learn if program is installed from Python

I just ran a small Python script of mine on a new computer and I got an error message that xsltproc isn't found. So I want to modify the script to test whether xsltproc is installed. (I should probably do this with several other scripts too.)

However, I don't know how to do this. How do I learn, from a Python script running on Linux, whether a program is installed?

>>7
1. >>5 didn't give any code.
2. 'which' is not portable to other operating systems.
3. I was answering >>1's question.