pvaPy provides Python bindings for EPICS pvAccess
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Latest commit 4e01e35 Jul 11, 2018


PvaPy - PvAccess for Python

The PvaPy package is a Python API for EPICS4. It supports both PVA and CA providers, all standard EPICS4 types (structures, scalars, unions, etc), standard set of channel operations (put/get, monitor), RPC client/server, PVA server, etc. It uses Boost/Python to wrap EPICS4 C++ libraries and APIs, and is currently supported on the Unix-like operating systems (Linux, OSX).


At the moment prebuilt PvaPy packages exist only for the Conda package management system. If you do not use Conda, or if you need 32-bit Linux Conda packages, you will have to build PvaPy from sources.


  • Supported platforms: Linux (64-bit), OSX
  • Supported Python versions: Python2, Python3

Make sure conda command is in your path, and execute the following command:

  $ conda install -c epics pvapy

This should result in installation of pvapy, pvapy-boost and epics-base Conda packages.


This section is intended for those users that would like to build PvaPy from the source code.


Building PvaPy from source requires recent versions of the following software:

  1. EPICS Base (>=v3.14.12.3)
  2. EPICS4 CPP release (>=v4.4.0); note that EPICS7 release includes all needed v4 modules
  3. Python development header files/libraries (>=v2.6.6)
  4. Boost (>=v1.41.0); must have the boost_python library built
  5. Standard development tools (gcc, make, autoconf, etc.)
  6. Boost.NumPy (optional, for retrieving numeric scalar arrays as numpy arrays; this is not needed for Boost v1.63.0 and later)
  7. Sphinx (optional, for generating documentation)

The version numbers mentioned above are the earliest that PvaPy was tested with. Any recent version of Python and the Boost libraries (such as those that come with current linux distributions) should work. Similarly, any recent version of EPICS Base that is supported by EPICS4 CPP should be fine.

Nothing special needs to be done when building the EPICS4 CPP modules. Ensure that the EPICS Base installation you use for this module is the same one that was used to build the EPICS4 modules.

This module has not been adapted for use on Microsoft Windows. Only Unix-like operating systems (e.g. Linux, MacOS, Solaris) are currently supported.

1. Configure PvaPy.

This can be done manually, or using autoconf.

For manual configuration: Read the comments in both the configure/RELEASE and configure/CONFIG_SITE files and follow the instructions given there.

For automatic configuration: In the top level directory run

  $ make configure EPICS_BASE=/epics/base/path [EPICS4_DIR=/epics/v4/path] [BOOST_NUM_PY_DIR=/boost.numpy/path] [BOOST_ROOT=/boost/path] [PYTHON_VERSION=3]

Note that you can only use the automatic configuration if the v4 modules have not been renamed. In the above command replace /epics/base/path with the full path to your EPICS Base directory, and /epics/v4/path with the full path to your top level directory containing the v4 modules pvDataCPP, pvAccessCPP, etc. If you are using EPICS7 release, you can omit EPICS4_DIR argument, as configuration scripts will find the required v4 libraries and header files in the EPICS Base directory.

The optional BOOST_NUM_PY_DIR argument enables NumPy array support for older Boost versions, as v1.63.0 and later releases already include NumPy libraries. The optional BOOST_ROOT argument is typically used for custom Boost installation directory. In case you are using custom python installation that uses shared object libraries, you may need to set PATH and LD_LIBRARY_PATH environment variables before running the above make configure command. Also, note that building with python 3.x requires PYTHON_VERSION=3 argument.

The make configure command will check for your Boost/Python libraries, and create suitable configure/RELEASE.local and configure/CONFIG_SITE.local files that are used by the build process. They should look roughly like the examples below:

  $ cat RELEASE.local
  PVACLIENT = /home/epics/v4/pvaClientCPP
  PVDATABASE = /home/epics/v4/pvDatabaseCPP
  NORMATIVETYPES = /home/epics/v4/normativeTypesCPP
  PVACCESS = /home/epics/v4/pvAccessCPP
  PVDATA = /home/epics/v4/pvDataCPP
  EPICS_BASE = /home/epics/base-3.15.5

