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ALMAACS & Pythonnamespace where the enumeration is defined. Please do not use thenative Python copy methods on CORBA enumerations.3.4StructAn IDL struct definition is mapped into a Python class or type. Foreach field in the struct, there is a corresponding attribute in the classwith the same name as the field. The constructor of the class expectsthe field values, from left to right.(Constants)An IDL constant definition maps to a Python variable initialized withthe value of the constant.(Exceptions)An IDL exception is translated into a Python class derived fromCORBA.UserException. System exceptions are derived fromCORBA.SystemException. Both base classes are derived fromCORBA.Exception. The parameters of the exception are mapped in thesame way as the fields of a struct definition. When raising anexception, a new instance of the class is created and the constructorexpects the exception parameters.in, out, and inout Parametersin parameters are mapped as one would expect in Python. That is if some interface Icontains a method, void methodA(in string parameterX), a client would invoke it like this– I. methodA(“not interesting”). However, things get much more interesting with theother two cases.Let’s see another method of the I interface which incorporates in, out, and inoutparameters in addition to a return value:double methodB(in string parameterX, inout boolean isOK, out long someValue);A client would invoke methodB like this:(methodBReturnValue, boolOutValue, longValue) I.methodB(“ ”, 0);Now for the explanation – since every type in Python is an object which is really just apointer, the concept of references to objects does not really exist. In short, parameterspassed to Python functions and methods are complete copies of the objects (i.e., pass byvalue). To deal with this, the CORBA mapping for Python specifies that out parameterswill be returned via a tuple. The first object in this tuple is the return value if the methodhas one, followed by inout/out parameters in the order they were specified from left toright. To make this clearer, the implementation of methodB in a Python servant could bethe following:def methodB(self, parameterX, isOK):# 3.14 corresponds to the return double value# 1 is for ‘inout Boolean isOK’# 34 is ‘out long someValue’return (3.14, 1, 34)Revision: 4.1Page 11 o23

ALMA3.5ACS & PythonNarrowing CORBA Objects to their Real TypeBecause ALMA has chosen to use omniORBPy as our Python ORB, 99 out of 100 times this isnot necessary and the ORB does it automatically. The only time this needs to be done is when anIDL interface uses complex inheritance and the object needs to be narrowed to a base interface.If this is ever the case, all CORBA objects have a “ narrow(I)” method where I is the PythonCORBA stub class for the interface you want to narrow the object to.4Utility and Helper PackagesPrecise examples in this section have yet to be determined. In the meantime, you canlook at the test scripts located in the ALMA CVS repository which should be pretty self explanatory.4.1ACS Configuration DatabaseSee ACS/LGPL/CommonSoftware/acspy/test/acspyTestCDB.py in the ALMA CVSrepository.4.2ACS Error SystemSee the ACS Error System document ([R10]).4.3ACS Logging SystemSee g.py in the ALMACVS repository.Revision: 4.1Page 12 o23

ALMA4.4ACS & PythonACS Time SystemSee the ACS Time System document ([R11]).5Client TutorialAll ACS Python clients must use the Acspy.Clients.SimpleClient module. This moduledefines a single class, PySimpleClient, which implements the Client IDL interface and isresponsible for logging into manager and providing ACS services to the developer.Some PySimpleClient Methods/Attributes Available to DevelopersMethodavailableComponents()Revision: 4.1DescriptionGets a list of ComponentInfo structures (see maci.idl for a definition)about all known components.Page 13 o23

ALMAACS & PythonParameters: NoneReturns: a list consisting of ComponentInfostructures for every component manager knowsof or None if this method fails.Raises: ?getComponent(name)Get a component reference from manager.It’s important to note this method narrows the reference for thedeveloper and even imports the proper IDL Python stub.Parameters: name is the component’s nameReturns: a narrowed reference to the component or None ifthat reference cannot be obtained.Raises: ?releaseComponent(name)Release the component defined by name.Parameters: name is the component’s nameReturns: the number of clients/components attached to thecomponent or None on failure.Raises: ?token.hSecurity token for this client. If PySimpleClient does not provide accessto some manager method that is needed, you can always use thegetManager function of the ACSCorba module directly.An ExampleThe example depicted here consists of a client which logs into manager, accesses anACS::CharacteristicComponent, and then creates a monitor for one of that component’sBACI properties. Please note error handling has been omitted.1:#!/usr/bin/env python2:3:from Acspy.Clients.SimpleClient import PySimpleClientRevision: 4.1Page 14 o23

