req=context.createSubRequest("active:python");
req.addArgument("operator", [resource representation, aspect, or URI] );
result=context.issueSubRequest(req);

The python accessor executes Python language scripts. It is based on Jython.

The operator specifies the resource to be executed as a program. This may contain zero or more classes plus some script code outside the context of any class which will be executed when the module is invoked. The script may either be a plain/text or an XML document with the script specified as the text of the root element. This later form is useful when embedding python snippets inside declarative languages.

When using XML it is strongly advisable to enclose your script inside a CDATA section, for example:

<script><![CDATA[ ...your script .. ]]></script>

Script Context

When executed, each script is provided an instance of the INKFConvenienceHelper class as the local variable context. The context variable has two purposes. It supports introspection of the request which initiated the script and it allows sub-requests to be issued. For more details see the NKF Reference Guide.

Once a response object is created:

response=context.createResponseFrom(...);

it will be returned via the context when the script exits.

Parameters

Parameters can be passed to a script using named arguments. Argument values are available within the script through the context using the this:param URI address space convention.

For example if the python accessor is requested with...

active:python+operator@myscript.py+myargument@data.xml

The myargument parameter can be obtained inside the script using the context and the argument URI "this:param:myargument".

context.source("this:param:myargument")

Python Libraries

There are many useful Python libraries. It can also be useful to structure your own code using multiple modules. The python accessor allows you to import Python modules into your Python scripts. Traditionally Python has been oriented around a filesystem and all imports were resolved against the fileystem either absolutely or relatively. Because NetKernel has a modular address space abstraction this accessor resolves imports not from the filesystem but from NetKernel modules. The principles are the same, modules are resolved either absolutely or against the current working URI of the "main" module. Module import specificiations are converted to URIs which are resolved against the address space of the request for execution of the module.

For example, the import statement:

from test.library.area import AreaClass

executed inside the module addressed by ffcpl:/scripts/myModule.py would be resolved against the following URIs with the first successful one used:

1. ffcpl:/scripts/test/library/area.py (relative to main)
2. ffcpl:/test/library/area.py (absolute)

Java Libraries

The Jython implementation has the ability to access native Java classes. Java classes are often needed when constructing requests and issuing results in NKF.

Java classes are imported with the following python syntax:

import sys
sys.add_package("org.ten60.netkernel.layer1.representation")
sys.add_package("com.ten60.netkernel.urii.aspect")

Exception Handling

Python contains good exception handling however during development unexpected and unhandled exceptions will occur. The Python accessor maps python exceptions into the NetKernel domain so they integrate well into the composite stack trace that NetKernel presents. Specifically you can actually see the Python stack trace with python-module names, functions and line numbers.

Example

Here's an example of a Python script for performing an XSLT, logging the result and returning the result as the response.

# Create and issue XSLT request
sr=context.createSubRequest("active:xslt)
sr.addArgument("operand","data1.xml")
sr.addArgument("operator","style1.xsl")
rep=context.issueSubRequest(sr)

# Log result
sr=context.createSubRequest()
sr.setURI("active:log")
sr.addArgument("operand",rep)
context.issueSubRequest(sr)

# Create response, set metadata and exit
resp=context.createResponseFrom(rep)
resp.setMimeType("text/plain")
resp.setExpired()