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Using ILU with Python
*********************

Introduction
============

   This tutorial will show how to use the ILU system with the
programming language Python, both as a way of developing software
libraries, and as a way of building distributed systems.  In an
extended example, we'll build an ILU module that implements a simple
four-function calculator, capable of addition, subtraction,
multiplication, and division.  It will signal an error if the user
attempts to divide by zero.  The example demonstrates how to specify
the interface for the module; how to implement the module in Python;
how to use that implementation as a simple library; how to provide the
module as a remote service; how to write a client of that remote
service; and how to use subtyping to extend an object type and provide
different versions of a module.  We'll also demonstrate how to use OMG
IDL with ILU, and discuss the notion of network garbage collection.

   Each of the programs and files referenced in this tutorial is
available as a complete program in a separate appendix to this
document; parts of programs are quoted in the text of the tutorial.

Specifying the Interface
========================

   Our first task is to specify more exactly what it is we're trying to
provide.  A typical four-function calculator lets a user enter a value,
then press an operation key, either +, -, /, or *, then enter another
number, then press = to actually have the operation happen.  There's
usually a CLEAR button to press to reset the state of the calculator.
We want to provide something like that.

   We'll recast this a bit more formally as the interface of our
module; that is, the way the module will appear to clients of its
functionality.  The interface typically describes a number of function
calls which can be made into the module, listing their arguments and
return types, and describing their effects.  ILU uses object-oriented
interfaces, in which the functions in the interface are grouped into
sets, each of which applies to an object type.  These functions are
called methods.

   For example, we can think of the calculator as an object type, with
several methods:  Add, Subtract, Multiply, Divide, Clear, etc.  ILU
provides a standard notation to write this down with, called ISL (which
stands for "Interface Specification Language").  ISL is a declarative
language which can be processed by computer programs.  It allows you to
define object types (with methods), other non-object types, exceptions,
and constants.

   The interface for our calculator would be written in ISL as:
     INTERFACE Tutorial;
     
     EXCEPTION DivideByZero;
     
     TYPE Calculator = OBJECT
       METHODS
         SetValue (v : REAL),
         GetValue () : REAL,
         Add (v : REAL),
         Subtract (v : REAL),
         Multiply (v : REAL),
         Divide (v : REAL) RAISES DivideByZero END
       END;
   This defines an interface `Tutorial', an exception `DivideByZero',
and an object type `Calculator'.  Let's consider these one by one.

   The interface, `Tutorial', is a way of grouping a number of type and
exception definitions.  This is important to prevent collisions between
names defined by one group and names defined by another group.  For
example, suppose two different people had defined two different object
types, with different methods, but both called `Calculator'!  It would
be impossible to tell which calculator was meant.  By defining the
`Calculator' object type within the scope of the `Tutorial' interface,
this confusion can be avoided.

   The exception, `DivideByZero', is a formal name for a particular
kind of error, division by zero.  Exceptions in ILU can specify an
exception-value type, as well, which means that real errors of that
kind have a value of the exception-value type associated with them.
This allows the error to contain useful information about why it might
have come about.  However, `DivideByZero' is a simple exception, and
has no exception-value type defined.  We should note that the full name
of this exception is `Tutorial.DivideByZero', but for this tutorial
we'll simply call our exceptions and types by their short name.

   The object type, `Calculator' (again, really `Tutorial.Calculator'),
is a set of six methods.  Two of those methods, `SetValue' and
`GetValue', allow us to enter a number into the calculator object, and
"read" the number.  Note that `SetValue' takes a single argument, V, of
type `REAL'.  `REAL' is a built-in ISL type, denoting a 64-bit floating
point number.  Built-in ISL types are things like `INTEGER' (32-bit
signed integer), `BYTE' (8-bit unsigned byte), and `CHARACTER' (16-bit
Unicode character).  Other more complicated types are built up from
these simple types using ISL type constructors, such as `SEQUENCE OF',
`RECORD', or `ARRAY OF'.

   Note also that `SetValue' does not return a value, and neither do
`Add', `Subtract', `Multiply', or `Divide'.  Rather, when you want to
see what the current value of the calculator is, you must call
`GetValue', a method which has no arguments, but which returns a `REAL'
value, which is the value of the calculator object.  This is an
arbitrary decision on our part; we could have written the interface
differently, say as
     TYPE NotOurCalculator = OBJECT
       METHODS
         SetValue () : REAL,
         Add (v : REAL) : REAL,
         Subtract (v : REAL) : REAL,
         Multiply (v : REAL) : REAL,
         Divide (v : REAL) : REAL RAISES DivideByZero END
       END;

- but we didn't.

   Our list of methods on `Calculator' is bracketed by the two keywords
`METHODS' and `END', and the elements are separated from each other by
commas.  This is pretty standard in ISL: elements of a list are
separated by commas; the keyword `END' is used when an explicit
list-end marker is needed (but not when it's not necessary, as in the
list of arguments to a method); the list often begins with some
keyword, like `METHODS'.  The raises clause (the list of exceptions
which a method might raise) of the method `Divide' provides another
example of a list, this time with only one member, introduced by the
keyword `RAISES'.

   Another standard feature of ISL is separating a name, like `v', from
a type, like `REAL', with a colon character.  For example, constants
are defined with syntax like
     CONSTANT Zero : INTEGER = 0;

Definitions, of interface, types, constants, and exceptions, are
terminated with a semicolon.

   We should expand our interface a bit by adding more documentation on
what our methods actually do.  We can do this with the docstring
feature of ISL, which allows the user to add arbitrary text to object
type definitions and method definitions.  Using this, we can write
     INTERFACE Tutorial;
     
     EXCEPTION DivideByZero
       "this error is signalled if the client of the Calculator calls
     the Divide method with a value of 0";
     
     TYPE Calculator = OBJECT
       COLLECTIBLE
       DOCUMENTATION "4-function calculator"
       METHODS
         SetValue (v : REAL) "Set the value of the calculator to `v'",
         GetValue () : REAL  "Return the value of the calculator",
         Add (v : REAL)      "Adds `v' to the calculator's value",
         Subtract (v : REAL) "Subtracts `v' from the calculator's value",
         Multiply (v : REAL) "Multiplies the calculator's value by `v'",
         Divide (v : REAL) RAISES DivideByZero END
           "Divides the calculator's value by `v'"
       END;

Note that we can use the `DOCUMENTATION' keyword on object types to add
documentation about the object type, and can simply add documentation
strings to the end of exception and method definitions.  These
docstrings are passed on to the Python docstring system, so that they
are available at runtime from Python.  Documentation strings cannot
currently be used for non-object types.