  $ cat CONFIG_SITE.local
  PVA_PY_CPPFLAGS = -I/usr/include -I/usr/include/python2.7
  PVA_PY_LDFLAGS = -L/usr/lib64 -lpython2.7
  PVA_PY_SYS_LIBS = boost_python  
  PVA_PY_PYTHON = /bin/python
  PVA_PY_PYTHONPATH = /home/epics/v4/pvaPy/lib/python/2.7/linux-x86_64
  PVA_PY_LD_LIBRARY_PATH = /usr/lib64
  PVA_PY_EPICS_BASE = /home/epics/base-3.15.5
  PVA_PY_EPICS4_DIR = /home/epics/v4
  PVA_PY_HOST_ARCH = linux-x86_64
  PVA_PY_SETUP_SH = /home/epics/pvaPy/bin/linux-x86_64/pvapy_setup_full.2.7.sh

The above files were created automatically on a 64-bit RHEL 7.4 machine, with the following boost/python packages installed:

  $ rpm -q boost-python python-devel

Note that the automatic configuration process also creates pvapy_setup_full.$PYTHON_VERSION.(c)sh and pvapy_setup_pythonpath.$PYTHON_VERSION.(c)sh files in the bin/$EPICS_HOST_ARCH directory. The full setup files modify PATH, LD_LIBRARY_PATH and PYTHONPATH environment variables, while the pythonpath setup files modify only PYTHONPATH variable. For example,

  $ cat pvapy_setup_pythonpath.2.7.sh
  # modifies PYTHONPATH environment variable
  if test -z "$PYTHONPATH" ; then
      export PYTHONPATH=/home/epics/v4/pvaPy/lib/python/2.7/linux-x86_64
      export PYTHONPATH=/home/epics/v4/pvaPy/lib/python/2.7/linux-x86_64:$PYTHONPATH

After building pvaPy, the environment setup files can be sourced to use the built python module, e.g.:

  $ . /home/epics/v4/pvaPy/bin/linux-x86_64/pvapy_setup_pythonpath.2.7.sh
  $ echo $PYTHONPATH

or for csh users:

  % source /home/epics/v4/pvaPy/bin/linux-x86_64/pvapy_setup_pythonpath.2.7.csh
  % echo $PYTHONPATH

2. Compile the pvaPy source.

In the top level package directory run:

  $ make

The above command will create and install a loadable library pvaccess.so under the lib/python directory which can be imported directly by Python.

3. Generate documentation.

This step is optional and requires Sphinx to be installed:

  $ make doc

If a sphinx-build script is present on the system, html pages will be generated in the documentation/sphinx/_build/html directory.

Basic Usage: PV put/get

For simple testing, do the following:

  1. In a separate terminal, start the testDbPv IOC:
  $ cd $EPICS4_DIR/pvaSrv/testTop/iocBoot/testDbPv
  $ ../../bin/$EPICS_HOST_ARCH/testDbPv st.cmd
  1. Source the appropriate setup file from pvaPy's bin/$EPICS_HOST_ARCH directory and start python (the Python PVA module is called pvaccess):
  $ python
  >>> import pvaccess
  >>> dir (pvaccess)
  ['BOOLEAN', 'BYTE', 'CA', 'Channel', 'DOUBLE', 'FLOAT', 'FieldNotFound',
  'INT', 'InvalidArgument', 'InvalidDataType', 'InvalidRequest', 'LONG',
  'NtTable', 'NtType', 'PVA', 'ProviderType', 'PvAlarm', 'PvBoolean', 'PvByte',
  'PvDouble', 'PvFloat', 'PvInt', 'PvLong', 'PvObject', 'PvScalar',
  'PvScalarArray', 'PvShort', 'PvString', 'PvTimeStamp', 'PvType', 'PvUByte',
  'PvUInt', 'PvULong', 'PvUShort', 'PvUnion', 'PvaException', 'RpcClient',
  'RpcServer', 'SHORT', 'STRING', 'UBYTE', 'UINT', 'ULONG', 'USHORT', '__doc__',
  '__file__', '__name__', '__package__']
  >>> c = pvaccess.Channel('int01')
  >>> print c.get()
      int value 0
  >>> c.putInt(7)
  >>> print c.get()
      int value 7
  >>> c.put(pvaccess.PvInt(5))
  >>> print c.get()
      int value 5

In the above, note that in addition to PV object classes like PvInt, one can also use standard Python types as arguments for channel puts.