ALMA4:5:6:7:8:9:10:11:12:13:14:ACS & Pythonimport ACS, ACS POA# class MyMonitor(ACS POA.CBdouble):def working(self, value, completion, desc):print "Working the value is: ", str(value)def done(self, value, completion, desc):print "Done. The value is: ", str(value)def negotiate (self, time to transmit, desc):return 1# simpleClient PySimpleClient()15:16:17:mount simpleClient.getComponent("MOUNT1")actAzProperty mount. get actAz()18:19:20:21:22:cbMon MyMonitor()cbMonServant simpleClient.activateOffShoot(cbMon)desc ACS.CBDescIn(0L, 0L, 0L) # Create the real monitor registered with MOUNT1actMon actAzProperty.create monitor(cbMonServant, desc)23:24:25:26:27:actMon.set timer trigger(10000000)from time import t()3PySimpleClient is the standard Python client capable of logging into manager.4Import the Python CORBA stubs for the BACI module. These are only necessary becausecallbacks are being used.6 15Since an asynchronous monitor will be created for a BACI double property, an implementationof the double callback IDL interface must be defined. The Acspy.Common.Callbacks modulecould have been used, but where’s the fun in that?6Our class must be derived from ACS POA.CBdouble which is generated by the IDL toPython compiler. MyMonitor is actually a servant!7 8The working method is invoked when a value changes for on change monitors or when thetiming trigger has been executed. Developers override this method to do the real monitoringof data values. The value parameter is the real data value (completion and desc parameters arenot too useful except for timestamps).9 10done is essentially identical to the working method except for the fact that done is invokedwhen the monitor is destroyed.Revision: 4.1Page 15 o23

ALMAACS & Python11 12The only useful thing to know about negotiate is that it should normally return a true value(i.e., 1). For developers really interested in understanding this method, please see the BACISpecifications ([R08]).14Standard way of instantiating a client which logs into manager to access common services andcomponents.16getComponent method of PySimpleClient is quite literally used to get a named component,activating it if necessary. Please take special note we never even had to import the CORBAPython stubs for the MOUNT1 component. PySimpleClient does this automatically!17Under the Python mapping, references to attributes of interfaces are obtained by prepending“ get ” to the attribute’s name and invoking that method on the interface.19 20An instance of the callback class is created. On line 20, the subclass becomes a CORBAobject by utilizing the activateOffShoot ContainerService method.21Explaining CBDescIn and similar IDL structs/interfaces is beyond the scope of this document.This line of code can be simply copied without really understanding what it does. Feel free toreview the BACI Specifications document ([R08]) if you’re really interested though.24Override the default monitor time using this method. It now monitors the property once persecond.25 27Allow the monitor to run for ten seconds and then destroy it.29Since we’re done with this component, release it. Depending on whether or not otherclients/components reference MOUNT1, it may or may not be deactivated.30Must always disconnect from manager when we’re through.6Servant TutorialACS provides a full implementation of the Container IDL interface in Python implyingthat any IDL interface derived from ACS::ACSComponent can be implemented inPython. Furthermore, specific support classes have been placed in the Acspy.Servantspackage to make the developers life easier. In this section, the process of creating aPython CORBA servant is covered from the ground up.Developing an IDL InterfaceRegardless of servant implementation language, the first thing that needs to be done is todefine an IDL interface. It’s assumed the developer has some background in CORBAand IDL so we will not go into too much detail here. Besides the guidelines presented inthe IDL to Python mapping section, there is only one rule which must be followed:developer defined IDL interfaces must be derived from the ACS::ACSComponentinterface in some form. This stipulation is imposed on all programming languages ACSRevision: 4.1Page 16 o23

ALMAACS & Pythonsupports! Now an example IDL will be presented which will be used throughout the restof the servant tutorial.1:2:#ifndef HELLODEMO IDL#define HELLODEMO IDL3:4:#include acscomponent.idl 5:6:#pragma prefix "alma"7:8:9:10:11:12:13:14:15:16:17:18:module demo{interface HelloDemo : ACS::ACSComponent{string sayHello();string sayHelloWithParameters(in string inString,inout double inoutDouble,out long outInt);};};#endif1, 2,& 13It is critical that IDL files use include guards.4Used to access the ACSComponent interface.6This line is standard for all ALMA IDL interfaces.8The module’s name is arbitrary except that it should not conflict with Python package names(i.e., “module sys” or “module Acspy” are extremely bad names).10Choose any name you want for the interface. Just make sure it is derived fromACSComponent.Revision: 4.1Page 17 o23

ALMA12 15ACS & PythonTwo methods are defined for this simple example. Both return strings and the differencebetween the two is that one uses parameters.The Servants PackageServant helper classes can be found in Acspy.Servants. They are designed to provideservices to servants, give the developer an implementation of the ACSComponentinterface, as well as provide methods which only the container can invoke when thecomponent is started, stopped, or ends up in some error state (LifeCycle interface). Fordetails, see the pydoc.A Python Implementation for the IDL InterfaceSo far an IDL interface has been defined in the “demo” module. Well the Pythonmapping specifies the IDL compiler will generate a Python package for that IDL moduleand name it “demo POA”. By convention, we will create a Python package which justconsists of “Impl” appended to the IDL module’s name. In our case, this will be the“demoImpl” package. Also by convention, we place the real implementation of the IDLinterface inside a Python module of an identical name. This must sound pretty confusing,but this class diagram should hopefully help some:Revision: 4.1Page 18 o23