   ILU provides a program, `islscan', which can be used to check the
syntax of an ISL specification.  `islscan' parses the specification and
summarizes it to standard output:
     % islscan Tutorial.isl
     Interface "Tutorial", imports "ilu"
       {defined on line 1
        of file /tmp/tutorial/Tutorial.isl (Fri Jan 27 09:41:12 1995)}
     
     Types:
       real                       {<built-in>, referenced on 10 11 12 13 14 15}
     
     Classes:
       Calculator                 {defined on line 17}
         methods:
           SetValue (v : real);                          {defined 10, id 1}
             "Set the value of the calculator to `v'"
           GetValue () : real;                           {defined 11, id 2}
             "Return the value of the calculator"
           Add (v : real);                               {defined 12, id 3}
             "Adds `v' to the calculator's value"
           Subtract (v : real);                          {defined 13, id 4}
             "Subtracts `v' from the calculator's value"
           Multiply (v : real);                          {defined 14, id 5}
             "Multiplies the calculator's value by `v'"
           Divide (v : real) {DivideByZero};             {defined 16, id 6}
             "Divides the calculator's value by `v'"
         documentation:
           "4-function calculator"
         unique id:  ilu:cigqcW09P1FF98gYVOhf5XxGf15
     
     Exceptions:
       DivideByZero               {defined on line 5, refs 15}
     %

`islscan' simply lists the types defined in the interface, separating
out object types (which it calls "classes"), the exceptions, and the
constants.  Note that for the `Calculator' object type, it also lists
something called its unique id.  This is a 160-bit number (expressed in
base 64) that ILU assigns automatically to every type, as a way of
distinguishing them.  While it might interesting to know that it exists
(:-), the ILU user never has know what it is; `islscan' supplies it for
the convenience of the ILU implementors, who sometimes do have to know
it.

Implementing the True Module
============================

   After we've defined an interface, we then need to supply an
implementation of our module.  Implementations can be done in any
language supported by ILU.  Which language you choose often depends on
what sort of operations have to be performed in implementing the
specific functions of the module.  Different languages have specific
advantages and disadvantages in different areas.  Another consideration
is whether you wish to use the implementation mainly as a library, in
which case it should probably be done in the same language as the rest
of your applications, or mainly as a remote service, in which case the
specific implementation language is less important.

   We'll demonstrate an implementation of the `Calculator' object type
in Python, which is one of the most capable of all the ILU-supported
languages.  This is just a matter of defining a Python class,
corresponding to the `Tutorial.Calculator' type.  Before we do that,
though, we'll explain how the names and signatures of the Python
functions are arrived at.

What the Interface Looks Like in Python
---------------------------------------

   For every programming language supported by ILU, there is a standard
mapping defined from ISL to that programming language.  This mapping
defines what ISL type names, exception names, method names, and so on
look like in that programming language.

   The mapping for Python is straightforward.  For type names, such as
`Tutorial.Calculator', the Python name of the ISL type `Interface.Name'
is `Interface.Name', with any hyphens replaced by underscores.  That
is, the name of the interface in ISL becomes the name of the module in
Python.  So the name of our `Calculator' type in Python would be
`Tutorial.Calculator', which is really the name of a Python class.

   The Python mapping for a method name such as `SetValue' is the
method name, with any hyphens replaced by underscores.  The return type
of this Python method is whatever is specified in the ISL specification
for the method, or `None' if no type is specified.  The arguments for
the Python method are the same as specified in the ISL; their types are
the Python types corresponding to the ISL types, *except* that one
extra argument is added to the beginning of each Python version of an
ISL method; it is an instance of the object type on which the method is
defined.  An instance is simply a value of that type.  Thus the Python
method corresponding to our ISL `SetValue' would have the prototype
signature
        def SetValue (self, v):

Similarly, the signatures for the other methods, in Python, are
        def GetValue (self):
     
        def Add (self, v):
     
        def Subtract (self, v):
     
        def Multiply (self, v):
     
        def Divide (self, v):

Note that even though the `Divide' method can raise an exception, the
signature looks like those of the other methods.  This is because the
normal Python exception signalling mechanism is used to signal
exceptions back to the caller.  The mapping of exception names is
similar to the mapping used for types.  So the exception
`Tutorial.DivideByZero' would also have the name
`Tutorial.DivideByZero', in Python.

   One way to see what all the Python names for an interface look like
is to run the program `python-stubber'.  This program reads an ISL
file, and generates the necessary Python code to support that interface
in Python.  One of the files generated is `INTERFACE.py', which
contains the definitions of all the Python types for that interface.
     % python-stubber Tutorial.isl
     client stubs for interface "Tutorial" to Tutorial.py ...
     server stubs for interface "Tutorial" to Tutorial__skel.py ...
     %

Building the Implementation
---------------------------

   To provide an implementation of our interface, we subclass the
generated Python class for our `Calculator' class:

     # CalculatorImpl.py
     
     import Tutorial, Tutorial__skel
     
     class Calculator (Tutorial__skel.Calculator):
     
             def __init__ (self):
                     self.the_value = 0.0
     
             def SetValue (self, v):
                     self.the_value = v
     
             def GetValue (self):
                     return self.the_value
     
             def Add (self, v):
                     self.the_value = self.the_value + v
     
             def Subtract (self, v):
                     self.the_value = self.the_value - v
     
             def Multiply (self, v):
                     self.the_value = self.the_value * v
     
             def Divide (self, v):
                     try:
                             self.the_value = self.the_value / v
                     except ZeroDivisionError:
                             raise Tutorial.DivideByZero

   Each instance of a `CalculatorImpl.Calculator' object inherits from
`Tutorial__skel.Calculator', which in turn inherits from
`Tutorial.Calculator'.  Each has an instance variable called
`the_value', which maintains a running total of the `accumulator' for
that instance.  We can create an instance of a `Tutorial.Calculator'
object by simply calling `CalculatorImpl.Calculator()'.

