TEP 1 - Device Server High Level API¶

TEP:	1
Title:	Device Server High Level API
Version:	2.2.0
Last-Modified:	10-Sep-2014
Author:	Tiago Coutinho <tcoutinho@cells.es>
Status:	Active
Type:	Standards Track
Content-Type:	text/x-rst
Created:	17-Oct-2012

Abstract¶

This TEP aims to define a new high level API for writting device servers.

Rationale¶

The code for Tango device servers written in Python often obey a pattern. It would be nice if non tango experts could create tango device servers without having to code some obscure tango related code. It would also be nice if the tango programming interface would be more pythonic. The final goal is to make writting tango device servers as easy as:

class Motor(Device):
    __metaclass__ = DeviceMeta

    position = attribute()

    def read_position(self):
        return 2.3

    @command()
    def move(self, position):
        pass

if __name__ == "__main__":
    server_run((Motor,))

Places to simplify¶

After looking at most python device servers one can see some patterns:

At <Device> class level:

<Device> always inherits from latest available DeviceImpl from pogo version

constructor always does the same:

calls super constructor

debug message

calls init_device

all methods have debug_stream as first instruction

init_device does additionaly get_device_properties()
read attribute methods follow the pattern:
def read_Attr(self, attr):
  self.debug_stream()
  value = get_value_from_hardware()
  attr.set_value(value)
write attribute methods follow the pattern:
def write_Attr(self, attr):
  self.debug_stream()
  w_value = attr.get_write_value()
  apply_value_to_hardware(w_value)

At <Device>Class class level:

A <Device>Class class exists for every <DeviceName> class

The <Device>Class class only contains attributes, commands and properties descriptions (no logic)

The attr_list description always follows the same (non explicit) pattern (and so does cmd_list, class_property_list, device_property_list)

the syntax for attr_list, cmd_list, etc is far from understandable

At main() level:

The main() method always does the same:

create Util

register tango class

when registering a python class to become a tango class, 99.9% of times the python class name is the same as the tango class name (example: Motor is registered as tango class “Motor”)

call server_init()

call server_run()

High level API¶

The goals of the high level API are:

Maintain all features of low-level API available from high-level API¶

Everything that was done with the low-level API must also be possible to do with the new API.

All tango features should be available by direct usage of the new simplified, cleaner high-level API and through direct access to the low-level API.

Automatic inheritance from the latest** `DeviceImpl`¶

Currently Devices need to inherit from a direct Tango device implementation (DeviceImpl, or Device_2Impl, Device_3Impl, Device_4Impl, etc) according to the tango version being used during the development.

In order to keep the code up to date with tango, every time a new Tango IDL is released, the code of every device server needs to be manually updated to ihnerit from the newest tango version.

By inheriting from a new high-level Device (which itself automatically decides from which DeviceImpl version it should inherit), the device servers are always up to date with the latest tango release without need for manual intervention (see PyTango.server).

Low-level way:

class Motor(PyTango.Device_4Impl):
    pass

High-level way:

class Motor(PyTango.server.Device):
    pass

Default implementation of `Device` constructor¶

99% of the different device classes which inherit from low level DeviceImpl only implement __init__ to call their init_device (see PyTango.server).

Device already calls init_device.

Low-level way:

class Motor(PyTango.Device_4Impl):

    def __init__(self, dev_class, name):
        PyTango.Device_4Impl.__init__(self, dev_class, name)
        self.init_device()

High-level way:

class Motor(PyTango.server.Device):

    # Nothing to be done!

    pass

Default implementation of `init_device()`¶

99% of different device classes which inherit from low level DeviceImpl have an implementation of init_device which at least calls get_device_properties() (see PyTango.server).

init_device() already calls get_device_properties().

Low-level way:

class Motor(PyTango.Device_4Impl):

    def init_device(self):
        self.get_device_properties()

High-level way:

class Motor(PyTango.server.Device):
    # Nothing to be done!
    pass

Remove the need to code `DeviceClass`¶

99% of different device servers only need to implement their own subclass of DeviceClass to register the attribute, commands, device and class properties by using the corresponding attr_list, cmd_list, device_property_list and class_property_list.

With the high-level API we completely remove the need to code the DeviceClass by registering attribute, commands, device and class properties in the Device with a more pythonic API (see PyTango.server)

Hide <Device>Class class completely
simplify main()

Low-level way:

class Motor(PyTango.Device_4Impl):

    def read_Position(self, attr):
        pass

class MotorClass(PyTango.DeviceClass):

    class_property_list = { }
    device_property_list = { }
    cmd_list = { }

    attr_list = {
        'Position':
            [[PyTango.DevDouble,
            PyTango.SCALAR,
            PyTango.READ]],
        }

    def __init__(self, name):
        PyTango.DeviceClass.__init__(self, name)
        self.set_type(name)

High-level way:

class Motor(PyTango.server.Device):

    position = PyTango.server.attribute(dtype=float, )

    def read_position(self):
        pass

Pythonic read/write attribute¶

With the low level API, it feels strange for a non tango programmer to have to write:

def read_Position(self, attr):
    # ...
    attr.set_value(new_position)

def read_Position(self, attr):
    # ...
    attr.set_value_date_quality(new_position, time.time(), AttrQuality.CHANGING)

