Wandering Thoughts archives

Using Python to test system behavior

One of the things I like about the Python interpreter is that it is just about right for doing little system behavior tests, like how various systems handled a TCP port binding issue. This is because Python is low level enough to be relatively close to the actual system, so you can be pretty sure that what you're seeing isn't Python-specific behavior, while at the same time being high level enough that you can do the tests in a couple of lines.

Strictly speaking you don't need an interpreter for this, just a language at the right level, but having an interpreter avoids the whole drag of editing and running and re-editing and re-running and so on.

(And theoretically you can write this stuff directly in C, but I'm certainly not familiar enough with things like the C socket API that I can toss off test programs for this in ten minutes or less. In Python I can bind a socket in three statements, and one of them is 'import socket'.)

So today when I wanted to see how bind() behaved on Solaris 9, I fired up two interpreters in two windows, and in the first I typed:

>>> import socket
>>> s = socket.socket(); s.bind(('', 4000))

and in the other I typed:

>>> import socket
>>> s = socket.socket(); s.bind(('127.0.0.1', 4000))

And the Solaris 9 machine promptly spewed a traceback complaining about 'socket.error: (125, 'Address already in use')', just as I expected. (Yeah, I could have done it all in one Python interpreter; I didn't feel like seeing if Solaris did something funny if the same process was doing both bind()s. (It turns out it doesn't.))