   So, a very simple program to use the `Tutorial' module might be the
following:

     # simple1.py, a simple program that demonstrates the use of the
     #  Tutorial true module as a library.
     #
     # run this with the command "python simple1.py NUMBER [NUMBER...]"
     #
     
     import Tutorial, CalculatorImpl, string, sys
     
     # A simple program:
     #  1)  make an instance of Tutorial.Calculator
     #  2)  add all the arguments by invoking the Add method
     #  3)  print the resultant value.
     
     def main (argv):
     
             c = CalculatorImpl.Calculator()
             if not c:
                     error("Couldn't create calculator")
     
             # clear the calculator before using it
     
             c.SetValue (0.0)
     
             # now loop over the arguments, adding each in turn */
     
             for arg in argv[1:]:
                     v = string.atof(arg)
                     c.Add (v)
     
             # and print the result
     
             print "the sum is", c.GetValue()
             sys.exit(0)
     
     main(sys.argv)

This program would be compiled and run as follows:
     % python simple1.py 34.9 45.23111 12
     the sum is 92.13111
     %

This is a completely self-contained use of the `Tutorial'
implementation; when a method is called, it is the true method that is
invoked.  The use of ILU in this program adds some overhead in terms of
included code, but has almost the same performance as a version of this
program that does not use ILU.

Checking for Exceptions
-----------------------

   Suppose, instead of the `Add' method, we'd called the `Divide'
method.  In that case, we might have had to handle a `DivideByZero'
exception; that is, notice the exception and do something sensible.  We
do this by establishing a handler for the exception:
             ...
             # now loop over the arguments, Dividing by each in turn */
     
             try:
                     for arg in argv[2:]:
                             v = string.atof(arg)
                             c.Divide (v)
             except:
                     print 'exception signalled:  ' + str(sys.exc_type)
                     sys.exit(1)
             ...

And here's an example of what we get when it runs:
     % python simple2.py 12345 6 7 8 9
     the sum is 4.08234126984
     % python simple2.py 12345 6 0 8 9
     exception signalled:  Tutorial: DivideByZero
   Actually, every method may return an exception, as there are a number
of standard system exceptions which may be signalled even by methods
which have no declared exceptions.  So we should check every method to
see if it succeeded, even simple ones like `GetValue'.

Providing the True Module as a Network Service
==============================================

   Now let's see what's involved in providing the calculator
functionality as a network service.  Basically, there are three things
to look at:
   * providing a "factory" to build calculator objects;

   * publishing the name of the factory; and

   * writing a server program.

Using Factories to Build Objects
--------------------------------

   When one program uses code from another address space, it has to get
its hands on an instance of an ILU object, to be able to call methods.
In our library application, we simply made a call into the true module,
to create an instance of the calculator object.  In the networked
world, we need to do the same kind of thing, but this time the call
into the true module has to be a method on an object type.  In short,
we need to have some object type which exports a method something like
       CreateCalculator () : Calculator

   There are several ways to provide this.  The standard way of doing
it is to add an object type to our `Tutorial' interface, which contains
this method.  This kind of object type is sometimes called a factory,
because it exists only in order to build instances of other object
types.  We'll add the following type definition to our `Tutorial.isl':
     TYPE Factory = OBJECT
       METHODS
         CreateCalculator () : Calculator
       END;

   Then we need to provide an implementation of the `Factory' object
type, just as we did with the `Calculator' type:

     import Tutorial, Tutorial__skel, CalculatorImpl
     
     class Factory (Tutorial__skel.Factory):
     
             # have the __init__ method take handle and server args
             # so that we can control which ILU kernel server is used,
             # and what the instance handle of the Factory object on
             # that server is.  This allows us to control the object ID
             # of the new Factory instance.
     
             def __init__(self, handle=None, server=None):
                     self.IluInstHandle = handle
                     self.IluServer = server
     
             def CreateCalculator (self):
                     return (CalculatorImpl.Calculator())

Now, to provide other programs a way of creating calculator objects,
we'll just create just one instance of `Tutorial.Factory', and let
programs call the `CreateCalculator' method on that at will, to obtain
new calculator objects.

Publishing a Well-Known Instance
--------------------------------

   The question then arises, how does a program that wants to use the
`Factory' object get its hands on that one well-known instance?  The
answer is to use the simple binding system built into ILU.  Simple
binding allows a program acting as a "server" to publish the location
of a well-known object, and allows programs acting as "clients" of that
server to look up the location, given the object's name.

   The name of an ILU object instance has two parts, which are the
instance handle of the object, and the name of its kernel server, called
the server ID.  (The kernel server is a data structure maintained by
the ILU kernel which takes care of all communication between different
address spaces.)  These two combined must form a universally unique ID
for the object.  Usually you can simply let the ILU system choose names
for your objects automatically, in which case it takes care to choose
names which will never conflict with names in use by others.  However,
for objects which we wish to publish, we need to specify what the name
of an object will be, so that users of the well-known object can find
it.

   When working with the Python programming language, this act of
explicitly specifying an object name is divided into two steps.  First,
we create a kernel server with a specified server ID.  Secondly, we
create an instance of an object on this new server, with a specified
instance handle.  Together, the server ID and the instance handle form
the name of the instance.

   For instance, we might use a server ID of `Tutorial.DOMAIN', where
DOMAIN is your Internet domain (typically something like
`DEPARTMENT.COMPANY.com', or `DEPARTMENT.UNIVERITY.edu').  This serves
to distinguish your server from other servers on the net.  Then we can
use a simple instance handle, like `theFactory'.  The name, or object
ID, of this object would then be `theFactory@Tutorial.DOMAIN', where
DOMAIN would vary from place to place.  Note that this implies that
only one instance of this object is going to exist in the whole domain.
If you have many people using different versions of this object in
your domain, you should introduce more qualifiers in the server ID so
that your kernel server can be distinguished from that run by others.

The Server Program
------------------

   Given this information, we can now write a complete program that will
serve as a provider of calculator objects to other programs.  It will
create a single `Factory' instance with a well-known name, publish that
instance, then hang out servicing methods invoked on its objects.
Here's what it looks like:
     # server.py -- a program that runs a Tutorial.Calculator server
     #
     
     import ilu, FactoryImpl, sys
     
     def main(argv):
     
             if (len(argv) < 2):
                     print "Usage:  python server.py SERVER-ID"
                     sys.exit(1)
     
             # Create a kernel server with appropriate server ID, which
             #  is passed in as the first argument
     
             theServer = ilu.CreateServer (argv[1])
     
             # Now create an instance of a Factory object on that server,
             #  with the instance handle "theFactory"
     
             theFactory = FactoryImpl.Factory ("theFactory", theServer)
     
             # Now make the Factory object "well-known" by publishing it.
     
             theFactory.IluPublish()
     
             # Now we print the string binding handle (the object's name plus
             # its location) of the new instance.
     
             print "Factory instance published."
             print "Its SBH is '" + theFactory.IluSBH() + "'."
     
             ilu.RunMainLoop ()
     
     
     main(sys.argv)

   When we run this program, we'll see something like:
     % ./server.py Tutorial.DEPT.COMPANY.COM &
     Factory instance published.
     Its SBH is 'theFactory@Tutorial.DEPT.COMPANY.COM@SOMEGIBBERISH'.
     %

This indicates that the object known as
`theFactory@Tutorial.DEPT.COMPANY.COM' is being exported in a
particular way, which is encoded in the SOMEGIBBERISH part of the
string binding handle.  Your specific numbers will vary, but it should
look similar.

