Thursday, November 23, 2006

IronPython and trace style debugging

When developing with IronPython under Windows, there is excellent debugger support via Visual Studio. But sometimes you just want to do simple trace style debugging, in other words, put some print statements in your code. This works well when developing an application you can run under the console but is problematic if the application is a service or ASP.NET handler. When developing similar applications with CPython under Windows, I used the win32trace.pyd/ to achieve this.

From the win32 python docs:

These modules allow one Python process to generate output via a "print" statement, and another unrelated Python process to display the data. This works even if the Python process generating the output is running under the context of a service, where more traditional means are not available. The module works on Windows 95 and Windows NT.

To enable this debugging, simply "import win32traceutil" somewhere in your program - thats it! This automatically re-directs the standard Python output streams to the remote collector (If that behaviour is undesirable, you will need to use win32trace directly.) To actually see the output as it is produced, start a DOS prompt, and run the file "" (eg, double-click on it from Explorer, or "python.exe") This will print all output from all other Python processes (that have imported win32traceutil) until you kill it with Ctrl+Break!)

So I decided you create something similar for IronPython. win32trace uses a memory mapped file for communication between the application and the trace collector. Since the only way I could find to use memory mapped files under CLI, relied on calls to unmanaged win32 api, I decided to use network UDP datagrams instead. Rather than writing something from scratch, I remembered a python script that I had used many years ago, by Jeff Bauer. The original link to the script is broken, so I finally tracked a copy down in the Zope CVS. So I wrapped this code in a wrapper

To use it, you just need to put this script in your IronPython or CPython path, and any output to stdout or stderr will be sent to the collector.

To start the collector:


If the following code is run as a script by IronPython or CPython:

import traceutil

print "start"
a = "A"
# Create an error
import monty

the following would appear in the collector console running under .NET:

creosote bucket waiting on port: 7739
Traceback (most recent call last):\r\n File E:\\IronPython\\IPCE-r4\\, line 6, in Initialize\r\n File , line 0, in __import__##4r\nImportErr
or: No module named monty\r\n

and the following would appear in the collector console running under Mono:

creosote bucket waiting on port: 7739
Traceback (most recent call last):\n File test, line unknown, in Initialize\nImportError: No module named monty\n

It is important to note that the creosote client code does not block if the collector isn't running which is a good behaviour.

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