Basic Usage: PV monitor

  1. In a separate terminal, start the testDbPv IOC:
  $ cd $EPICS4_DIR/pvaSrv/testTop/iocBoot/testDbPv
  $ ../../bin/$EPICS_HOST_ARCH/testDbPv st.cmd
  1. PV values can be changed using the IOC shell command dbpf, e.g:
  epics> dbpr 'float01'
  ASG:                DESC:               DISA: 0             DISP: 0
  DISV: 1             NAME: float01       SEVR: MAJOR         STAT: LOLO
  TPRO: 0             VAL: 0
  epics> dbpf 'float01' 11.1
  DBR_FLOAT:          11.1
  1. Monitor a channel in Python, passing in a subscriber object (function that processes PvObject instance):
  >>> c = pvaccess.Channel('float01')
  >>> def echo(x):
  ...     print 'New PV value:', x
  >>> c.subscribe('echo', echo)
  >>> c.startMonitor()
  >>> New PV value: epics:nt/NTScalar:1.0
      float value 11.1

  New PV value: epics:nt/NTScalar:1.0
      float value 11.2

  New PV value: epics:nt/NTScalar:1.0
      float value 11.3

  >>> c.stopMonitor()

Advanced Usage: PVA Server Class

  1. In terminal 1, create a simple 'pair' channel:
  $ python 
  >>> pv = PvObject({'x': INT, 'y' : INT})
  >>> pvaServer = PvaServer('pair', pv)
  1. In terminal 2, start monitoring this channel:
  $ pvget -m pair
  1. In terminal 1, update one of the object's fields:
  >>> pv['x'] = 1

This change should appear in terminal 2.

Advanced Usage: RPC Client Class

  1. In a separate terminal, start the v4 test RPC service:
  $ cd $EPICS4_DIR/pvAccessCPP/bin/$EPICS_HOST_ARCH
  $ ./rpcServiceExample # in terminal 2
  1. RPC test channel is 'sum':
  >>> rpc = pvaccess.RpcClient('sum')
  >>> request = pvaccess.PvObject({'a': pvaccess.STRING, 'b': pvaccess.STRING})
  >>> request.set({'a': '11', 'b': '22' })
  >>> print request
      string a 11
      string b 22
  >>> response = rpc.invoke(request)
  >>> print response
      double c 33

Advanced Usage: RPC Server Class

Example 1

  1. In a separate terminal, source the environment file and start python:
  $ python # in terminal 2
  >>> import pvaccess
  >>> srv = pvaccess.RpcServer()
  >>> def echo(x):    # x is an instance of PvObject
  ...     print 'Got object: ', x
  ...     return x    # service must return an instance of PvObject
  >>> srv.registerService('echo', echo)
  >>> srv.listen()
  1. In terminal 1, reuse previous request object
  >>> rpc = pvaccess.RpcClient('echo')
  >>> response = rpc.invoke(request)
  >>> print response
      string a 11
      string b 22

Example 2

  1. In terminal 2:
  $ python
  >>> import pvaccess
  >>> srv = pvaccess.RpcServer()
  >>> def sum(x):
  ...     a = x.getInt('a')
  ...     b = x.getInt('b')
  ...     return pvaccess.PvInt(a+b)
  >>> srv.registerService('sum', sum)
  >>> srv.listen()
  1. In terminal 1:
  >>> rpc = pvaccess.RpcClient('sum')
  >>> request = pvaccess.PvObject({'a': pvaccess.INT, 'b': pvaccess.INT})
  >>> request.set({'a': 11, 'b': 22})
  >>> print request
      int a 11
      int b 22
  >>> response = rpc.invoke(request)
  >>> print response
      int value 33

Example 3

  1. In terminal 2:
  >>> import pvaccess
  >>> srv = pvaccess.RpcServer()
  >>> def hash(x):
  ...     import hashlib
  ...     md5 = hashlib.md5()
  ...     md5.update(str(x))
  ...     h = md5.hexdigest()
  ...     dict = x.getStructureDict()
  ...     dict['hash'] = pvaccess.STRING
  ...     response = pvaccess.PvObject(dict)
  ...     response.setString('hash', h)
  ...     return response
  >>> srv.registerService('hash', hash)
  >>> srv.listen()
  1. In terminal 1:
  >>> rpc = pvaccess.RpcClient('hash')
  >>> request = pvaccess.PvString('abcd')
  >>> print rpc.invoke(request)
      string hash 0a380e7375d8c3f68d1bbe068141d6ce
      string value