Using the Network Service
=========================

   Given that someone has exported a module as a network service, by
publishing the location of a well-known instance of an object type, a
potential client of that module can then use the module by binding to
that well-known instance.  It does this by calling the standard ILU
routine `ilu.LookupObject()', which takes the name and type of an
instance, and attempts to find that instance on the net.  The name of
the object is specified as a pair of strings, the server ID of the
object's kernel server, and the instance handle of the object on that
kernel server.

   So, in our first example, we could replace the call to
`Create_Tutorial_Calculator' with a routine that calls
`ilu.LookupObject()' to find the factory, then creates an instance of a
`Calculator'.  The full code of the revised example, `simple3.py', is
available as *Note simple3.py::, but here's what the new code for
obtaining an instance of a `Calculator' looks like:
     def Get_Tutorial_Calculator (factoryObjectSID, factoryObjectIH):
     
             # We have to call ilu.LookupObject() with the object ID of
             # the factory object, and the ``type'' of the object we're looking
             # for, which is always available as MODULE.TYPENAME
     
             f = ilu.LookupObject (factoryObjectSID, factoryObjectIH, Tutorial.Factory)
             if not f:
                     print "Can't find Tutorial.Factory instance " + factoryObjectSID + factoryObjectIH
                     sys.exit(1)
             c = f.CreateCalculator()
             return (c)
   We then can use the `simple3' program:
     % python simple3.py Tutorial.DEPT.COMPANY.COM theFactory 1 2 3 4 5 6
     the sum is 2.10000
     %

Subtyping and Other ISL Types
=============================

   ILU ISL contains support for a number of types other than object
types and `REAL'.  The primitive ISL types include 16, 32, and 64 bit
signed and unsigned integers, bytes, 8 and 16 bit characters, a boolean
type, and 32, 64, and 128 bit floating point types.  A number of type
constructors allow specification of arrays, sequences, records, unions,
and enumerations, as well as object types.  The ISL `OPTIONAL' type
constructor provides an implicit union of some type with `NULL', which
is useful for building recursive data structures such as linked lists
or binary trees.

   To illustrate some of these types, we'll extend the
`Tutorial.Calculator' type.  Many real-world desktop calculators
include a register tape, a printed listing of all the operations that
have been performed, with a display of what the value of the calculator
was after each operation.  We'll add a register tape to
`Tutorial.Calculator'.

   We could do it by adding a new method to `Tutorial.Calculator',
called `GetTape'.  Unfortunately, this would break our existing code,
because it would change the `Tutorial.Calculator' object type, and
existing compiled clients wouldn't be able to recognize the new object
type.  Instead, we'll extend the object type by subtyping; that is, by
creating a new object type which uses `Tutorial.Calculator' as a
supertype, but adds new methods of its own.  This subtype will actually
have two types; both its own new type, and `Tutorial.Calculator'.
We'll also define a subtype of the `Tutorial.Factory' type, to allow us
to create new instances of the new `Calculator' subtype.  Finally,
we'll define a new module interface for the new types, so that we don't
have to modify the `Tutorial' interface.

   First, let's define the necessary type to represent the operations
performed on the calculator:

     INTERFACE Tutorial2 IMPORTS Tutorial END;
     
     TYPE OpType = ENUMERATION
         SetValue, Add, Subtract, Multiply, Divide END;
     
     TYPE Operation = RECORD
         op : OpType,
         value : REAL,
         accumulator : REAL
       END;
     
     TYPE RegisterTape = SEQUENCE OF Operation;
   The enumerated type `OpType' defines an abstract type with five
possible values.  The type `Operation' defines a record type (in Python,
a dictionary) with 3 fields:  the OP field, which tells us which of the
five possible calculator operations was performed, the VALUE field,
which tells us the value of the operand for the operation, and the
ACCUMULATOR field, which tells us what the value of the calculator was
after the operation had been performed.  Finally, the `Operation' type
is a simple sequence, or list, of `Operation'.  Note that `Tutorial2'
imports `Tutorial'; that is, it allows the use of the `Tutorial' types,
exceptions, and constants, in the specifications in `Tutorial2'.

   Now we define the new object types (in the same file):
     TYPE TapeCalculator = OBJECT COLLECTIBLE
       SUPERTYPES Tutorial.Calculator END
       DOCUMENTATION "4 function calculator with register tape"
       METHODS
         GetTape () : RegisterTape
       END;
     
     TYPE Factory = OBJECT SUPERTYPES Tutorial.Factory END
       METHODS
         CreateTapeCalculator () : TapeCalculator
       END;

The `SUPERTYPES' attribute of an object type may take multiple object
type names, so ISL supports multiple inheritance.  The
`Tutorial2.TapeCalculator' type will now support the six methods of
`Tutorial.Calculator', as well as its own method, `GetTape'.

   We then need to provide an implementation for
`Tutorial2.TapeCalculator' (see *Note TapeCalculatorImpl.py:: for the
actual code).  We modify each method on the `TapeCalculator' object to
record its invocation, and add a slot to hold the contents of the
`tape'.

   We also provide an implementation for `Tutorial2.Factory':
     import Tutorial2, Tutorial2__skel, TapeCalculatorImpl
     
     class Factory (Tutorial2__skel.Factory):
     
             # have the __init__ method take handle and server args
             # so that we can control which ILU kernel server is used,
             # and what the instance handle of the Factory object on
             # that server is.  This allows us to control the object ID
             # of the new Factory instance.
     
             def __init__(self, handle=None, server=None):
                     self.IluInstHandle = handle
                     self.IluServer = server
     
             def CreateCalculator (self):
                     return (TapeCalculatorImpl.TapeCalculator())
     
             CreateTapeCalculator = CreateCalculator

Note that both the `Tutorial2.Factory.CreateCalculator' and
`Tutorial2.Factory.CreateTapeCalculator' methods create and return
instances of `Tutorial2.TapeCalculator'.  This is valid, because
instances of `Tutorial2.TapeCalculator' are also instances of
`Tutorial.Calculator'.

   Now we modify `server.py' to create an instance of
`Tutorial2.Factory', instead of `Tutorial.Factory', and to initialize
the `Tutorial2' true-side code (See *Note server2.py:: for the actual
code).

