best way of testing a program exists before using it?

[Hari Sekhon]

I am writing a wrapper to a binary command to run it and then do 
something with the xml output from it.

What is the best way of making sure that the command is installed on the 
system before I try to execute it, like the python equivalent of the 
unix command "which"?

Otherwise I'd have to do something like:

if os.system('which somecommand') != 0:
    print "you don't have %s installed" % somecommand
    sys.exit(1)

I know that isn't portable which is why a python solution would be 
better (although this will run on unix anyway, but it'd be nice if it 
ran on windows too).


-h

-- 
Hari Sekhon