Implementation of Unix's which command in python

which.py

def which(exe=None):
    '''
    Python clone of POSIX's /usr/bin/which
    '''
    if exe:
        (path, name) = os.path.split(exe)
        if os.access(exe, os.X_OK):
            return exe
        for path in os.environ.get('PATH').split(os.pathsep):
            full_path = os.path.join(path, exe)
            if os.access(full_path, os.X_OK):
                return full_path
    return None

Why not shutil.which?

Not available before python 3.3