   Finally, see *Note simple4.py::, for an example of a client of the
`TapeCalculator' type.

   Note that one nice result of this approach to versioning is that old
clients, which know nothing about the new `TapeCalculator' class, or
about the whole `Tutorial2' interface in general, will continue to
function, since every instance of `Tutorial2.TapeCalculator' is also an
instance of `Tutorial.Calculator', and every instance of
`Tutorial2.Factory' is also an instance of `Tutorial.Factory'.

Using OMG IDL with ILU
======================

   ILU also supports the use of the interface definition language OMG
IDL, defined by the Object Management Group (OMG) for their Common
Object Request Broker Architecture (CORBA).  OMG IDL uses a C++-like
syntax, so it may be easier for Python and C++ programmers to use than
ILU ISL.  Unfortunately, CORBA doesn't include some of the concepts in
ILU, such as garbage collection for transient objects, or `OPTIONAL'
types, so not every ILU interface can be expressed in OMG IDL, but many
of them can.  For example, here is the OMG IDL version of the
`Tutorial' interface:
     module Tutorial {
     
       exception DivideByZero {};
     
       interface Calculator {
         // Set the value of the calculator to `v'
         void SetValue (in double v);
         // Return the value of the calculator
         double GetValue ();
         // Adds `v' to the calculator's value
         void Add (in double v);
         // Subtracts `v' from the calculator's value
         void Subtract (in double v);
         // Multiplies the calculator's value by `v'
         void Multiply (in double v);
         // Divides the calculator's value by `v'
         void Divide (in double v) raises (DivideByZero);
       };
     
       interface Factory {
         // Create and return an instance of a Calculator object
         Calculator CreateCalculator();
       };
     };

   This can be used with the `python-stubber':

     % python-stubber Tutorial.idl
     client stubs for interface "Tutorial" to Tutorial.py ...
     server stubs for interface "Tutorial" to Tutorial__skel.py ...
     %

   This will be a bit slower than running the `python-stubber' on the
equivalent ISL file, as the program works by converting the OMG IDL
into ISL, then compiling from the ISL description.  OMG IDL interfaces
can be checked by running the OMG IDL-to-ILU ISL converter, `idl2isl',
directly:

     % idl2isl Tutorial.idl
     INTERFACE Tutorial;
     
     EXCEPTION DivideByZero;
     TYPE Calculator = OBJECT OPTIONAL
             METHODS
                     SetValue (v : REAL),
                     GetValue () : REAL,
                     Add (v : REAL),
                     Subtract (v : REAL),
                     Multiply (v : REAL),
                     Divide (v : REAL)
                             RAISES DivideByZero END
             END;
     TYPE Factory = OBJECT OPTIONAL
             METHODS
                     CreateCalculator () : Calculator
             END;
     %

   You will notice that the ISL interface generated by `idl2isl' is a
bit different, in that the object type modifier `OPTIONAL' is used in
the description of the `Calculator' and `Factory' types.  This is
because CORBA has the notion that any object type instance passed as a
parameter or return value (or field in an array, or element of a
sequence, etc.) may be `None', instead of being a valid instance
pointer.  Thus, when working with OMG IDL descriptions of your
interfaces, it is necessary to check the return type of methods like
`Tutorial.Factory.CreateCalculator' to see that a valid object
reference has been returned, before using the object.  ISL allows you
to have these CORBA-style objects, by using the `OPTIONAL' modifier in
the declaration of an object type, but it also allows object pointers
which can't be `None'.  By default ILU object instances may not be
`None'.

   The OMG IDL version of `Tutorial2' can be found in *Note
Tutorial2.idl::.

Transient and Permanent Objects
===============================

   Keen-eyed readers will have seen that the `Calculator' type in
`Tutorial.isl' is marked with the modifier `COLLECTIBLE', while the
`Factory' type is not.  ILU differentiates between two kinds of
objects, which we call transient and permanent.

   Transient objects are typically created for a specific use, and
typically have a fixed lifetime.  They often have state associated with
them.  They are frequently used to represent some ongoing computation
or operation.

   Permanent objects are usually expected to be around forever (or
until some activity not captured by a programming concept terminates).
Permanent objects are often gateways to some sort of service, or
represent some external piece of data, such as a cell in a spreadsheet,
or a file on a disk somewhere.  They are often stateless; when they do
have state, it is typically backed by some form of stable storage, as
they have to be available across crashes of their server program.

   ILU provides an automatic system for distributed garbage collection
of transient objects; that is, of removing them from everyone's address
space when everyone is done with them.  This uses the underlying
garbage collection techniques of the particular programming language
being used.  With Python, the ILU garbage collector works in
conjunction with the Python collector; with ANSI C, we use the
extremely primitive collector provided for C; that is, when the process
terminates, the objects are collected.  To indicate to ILU that a
particular object type is to be treated as transient, you mark the
object type specification, in the ISL, with the keyword `COLLECTIBLE'.

   For permanent objects, like the `Tutorial.Factory' object type, the
ILU Python support will maintain a reference to the instance,
preventing the Python collector from collecting it.

Using ILU with Tkinter
======================

   Python comes with a module called `Tkinter', which is an interface
to John Ousterhout's Tk user interface system.  When you use both ILU
and Tkinter together in a program, you should call the `ilu_tk' main
loop to block and dispatch requests, instead of the `ilu' main loop.
To illustrate this, here is a version of the `server2.py' program which
puts up a Tk button.  When the user presses the button, the server
exits.  Notice that instead of calling `ilu.RunMainLoop()' to block
indefinitely, the program calls `ilu_tk.RunMainLoop()'.  This call
allows both Tk and ILU events to be handled when neither a Tk callback
nor an ILU method call is active.
     # server3.py -- a program that runs a Tutorial.Calculator server
     #  Puts up a Tk button to kill it with.
     
     import ilu, FactoryImpl2, sys, Tkinter, ilu_tk
     
     def main(argv):
     
             def quit():
                     sys.exit(0)
     
             if (len(argv) < 2):
                     print "Usage:  python server3.py SERVER-ID"
                     sys.exit(1)
     
             theServer = ilu.CreateServer (argv[1])
             theFactory = FactoryImpl2.Factory ("theFactory", theServer)
             theFactory.IluPublish()
     
             # Now we put up a Tk button so that the user can kill the
             #  server by pressing the button
     
             f = Tkinter.Frame() ; Tkinter.Pack.config(f)
             b = Tkinter.Button (f, {'text' : theFactory.IluObjectID(),\
                                     'command': quit})
             b.pack ({'side': 'left', 'fill': 'both'})
     
             # Then we wait in the ilu_tk mainloop, instead of either
             #  the ILU mainloop or the Tkinter mainloop
     
             ilu_tk.RunMainLoop()
     
     
     main(sys.argv)


File: 826750876202273301,  Node: General ILU Info

General ILU Info
****************

   The Inter-Language Unification system (ILU) is a multi-language
object interface system.  The object interfaces provided by ILU hide
implementation distinctions between different languages, between
different address spaces, and between operating system types.  ILU can
be used to build multi-lingual object-oriented libraries ("class
libraries") with well-specified language-independent interfaces.  It
can also be used to implement distributed systems.  It can also be used
to define and document interfaces between the modules of non-distributed
programs.

