1. f Fermilab Python Channel Access Interface (CaChannel) Geoff Savage EPICS Collaboration Meeting 16 November 2000

2. 11/16/00Python/CA2 Why Python?  “Python is an interpreted, interactive, object-oriented programming language”  Fermilab experience u Reduced development time u Portable (Linux, NT, OSF1) u Graphics (Tcl/Tk) u Powerful extension library u Easy to extend (SWIG) u Short learning curve  www.python.org

3. 11/16/00Python/CA3 Introduction  Implement channel access in a Python class for use in control system applications  First the channel access library must be wrapped u Each Python function wraps a CA “C” function u Additional C functions are needed to complete the wrap  Internal functions –callbacks –file descriptor manager  Helper functions –ca macros –db macros

4. 11/16/00Python/CA4 Software Layers EPICS channel access C library  C functions  Macros  Data structures  Constants caPython  Python wrapper around the C library  Collection of python functions  Software wrapper and interface generator (SWIG) CaChannel  Python class  Implemented using caPython functions and SWIG pointer library

5. 11/16/00Python/CA5 Examples Interactive >>> from CaChannel import * >>> ch = CaChannel() >>> ch.searchw('catest') >>> ch.putw(123.456) >>> ch.getw() 123.456 Script from CaChannel import * def main(): try: catest = CaChannel() catest.searchw('catest') catest.putw(123.456) print catest.getw() except CaChannelException, status: print ca.message(status) main()

6. 11/16/00Python/CA6 CaChannel Methods  Connection to a process variable (PV) u search_and_connect u clear_channel  Write to a PV u array_put u array_put_callback  Read from a PV u array_get u array_get_callback  Monitoring a PV u add_masked_array_event u clear_event

7. 11/16/00Python/CA7 CaChannel Methods  Send the requests (search, get, put) to an IOC (server) u pend_io, pend_event, flush_io  PV information u field_type, element_count, name, state, host_name, read_access, write_access  Convenient PV access u Added by us to make PV access easier u searchw = search + pend_io u putw = array_put + pend_io u getw = array_get + pend_io

8. 11/16/00Python/CA8 Callbacks  Users write callback functions in Python  The standard set of EPICS callbacks is supported u Connection u Put u Get  Value, Status, Time, Graphic, Control u Monitors  Value, Log, Alarm u Internal functions move data from C to Python

9. 11/16/00Python/CA9 Monitor Callback def eventCb(epics_args, user_args): print ca.message(epics_args['status']) print "new value = ",epics_args['pv_value'] print ca.alarmSeverityString( epics_args['pv_severity']) print ca.alarmStatusString( epics_args['pv_status']) try: ch = CaChannel() ch.searchw('catest') ch.add_masked_array_event( ca.dbf_type_to_DBR_STS(ch.field_type()), None, ca.DBE_VALUE | ca.DBE_ALARM, eventCb) ch.pend_io() except CaChannelException, status: print ca.message(status)

10. 11/16/00Python/CA10 CA Functions Not Wrapped  ca_change_connection_event u use ca_search_and_connect  ca_add_exception_event  ca_replace_printf_handler  ca_replace_access_rights_e vent  ca_puser macro u not used  ca_test_event

11. 11/16/00Python/CA11 Issues and Plans  Issues u Scan groups included but not tested u fd manager implementation appears to have a memory leak (Janousch Markus)  Plans u Complete the wrap u Suitability of next CA version for use in Python

12. 11/16/00Python/CA12 Summary  Extensive experience using CaChannel u CaChannel used in all control and monitoring GUIs at DZero  Easy for novices to use Python and CaChannel

13. 11/16/00Python/CA13 Downloading CaChannel www.aps.anl.gov/epics/ ->host software->CaPython