python一括変更ファイル拡張子の実装

import os
def swapextensions(dir, before, after):
    #0<=x<1
    if before[:1] != '.':
        before = '.'+before
    thelen = -len(before)
    if after[:1] != '.':
        after = '.'+after
    for path, subdirs, files in os.walk(dir):
        for oldfile in files:
            if oldfile[thelen:] == before:
                oldfile = os.path.join(path, oldfile)
                newfile = oldfile[:thelen]+after
                os.rename(oldfile, newfile)

if __name__ == '__main__':
    import sys
    if len(sys.argv) != 4:
        print "Usage: swapext rootdir before after"
        sys.exit(100)
    swapextensions(sys.argv[1], sys.argv[2], sys.argv[3])