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 tango.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 tango.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 tango.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 tango.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
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.