This takes a given filename; tries to find it in the environment path;
    then checks if it is executable. This returns the full path to the filename
    if found and executable. Otherwise this returns None.

def which(filename, env=None):
    '''This takes a given filename; tries to find it in the environment path;
    then checks if it is executable. This returns the full path to the filename
    if found and executable. Otherwise this returns None.'''

    # Special case where filename contains an explicit path.
    if os.path.dirname(filename) != '' and is_executable_file(filename):
        return filename
    if env is None:
        env = os.environ
    p = env.get('PATH')
    if not p:
        p = os.defpath
    pathlist = p.split(os.pathsep)
    for path in pathlist:
        ff = os.path.join(path, filename)
        if is_executable_file(ff):
            return ff
    return None