Check if a command is valid

[Kenny Meyer]

Hello,

I have to figure out if a string is callable on a Linux system. I'm
actually doing this:

    def is_valid_command(command):
        retcode = 100 # initialize
        if command:
            retcode = subprocess.call(command, shell=True)
        if retcode is 0:
            print "Valid command."
        else:
            print "Looks not so good..."

    is_valid_command("ls")

Never mind the code, because this is not the original.
The side effect of subprocess.call() is that it *actually* executes
it, but I just need the return code. What are better ways of doing
this?