A more pythonic away would be:

def read_position(self):
    # ...
    self.position = new_position

def read_position(self):
    # ...
    self.position = new_position, time.time(), AttrQuality.CHANGING

Or even:

def read_position(self):
    # ...
    return new_position

def read_position(self):
    # ...
    return new_position, time.time(), AttrQuality.CHANGING

Simplify main()¶

the typical main() method could be greatly simplified. initializing tango, registering tango classes, initializing and running the server loop and managing errors could all be done with the single function call to server_run()

Low-level way:

def main():
    try:
        py = PyTango.Util(sys.argv)
        py.add_class(MotorClass,Motor,'Motor')

        U = PyTango.Util.instance()
        U.server_init()
        U.server_run()

    except PyTango.DevFailed,e:
        print '-------> Received a DevFailed exception:',e
    except Exception,e:
        print '-------> An unforeseen exception occured....',e

High-level way:

def main():
    classes = Motor,
    PyTango.server_run(classes)

In practice¶

Currently, a pogo generated device server code for a Motor having a double attribute position would look like this:

#!/usr/bin/env python
# -*- coding:utf-8 -*-


##############################################################################
## license :
##============================================================================
##
## File :        Motor.py
##
## Project :
##
## $Author :      t$
##
## $Revision :    $
##
## $Date :        $
##
## $HeadUrl :     $
##============================================================================
##            This file is generated by POGO
##    (Program Obviously used to Generate tango Object)
##
##        (c) - Software Engineering Group - ESRF
##############################################################################

""""""

__all__ = ["Motor", "MotorClass", "main"]

__docformat__ = 'restructuredtext'

import PyTango
import sys
# Add additional import
#----- PROTECTED REGION ID(Motor.additionnal_import) ENABLED START -----#

#----- PROTECTED REGION END -----#  //      Motor.additionnal_import

##############################################################################
## Device States Description
##
## No states for this device
##############################################################################

class Motor (PyTango.Device_4Impl):

#--------- Add you global variables here --------------------------
#----- PROTECTED REGION ID(Motor.global_variables) ENABLED START -----#

#----- PROTECTED REGION END -----#  //      Motor.global_variables
#------------------------------------------------------------------
#    Device constructor
#------------------------------------------------------------------
    def __init__(self,cl, name):
        PyTango.Device_4Impl.__init__(self,cl,name)
        self.debug_stream("In " + self.get_name() + ".__init__()")
        Motor.init_device(self)

#------------------------------------------------------------------
#    Device destructor
#------------------------------------------------------------------
    def delete_device(self):
        self.debug_stream("In " + self.get_name() + ".delete_device()")
        #----- PROTECTED REGION ID(Motor.delete_device) ENABLED START -----#

        #----- PROTECTED REGION END -----#  //      Motor.delete_device

#------------------------------------------------------------------
#    Device initialization
#------------------------------------------------------------------
    def init_device(self):
        self.debug_stream("In " + self.get_name() + ".init_device()")
        self.get_device_properties(self.get_device_class())
        self.attr_Position_read = 0.0
        #----- PROTECTED REGION ID(Motor.init_device) ENABLED START -----#

        #----- PROTECTED REGION END -----#  //      Motor.init_device

#------------------------------------------------------------------
#    Always excuted hook method
#------------------------------------------------------------------
    def always_executed_hook(self):
        self.debug_stream("In " + self.get_name() + ".always_excuted_hook()")
        #----- PROTECTED REGION ID(Motor.always_executed_hook) ENABLED START -----#

        #----- PROTECTED REGION END -----#  //      Motor.always_executed_hook

#==================================================================
#
#    Motor read/write attribute methods
#
#==================================================================

#------------------------------------------------------------------
#    Read Position attribute
#------------------------------------------------------------------
    def read_Position(self, attr):
        self.debug_stream("In " + self.get_name() + ".read_Position()")
        #----- PROTECTED REGION ID(Motor.Position_read) ENABLED START -----#
        self.attr_Position_read = 1.0
        #----- PROTECTED REGION END -----#  //      Motor.Position_read
        attr.set_value(self.attr_Position_read)

#------------------------------------------------------------------
#    Read Attribute Hardware
#------------------------------------------------------------------
    def read_attr_hardware(self, data):
        self.debug_stream("In " + self.get_name() + ".read_attr_hardware()")
        #----- PROTECTED REGION ID(Motor.read_attr_hardware) ENABLED START -----#

        #----- PROTECTED REGION END -----#  //      Motor.read_attr_hardware


#==================================================================
#
#    Motor command methods
#
#==================================================================


#==================================================================
#
#    MotorClass class definition
#
#==================================================================
class MotorClass(PyTango.DeviceClass):

