Writing a CASA Task

Overview

In CASA, it is relatively simple to create your own tasks -- these can be as simple or as complex as you desire, and with the ease of scripting in Python, the possibilities are almost endless. You can also configure your CASA setup to automatically load these tasks on startup, so that they're always available to you.

Writing a task isn't all that much more work than writing a script, and it gives a nice interface through which to interact (along with the same functionality).

This tutorial will step you through the process of creating a task, and also point you to some examples of other tasks (some within CASA itself, and others written by non-developers). We hope you find this useful in creating your own!

The structure of a task

A task comprises an XML file, in which the parameter interface and help file are coded, and a Python file, in which the main body of code lives. They must follow a particular naming scheme, so that the program which creates all the task files for CASA to read (called buildmytasks) knows what it's looking for. So, if you want to write a task called newtask, you will need to have:

newtask.xml
task_newtask.py

The XML file

Getting the XML file right can sometimes be a bit tricky, since the parser is very particular. In its simplest form, it is this:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" ?>

<casaxml xmlns="http://casa.nrao.edu/schema/psetTypes.html"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://casa.nrao.edu/schema/casa.xsd
file:///opt/casa/code/xmlcasa/xml/casa.xsd">

<task type="function" name="newtask" category="editing">

  <shortdescription>Does nothing in particular.</shortdescription>
      
  <description>As I said, this task doesn't really do much.</description>

  <input>

    <param type="string" name="imagein" mustexist="true">
      <description>Input image name</description>
      <value></value>
    </param>

  <constraints>
  </constraints>

  </input>

  <returns type="void"/>

  <example>

   As I said, this task doesn't do much.

   Keyword arguments:

	imagein -- name of input image file

  </example>

</task>

</casaxml>

• <shortdescription>: appears in the 'taskhelp' listing
• <description>: I'm really not sure where this goes. If anyone can figure it out, let me know!
• <input>: this is where the parameters which appear when you type "inp newtask" go
  • <param>: each param/description/value set defines a particular parameter.
    • In this case, we have defined that the parameter input needs to be of type "string" (other options include "int", for integer; "bool", for boolean True/False; "double" for double-precision floating point, or "any" for anything)
    • Setting mustexist="true" means that CASA will check that this file actually exists before proceeding (and exit with an error if it doesn't)