   The 2.0 release of ILU contains support for the programming
languages ANSI C, C++, Modula-3, Python, and Common Lisp.  It has been
installed on many flavors of UNIX, including SPARC machines running
SunOS 4.1.3 and Solaris 2, SGI MIPS machines running IRIX 5.2, Intel
486 machines running Linux 1.1.78, DEC Alpha machines with OSF/1, IBM
RS/6000 machines running AIX, and HP machines running HP/UX.  It runs
on Microsoft Windows 3.1, Windows 95, and Windows NT environments.  It
supports both threaded and non-threaded operation.  Since one of the
implementation goals of ILU is to maximize compatibility with existing
open standards, ILU provides support for use of the OMG CORBA IDL
interface description language, and can be thought of as a CORBA ORB
system (though with omissions from and extensions to the CORBA spec).
As another result, ILU includes a self-contained implementation of ONC
RPC.

   ILU is available free from
`ftp://ftp.parc.xerox.com/pub/ilu/ilu.html'.


File: 826750876202273301,  Node: Tutorial.isl

Tutorial.isl
************

     INTERFACE Tutorial;
     
     EXCEPTION DivideByZero
       "this error is signalled if the client of the Calculator calls
     the Divide method with a value of 0";
     
     TYPE Calculator = OBJECT COLLECTIBLE
       DOCUMENTATION "4-function calculator"
       METHODS
         SetValue (v : REAL) "Set the value of the calculator to `v'",
         GetValue () : REAL  "Return the value of the calculator",
         Add (v : REAL)      "Adds `v' to the calculator's value",
         Subtract (v : REAL) "Subtracts `v' from the calculator's value",
         Multiply (v : REAL) "Multiplies the calculator's value by `v'",
         Divide (v : REAL) RAISES DivideByZero END
           "Divides the calculator's value by `v'"
       END;
     
     TYPE Factory = OBJECT
       METHODS
         CreateCalculator () : Calculator
       END;


File: 826750876202273301,  Node: CalculatorImpl.py

CalculatorImpl.py
*****************

     import Tutorial, Tutorial__skel
     
     class Calculator (Tutorial__skel.Calculator):
     
             def __init__ (self):
                     self.the_value = 0.0
     
             def SetValue (self, v):
                     self.the_value = v
     
             def GetValue (self):
                     return self.the_value
     
             def Add (self, v):
                     self.the_value = self.the_value + v
     
             def Subtract (self, v):
                     self.the_value = self.the_value - v
     
             def Multiply (self, v):
                     self.the_value = self.the_value * v
     
             def Divide (self, v):
                     try:
                             self.the_value = self.the_value / v
                     except ZeroDivisionError:
                             raise Tutorial.DivideByZero


File: 826750876202273301,  Node: FactoryImpl.py

FactoryImpl.py
**************

     import Tutorial, Tutorial__skel, CalculatorImpl
     
     class Factory (Tutorial__skel.Factory):
     
             # have the __init__ method take handle and server args
             # so that we can control which ILU kernel server is used,
             # and what the instance handle of the Factory object on
             # that server is.  This allows us to control the object ID
             # of the new Factory instance.
     
             def __init__(self, handle=None, server=None):
                     self.IluInstHandle = handle
                     self.IluServer = server
     
             def CreateCalculator (self):
                     return (CalculatorImpl.Calculator())


File: 826750876202273301,  Node: simple1.py

simple1.py
**********

     # simple1.py, a simple program that demonstrates the use of the
     #  Tutorial true module as a library.
     #
     # run this with the command "python simple1.py NUMBER [NUMBER...]"
     #
     
     import Tutorial, CalculatorImpl, string, sys
     
     # A simple program:
     #  1)  make an instance of Tutorial.Calculator
     #  2)  add all the arguments by invoking the Add method
     #  3)  print the resultant value.
     
     def main (argv):
     
             c = CalculatorImpl.Calculator()
             if not c:
                     error("Couldn't create calculator")
     
             # clear the calculator before using it
     
             c.SetValue (0.0)
     
             # now loop over the arguments, adding each in turn */
     
             for arg in argv[1:]:
                     v = string.atof(arg)
                     c.Add (v)
     
             # and print the result
     
             print "the sum is", c.GetValue()
             sys.exit(0)
     
     main(sys.argv)


File: 826750876202273301,  Node: simple2.py

simple2.py
**********

     # simple2.py, a simple program that demonstrates the use of the
     #  Tutorial true module as a library.
     #
     # run this with the command "python simple1.py NUMBER [NUMBER...]"
     #
     
     import Tutorial, CalculatorImpl, string, sys
     
     # A simple program:
     #  1)  make an instance of Tutorial.Calculator
     #  2)  add all the arguments by invoking the Add method
     #  3)  print the resultant value.
     
     def main (argv):
     
             c = CalculatorImpl.Calculator()
             if not c:
                     error("Couldn't create calculator")
     
             # clear the calculator before using it
     
             if (len(sys.argv) < 2):
                     c.SetValue (0.0)
             else:
                     c.SetValue (string.atof(argv[1]))
     
             # now loop over the arguments, Dividing by each in turn */
     
             try:
                     for arg in argv[2:]:
                             v = string.atof(arg)
                             c.Divide (v)
             except:
                     print 'exception signalled:  ' + str(sys.exc_type)
                     sys.exit(1)
     
             # and print the result
     
             print "the sum is", c.GetValue()
             sys.exit(0)
     
     main(sys.argv)


File: 826750876202273301,  Node: server.py

server.py
*********

     # server.py -- a program that runs a Tutorial.Calculator server
     #
     
     import ilu, FactoryImpl, sys
     
     def main(argv):
     
             if (len(argv) < 2):
                     print "Usage:  python server.py SERVER-ID"
                     sys.exit(1)
     
             # Create a kernel server with appropriate server ID, which
             #  is passed in as the first argument
     
             theServer = ilu.CreateServer (argv[1])
     