    #    Class Properties
    class_property_list = {
        }


    #    Device Properties
    device_property_list = {
        }


    #    Command definitions
    cmd_list = {
        }


    #    Attribute definitions
    attr_list = {
        'Position':
            [[PyTango.DevDouble,
            PyTango.SCALAR,
            PyTango.READ]],
        }


#------------------------------------------------------------------
#    MotorClass Constructor
#------------------------------------------------------------------
    def __init__(self, name):
        PyTango.DeviceClass.__init__(self, name)
        self.set_type(name);
        print "In Motor Class  constructor"

#==================================================================
#
#    Motor class main method
#
#==================================================================
def main():
    try:
        py = PyTango.Util(sys.argv)
        py.add_class(MotorClass,Motor,'Motor')

        U = PyTango.Util.instance()
        U.server_init()
        U.server_run()

    except PyTango.DevFailed,e:
        print '-------> Received a DevFailed exception:',e
    except Exception,e:
        print '-------> An unforeseen exception occured....',e

if __name__ == '__main__':
    main()

To make things more fair, let’s analyse the stripified version of the code instead:

import PyTango
import sys

class Motor (PyTango.Device_4Impl):

    def __init__(self,cl, name):
        PyTango.Device_4Impl.__init__(self,cl,name)
        self.debug_stream("In " + self.get_name() + ".__init__()")
        Motor.init_device(self)

    def delete_device(self):
        self.debug_stream("In " + self.get_name() + ".delete_device()")

    def init_device(self):
        self.debug_stream("In " + self.get_name() + ".init_device()")
        self.get_device_properties(self.get_device_class())
        self.attr_Position_read = 0.0

    def always_executed_hook(self):
        self.debug_stream("In " + self.get_name() + ".always_excuted_hook()")

    def read_Position(self, attr):
        self.debug_stream("In " + self.get_name() + ".read_Position()")
        self.attr_Position_read = 1.0
        attr.set_value(self.attr_Position_read)

    def read_attr_hardware(self, data):
        self.debug_stream("In " + self.get_name() + ".read_attr_hardware()")


class MotorClass(PyTango.DeviceClass):

    class_property_list = {
        }


    device_property_list = {
        }


    cmd_list = {
        }


    attr_list = {
        'Position':
            [[PyTango.DevDouble,
            PyTango.SCALAR,
            PyTango.READ]],
        }

    def __init__(self, name):
        PyTango.DeviceClass.__init__(self, name)
        self.set_type(name);
        print "In Motor Class  constructor"


def main():
    try:
        py = PyTango.Util(sys.argv)
        py.add_class(MotorClass,Motor,'Motor')

        U = PyTango.Util.instance()
        U.server_init()
        U.server_run()

    except PyTango.DevFailed,e:
        print '-------> Received a DevFailed exception:',e
    except Exception,e:
        print '-------> An unforeseen exception occured....',e

if __name__ == '__main__':
    main()

And the equivalent HLAPI version of the code would be:

#!/usr/bin/env python

from PyTango import DebugIt, server_run
from PyTango.server import Device, DeviceMeta, attribute


class Motor(Device):
    __metaclass__ = DeviceMeta

    position = attribute()

    @DebugIt()
    def read_position(self):
        return 1.0

def main():
    server_run((Motor,))

if __name__ == "__main__":
    main()

References¶

PyTango.server

Changes¶

from 2.1.0 to 2.2.0¶

Changed module name from hlapi to server

from 2.0.0 to 2.1.0¶

Changed module name from api2 to hlapi (High Level API)

From 1.0.0 to 2.0.0¶

API Changes
- changed Attr to attribute
- changed Cmd to command
- changed Prop to device_property
- changed ClassProp to class_property
Included command and properties in the example
Added references to API documentation

Copyright¶

This document has been placed in the public domain.

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TEP 1 - Device Server High Level API¶

Abstract¶

Rationale¶

Places to simplify¶

High level API¶

Maintain all features of low-level API available from high-level API¶

Automatic inheritance from the latest** `DeviceImpl`¶

Default implementation of `Device` constructor¶

Default implementation of `init_device()`¶

Remove the need to code `DeviceClass`¶

Pythonic read/write attribute¶

Simplify main()¶

In practice¶

References¶

Changes¶

from 2.1.0 to 2.2.0¶

from 2.0.0 to 2.1.0¶

From 1.0.0 to 2.0.0¶

Copyright¶

TEP 1 - Device Server High Level API¶

Abstract¶

Rationale¶

Places to simplify¶

High level API¶

Maintain all features of low-level API available from high-level API¶

Automatic inheritance from the latest** DeviceImpl¶

Default implementation of Device constructor¶

Default implementation of init_device()¶

Remove the need to code DeviceClass¶

Pythonic read/write attribute¶

Simplify main()¶

In practice¶

References¶

Changes¶

from 2.1.0 to 2.2.0¶

from 2.0.0 to 2.1.0¶

From 1.0.0 to 2.0.0¶

Copyright¶

Automatic inheritance from the latest** `DeviceImpl`¶

Default implementation of `Device` constructor¶

Default implementation of `init_device()`¶

Remove the need to code `DeviceClass`¶