=DbService.DbTool()
>>> dd.printPurpose()
>>> print( dd.getResult() )

      PID    create_user        create_date              comment
PID     1                TEST         rlc   2018-10-12 08:58:26.792518-05:00                            testing
PID     2               EMPTY         rlc   2018-10-12 08:58:26.797844-05:00                          no tables