             # Now create an instance of a Factory object on that server,
             #  with the instance handle "theFactory"
     
             theFactory = FactoryImpl.Factory ("theFactory", theServer)
     
             # Now make the Factory object "well-known" by publishing it.
     
             theFactory.IluPublish()
     
             # Now we print the string binding handle (the object's name plus
             # its location) of the new instance.
     
             print "Factory instance published."
             print "Its SBH is '" + theFactory.IluSBH() + "'"
     
             ilu.RunMainLoop ()
     
     
     main(sys.argv)


File: 826750876202273301,  Node: simple3.py

simple3.py
**********

     # simple3.py -- a simple client program that finds the Calculator Factory,
     #   creates a calculator, and adds up its arguments as before
     #
     # to run:  python simple4.py ARG [ARG...]
     
     import Tutorial, ilu, sys, string
     
     # We define a new routine, "Get_Tutorial_Calculator", which
     # finds the tutorial factory, then creates a new Calculator
     # object for us.
     
     def Get_Tutorial_Calculator (factoryObjectID):
     
             # We have to call ilu.LookupObject() with the object ID of
             # the factory object, and the ``type'' of the object we're looking
             # for, which is always available as MODULE.TYPENAME
     
             f = ilu.LookupObject (factoryObjectID, Tutorial.Factory)
             if not f:
                     print "Can't find Tutorial.Factory instance " + factoryObjectID
                     sys.exit(1)
             c = f.CreateCalculator()
             return (c)
     
     def main (argv):
     
             # A simple program:
             #  1)  make an instance of Tutorial.Calculator
             #  2)  add all the arguments by invoking the Add method
             #  3)  print the resultant value.
     
             if (len(argv) < 2):
                     print "Usage:  python simple3.py FACTORY-OBJECT-ID NUMBER [NUMBER...]\n",
                     sys.exit(1)
     
             c = Get_Tutorial_Calculator(argv[1])
             if not c:
                     print "Couldn't create calculator"
                     sys.exit(1)
     
             # clear the calculator before using it
     
             c.SetValue (0.0)
     
             # now loop over the arguments, adding each in turn
     
             for arg in argv[2:]:
                     v = string.atof (arg)
                     c.Add (v)
     
             # and print the result
     
             print "the sum is " + str(c.GetValue())
     
             sys.exit (0);
     
     
     main(sys.argv)


File: 826750876202273301,  Node: Tutorial2.isl

Tutorial2.isl
*************

     INTERFACE Tutorial2 IMPORTS Tutorial END;
     
     TYPE OpType = ENUMERATION
         SetValue, Add, Subtract, Multiply, Divide END;
     
     TYPE Operation = RECORD
         op : OpType,
         value : REAL,
         accumulator : REAL
       END;
     
     TYPE RegisterTape = SEQUENCE OF Operation;
     
     TYPE TapeCalculator = OBJECT COLLECTIBLE
       SUPERTYPES Tutorial.Calculator END
       DOCUMENTATION "4 function calculator with register tape"
       METHODS
         GetTape () : RegisterTape
       END;
     
     TYPE Factory = OBJECT SUPERTYPES Tutorial.Factory END
       METHODS
         CreateTapeCalculator () : TapeCalculator
       END;


File: 826750876202273301,  Node: TapeCalculatorImpl.py

TapeCalculatorImpl.py
*********************

     import Tutorial, Tutorial2, Tutorial2__skel
     
     class TapeCalculator (Tutorial2__skel.TapeCalculator):
     
             def __init__ (self):
                     self.value = 0.0
                     self.tape = []
     
             def SetValue (self, v):
                     global value
                     self.value = v
                     self.tape.append({'op' : Tutorial2.OpType__SetValue, 'value' : v, 'accumulator' : self.value})
     
             def GetValue (self):
                     return self.value
     
             def Add (self, v):
                     self.value = self.value + v
                     self.tape.append({'op' : Tutorial2.OpType__Add, 'value' : v, 'accumulator' : self.value})
     
     
             def Subtract (self, v):
                     self.value = self.value - v
                     self.tape.append({'op' : Tutorial2.OpType__Subtract, 'value' : v, 'accumulator' : self.value})
     
     
             def Multiply (self, v):
                     self.value = self.value * v
                     self.tape.append({'op' : Tutorial2.OpType__Multiply, 'value' : v, 'accumulator' : self.value})
     
     
             def Divide (self, v):
                     try:
                             self.value = self.value / v
                     except ZeroDivisionError:
                             raise Tutorial.DivideByZero
                     self.tape.append({'op' : Tutorial2.OpType__Divide, 'value' : v, 'accumulator' : self.value})
     
             def GetTape (self):
                     return (self.tape)


File: 826750876202273301,  Node: FactoryImpl2.py

FactoryImpl2.py
***************

     import Tutorial2, Tutorial2__skel, TapeCalculatorImpl
     
     class Factory (Tutorial2__skel.Factory):
     
             # have the __init__ method take handle and server args
             # so that we can control which ILU kernel server is used,
             # and what the instance handle of the Factory object on
             # that server is.  This allows us to control the object ID
             # of the new Factory instance.
     
             def __init__(self, handle=None, server=None):
                     self.IluInstHandle = handle
                     self.IluServer = server
     
             def CreateCalculator (self):
                     return (TapeCalculatorImpl.TapeCalculator())
     
             CreateTapeCalculator = CreateCalculator


File: 826750876202273301,  Node: server2.py

server2.py
**********

     # server2.py -- a program that runs a Tutorial2.TapeCalculator factory
     #
     
     import ilu, FactoryImpl2, sys
     
     def main(argv):
     
             if (len(argv) < 2):
                     print "Usage:  python server2.py SERVER-ID"
                     sys.exit(1)
     
             theServer = ilu.CreateServer (argv[1])
             theFactory = FactoryImpl2.Factory ("theFactory", theServer)
     
             # Now make the Factory object "well-known" by publishing it.
     
             theFactory.IluPublish()
     
             # Now we print the string binding handle (the object's name plus
             # its location) of the new instance.
     
             print "Factory2 instance published."
             print "Its SBH is '" + theFactory.IluSBH() + "'"
     
             ilu.RunMainLoop ()
     
     
     main(sys.argv)


File: 826750876202273301,  Node: simple4.py

simple4.py
**********

     # simple4.py -- a simple client program that finds the TapeCalculator Factory,
     #   creates a calculator, and provides a simple interactive calculator
     #
     # to run:  python simple4.py ARG [ARG...]
     
