human
Here's a good function header given the following:
I wrote the below code

    import os
    def is_exe(fpath):
        return (os.path.isfile(fpath) and os.access(fpath, os.X_OK))
    (fpath, fname) = os.path.split(program)
    if fpath:
        if is_exe(program):
            return program
    else:
        for path in path_env.split(os.pathsep):
            path = path.strip('"')
            exe_file = os.path.join(path, program)
            if is_exe(exe_file):
                return exe_file
    return None

What's a good function header?
assistant
#Here's a good function header.

def which(program, path_env):