New Python EPICS Interface

Motivation

History

Geoff Savage created caPython(CaChannel) and Mark Rivers extended it to epicsPV and upon that created device support modules, epicsMotor, epicsLogger, epicsScaler, epicsMed, epicsMca. In today's point of view, it has the following limitations:

• Based on EPICS 3.13. Using APIs obsoleted in EPICS 3.14.
• For the same reason, no thread support.
• It is not actively maintained.
• The code is wrapped by SWIG-1.1 and requires extra work to be used by SWIG-1.3.

On the other hand, Noboru Yamamoto created and still maintains PythonCA. It has been brought update to date with EPICS 3.14. It has the following advantages:

• With thread support.
• Preemptive callback is enabled by default, which does not require periodical call of poll() to process background CA activities.
• Wrapped with Python/C API, no SWIG dependence.

Indeed there is a third development from Matthew Newville, formerly known as Ezca and has evolved into EpicsCA.

• EPICS 3.14 compatible.
• Lacks thread support and disables preemptive callback.
• Caches all connected PV channels in the C extension level.
• Wrapped with SWIG 1.3.

What is in SLS

• All Python developments are based on CaChannel/!epicsPV classes. Most of them are command line programs without using multi-threads. Rarely but certainly true, in a GUI program like CVS:G/KEITHLEY, thread support is necessary.
• For historical reasons, many different copies have be installed to:

  /work/sls/lib/python
  /work/sls/lib/python15
  /work/sls/lib/python22
  /work/sls/lib/SL-3/python15
  /work/sls/lib/SL-3/python22
  /work/sls/lib/SL-5/epics-python2.4
  

  But with the upgrade to SL-5 system, the only valid place would be /usr/local/epics/extensions/lib/SL5-x86/python2.4/.
• As of Oct. 24, 2008, this new implementation has been installed to the above mentioned path, and replaces the old files.

Our Approach

PythonCA is chosen as the basic layer to provide Python EPICS interface. After a thorough analysis of the API of both CaPython and PythonCA, we rebuild the CaChannel interfaces. The results are the following files:

• _ca_fnal.py: holds all namespace from _ca and add necessary functions(mostly data type related) not directly implemented in _ca but existing in CaPython.
• CaChannel.py: CaChannel Class interface

Our modification of PythonCA has been kindly accepted into the main release, since 1.20.1.beta2.

Regression Test

ca_cb_gr.py and ca_wf.py are two unmodified test programs from CaPython. The new interface passes all test. However there are subtle differences, see blow.

How to Convert Old Codes

If you only use searchw(), getw(), putw() function calls either using CaChannel or epicsPV classes, then you can safely skip the following discussions. Continue reading if you worry about the internals.

• In CaPython one needs to call poll() periodically for CA library to process callback functions. This is not needed in PythonCA because preemptive callback is enabled by default.
• In CaPython, CaChannelException has a member variable status to hold the integer CA error code, ECA_NORMAL etc. In PythonCA, it has been used to hold the string message. But in both cases, print will show the error message.
• In PythonCA, add_masked_array_event ignores the req_type parameter, which means only native field type is supported.

Test Suite

Here we demonstrate two typical usage, one is for command line based program and one is for graphical based program with multithread capability.

• Before Start
  1. Load CVS:/G/EPICS/extensions/src/PythonCA/test/test.db to an IOC:

     iocsh>  dbLoadRecords("test.db")

     It provides the following records,
     • catest - ao
     • castr - stringout
     • cabo - bo
     • cawave - waveform of double
     • cawavec - waveform of char
     • cawaves - waveform of string
  2. Point your Python package search path to the new module,

     export PYTHONPATH=/somewhere/to/be/decided:$PYTHONPATH

• CVS:G/EPICS/extensions/src/PythonCA/test/test.py is a command line program doing synchronous CA get/put calls.
• CVS:G/EPICS/extensions/src/PythonCA/test/thread.py is a multithread GUI program using asynchronous CA calls. It spawns a thread to calculate the Fibonacci numbers and put it to PV catest. In the main thread PV catest is monitored and displayed in a Tkinter Label widget.
• CaChannel has doctest built in. See the test results with command,

  python CaChannel.py -v

Documentation

Basically the API is the same as CaPython, so the documentation of CaChannel is still valid.

Install

The releases can be downloaded from https://sourceforge.net/project/showfiles.php?group_id=249697&package_id=312334.

Binary: CaChannel_bin.tar.gz

OS	Python	EPICS	GCC
Windows	2.5	3.14.9	MinGW 3.4.5
Linux	2.4	3.14.8.2	GCC 4.1.2

Sources: CaChannel_src.tar.gz

• Download PythonCA from http://www-acc.kek.jp/EPICS_Gr/products.html and untar it, resulting a directory like PythonCA-1.20.1.
• Download CaChannel_src.tar.gz and extract the contents to PythonCA-1.20.1.
• Edit EPICS_config_XXX.py to setup your EPICS environment variables.
• Edit ca.py, at the bottom of the file, replace from _ca_kek import * with from _ca_fnal import *
• Run python setup.py build.

-- XiaoqiangWang - 22 Aug 2008

keywords: Python