     import Tutorial2, ilu, sys, string
     
     # We define a new routine, "Get_Tutorial_Calculator", which
     # finds the tutorial factory, then creates a new Calculator
     # object for us.
     
     def Get_Tutorial_Calculator (factoryObjectID):
     
       # We have to call ilu.LookupObject() with the object ID of
       # the factory object, and the ``type'' of the object we're looking
       # for, which is always available as MODULE.TYPENAME
     
       f = ilu.LookupObject (factoryObjectID, Tutorial2.Factory)
       if not f:
         print "Can't find Tutorial.Factory instance " + factoryObjectID
         sys.exit(1)
       c = f.CreateTapeCalculator()
       return (c)
     
     
     opname = ['SetValue', 'Add', 'Subtract', 'Divide', 'Multiply']
     
     def Print_Tape (tape):
     
       # print out the Calculator tape nicely
     
       for op in tape:
         print "  %s(%f) => %f" % (opname[op['op']], op['value'], op['accumulator'])
     
     def main (argv):
     
       if (len(argv) < 2):
         print "Usage:  python simple3.py FACTORY-OBJECT-ID NUMBER [NUMBER...]\n",
         sys.exit(1)
     
       c = Get_Tutorial_Calculator(argv[1])
       if not c:
         print "Couldn't create calculator"
         sys.exit(1)
     
       # clear the calculator before using it
     
       newval = 0.0
       c.SetValue (newval)
     
       quitflag = 0
     
       while not quitflag:
     
         sys.stdout.write("%.5f\n> " % newval)
         sys.stdout.flush()
     
         line = sys.stdin.readline()
     
         if (not line):
           sys.exit(0)
     
         try:
           if (line[0] == '\n'):
             pass
           elif (line[0] == '+'):
             val = string.atof(line[1:-1])
             c.Add(val)
           elif (line[0] == '-'):
             val = string.atof(line[1:-1])
             c.Subtract(val)
           elif (line[0] == '*'):
             val = string.atof(line[1:-1])
             c.Multiply(val)
           elif (line[0] == '/'):
             val = string.atof(line[1:-1])
             c.Divide(val)
           elif (line[0] == 'q'):
             quitflag = 1
           elif (line[0] == 'c'):
             c.SetValue(0.0)
           elif (line[0] == 't'):
             Print_Tape ( c.GetTape() )
           else:
             print "Invalid operation <" + line[:-1] + ">"
             print "Valid ops are +, -, *, /, tape, clear, quit"
     
           newval = c.GetValue()
     
         except:
           print "Operation <%s> signals error <%s>." % (line[:-1], sys.exc_type)
     
       sys.exit(0)
     
     
     main(sys.argv)


File: 826750876202273301,  Node: Tutorial.idl

Tutorial.idl
************

     module Tutorial {
     
       exception DivideByZero {};
     
       interface Calculator {
     
         // Set the value of the calculator to `v'
         void SetValue (in double v);
         // Return the value of the calculator
         double GetValue ();
         // Adds `v' to the calculator's value
         void Add (in double v);
         // Subtracts `v' from the calculator's value
         void Subtract (in double v);
         // Multiplies the calculator's value by `v'
         void Multiply (in double v);
         // Divides the calculator's value by `v'
         void Divide (in double v) raises (DivideByZero);
       };
     
       interface Factory {
     
         // Create and return an instance of a Calculator object
         Calculator CreateCalculator();
       };
     };


File: 826750876202273301,  Node: Tutorial2.idl

Tutorial2.idl
*************

     #include "Tutorial.idl"
     
     module Tutorial2 {
     
       enum OpType { SetValue, Add, Subtract, Multiply, Divide };
     
       struct Operation {
         OpType op;
         double value;
         double accumulator;
       };
     
       typedef sequence<Operation> RegisterTape;
     
       // A four function calculator with a register tape
       interface TapeCalculator : Tutorial::Calculator {
     
         RegisterTape GetTape ();
       };
     
       // A factory that produces TapeCalculators
       interface Factory : Tutorial::Factory {
     
         TapeCalculator CreateTapeCalculator ();
       };
     };


File: 826750876202273301,  Node: server3.py

server3.py
**********

     # server3.py -- a program that runs a Tutorial.Calculator server
     #  Puts up a Tk button to kill it with.
     
     import ilu, FactoryImpl2, sys, Tkinter, ilu_tk
     
     def main(argv):
     
             def quit():
                     sys.exit(0)
     
             if (len(argv) < 2):
                     print "Usage:  python server3.py SERVER-ID"
                     sys.exit(1)
     
             theServer = ilu.CreateServer (argv[1])
             theFactory = FactoryImpl2.Factory ("theFactory", theServer)
             theFactory.IluPublish()
     
             # Now we put up a Tk button so that the user can kill the
             #  server by pressing the button
     
             f = Tkinter.Frame() ; Tkinter.Pack.config(f)
             b = Tkinter.Button (f, {'text' : theFactory.IluObjectID(),\
                                     'command': quit})
             b.pack ({'side': 'left', 'fill': 'both'})
     
             # Then we wait in the ilu_tk mainloop, instead of either
             #  the ILU mainloop or the Tkinter mainloop
     
             ilu_tk.RunMainLoop()
     
     
     main(sys.argv)

Index of Concepts
*****************

* Menu:

* distributed garbage collection:       Top.
* docstring:                            Top.
* exception-value type:                 Top.
* factory:                              Top.
* garbage collection:                   Top.
* handle:                               Top.
* import:                               Top.
* instance:                             Top.
* instance handle:                      Top.
* interface:                            Top.
* ISL:                                  Top.
* kernel server:                        Top.
* mapping:                              Top.
* methods:                              Top.
* object ID:                            Top.
* object type:                          Top.
* object-oriented:                      Top.
* permanent:                            Top.
* publish:                              Top.
* raises clause:                        Top.
* register tape:                        Top.
* server ID:                            Top.
* simple binding:                       Top.
* subclass:                             Top.
* subtyping:                            Top.
* supertype:                            Top.
* transient:                            Top.
* type constructors:                    Top.
* unique id:                            Top.



Tag Table:
Node: Top115
Node: General ILU Info39414
Node: Tutorial.isl41044
Node: CalculatorImpl.py41969
Node: FactoryImpl.py42934
Node: simple1.py43716
Node: simple2.py44808
Node: server.py46195
Node: simple3.py47385
Node: Tutorial2.isl49442
Node: TapeCalculatorImpl.py50198
Node: FactoryImpl2.py51892
Node: server2.py52751
Node: simple4.py53686
Node: Tutorial.idl56619
Node: Tutorial2.idl57509
Node: server3.py58236

End Tag Table
