human
Solve the following python programming problem given the following code:
@memoize
def which(program):
assistant
#Here is the Python program you requested.

    """Returns `program` path or `None`.
    """
    try:
        from shutil import which
        return which(program)
    except ImportError:
        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 os.environ['PATH'].split(os.pathsep):
                path = path.strip('"')
                exe_file = os.path.join(path, program)
                if is_exe(exe_file):
                    return exe_file
        return None