ALMAACS & PythonNow the src directory you’re working in should hopefully contain the followingdeveloper defined Python package demoImpl/HelloDemo.py. Now we can finally seewhat the real implementation looks like:1:2:3:4:5:6:7:8:9:10:11:12:13:14:import demo POAfrom Acspy.Servants.ContainerServicesimport ContainerServicesfrom Acspy.Servants.ComponentLifecycle import ComponentLifecyclefrom Acspy.Servants.ACSComponentimport ACSComponent# class HelloDemo(demo nentLifecycle):# def init (self):ACSComponent. init (self)ContainerServices. init (self)# Override ComponentLifecycle methods Revision: 4.1Page 19 o23

ALMAACS & Pythondef initialize(self):try:lamp self.getComponent("LAMP1")self.brightness lamp. get brightness()except Exception, e:print "LAMP1 unavailable:", e# def cleanUp(self):self.releaseComponent("LAMP1")# Implementation of IDL methods def sayHello(self):return "hello"# def sayHelloWithParameters(self, inString, inoutDouble):self.getLogger().logInfo("sayHello called with arguments inString " inString "; inoutDouble " str(inoutDouble) ". Will return 'hello'.")return ("hello", inoutDouble, :31:32:33:34:35:36:37:38:39:# Main defined only for generic testing if name " main ":g HelloDemo()1Must import the package created by the IDL compiler for the IDL interface we will bedeveloping a servant implementation for.2 4ACS servant helper modules are imported.5 35Class HelloDemo is the concrete implementation of the HelloDemo IDL interface. It should benoted that the container requires the implementing Python class to have the same name as theIDL interface (i.e., HelloDemo).6The implementing class must be derived from the CORBA generated class defining theinterface’s methods and attributes.7ACSComponent provides an implementation of the ACS::ACSComponent interface and it ismandatory that components be derived from this class.8Derivation from ContainerServices is highly recommended as this class provides access toother components, ACS services, etc.9The ComponentLifecycle class contains methods (to be overridden) that are only accessible bythe Container. Components should not invoke ComponentLifecycle methods on themselves.Revision: 4.1Page 20 o23

ALMAACS & Python10 13If a developer defines a constructor for the servant implementation, it must callACSComponent and ContainerService’s constructors!14 23Two ComponentLifecycle methods are overridden and show usage of a couple ofContainerServices methods. These are called at object creation and destruction times by thecontainer.15 20In the initialize method, a reference to the “LAMP1” component is obtained and from that weget the “brightness” BACI property.22 23A properly written servant/client will always release other components when done using them.24 35Implementations of the various IDL methods that were defined for the HelloDemo interface.While this should be pretty straightforward, please review the section on parameter ordering inthe Python mapping chapter if any of this seems confusing.37 39Its good practice to ensure your component can at least be instantiated as a Python object.These three lines of code have saved the author an incredible amount of debugging time andshould prove valuable to other developers.Unlike C and Java servants, there are no macros or helper classes that must be created by thedeveloper to make an instance of the servant. In essence, these 38 lines consist of the onlyPython code that needs to be created!Editing the Configuration DatabaseSpecific instructions on creating an ACS Configuration Database can be found in variousother documents, so we will assume only modifications to an existing CDB need to bemade. There is only one file that needs to be modified ACS CDB/CDB/MACI/Components/Components.xml. Here, we just add one entry ofthe “ ” element per component we wish to create:1: Name "HELLODEMO1"2:Code "demoImpl.HelloDemo"3:Type "IDL:alma/demo/HelloDemo:1.0"4:Container "aragornContainer"/ 1Name of the component.2Name of the Python package containing an implementation of the IDL interface defined online 3.Revision: 4.1Page 21 o23

ALMA34ACS & PythonThe interface repository location of the servant’s IDL interface determined from the IDL fileitself. In this case: alma corresponds to #pragma prefix “alma” demo comes from module demo HelloDemo is the IDL interface’s name. For all intensive purposes, the version number will always be 1.0.The name of the Python container responsible for the lifecyle of this component.Makefile SupportThere are several Makefile targets dealing with IDL files, Python packages, etc.developers should familiarize themselves with:TargetIDL FILESDescriptionList of IDL files (without their ‘.idl’ postfix) separated by spaces thatcan be found in the ./idl directory. Specifying this target means thePython CORBA stubs will be created when doing a ‘make all’ from thesr

language. Python is great for scripting, prototyping, web development, and high level applications. It's also a wonderful language for non programmers to use because of its simplicity. Most importantly for ALMA, OMG has defined an IDL to Python CORBA mapping and several vendors offer free ORBs. This implies that anything done in C