python循环解压rar文件

C:.
│  main.py
│
├─1_STL_算法简介
│      STL_算法简介.rar
│
└─2_STL_算法_填充新值
        STL_算法_填充新值.rar

事情是这样的，下载了一套教学视频，文件夹命名规则是：“序号+视频名”，文件夹内是“视频名.rar”，上面选取了其中两个文件作为示例。这样看视频的时候就出现一个问题，每看一次都需要切换一次目录，解压一次文件，烦！
遂写main.py

import rarfile
import shutil
import os

path = os.getcwd()
allFile = os.listdir(path)

for dir in allFile:
    if '.' not in dir:
        dir2 = os.listdir(path+'\\'+dir)
        for file in dir2:
            if '.rar' in file:
                rar = rarfile.RarFile(path+'\\'+dir+'\\'+file)
                rar.extractall(path+'\\'+dir)
        dir2 = os.listdir(path+'\\'+dir)
        for file in dir2:
            if '.wmv' in file:
                print(dir)
                print(file)
                os.chdir(path+'\\'+dir)
                os.rename(file, dir+'.wmv')
                shutil.copy2(dir+'.wmv', path)
                os.chdir(path)

出现问题：

Traceback (most recent call last):
  File "C:\Python35\lib\site-packages\rarfile.py", line 2813, in custom_popen
    creationflags=creationflags)
  File "C:\Python35\lib\subprocess.py", line 676, in __init__
    restore_signals, start_new_session)
  File "C:\Python35\lib\subprocess.py", line 955, in _execute_child
    startupinfo)
FileNotFoundError: [WinError 2] 系统找不到指定的文件。

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\Administrator\Desktop\新建文件夹 (2)\main.py", line 14, in <module>
    rar.extractall(path+'\\'+dir)
  File "C:\Python35\lib\site-packages\rarfile.py", line 820, in extractall
    self._extract(fnlist, path, pwd)
  File "C:\Python35\lib\site-packages\rarfile.py", line 885, in _extract
    p = custom_popen(cmd)
  File "C:\Python35\lib\site-packages\rarfile.py", line 2816, in custom_popen
    raise RarCannotExec("Unrar not installed? (rarfile.UNRAR_TOOL=%r)" % UNRAR_TOOL)
rarfile.RarCannotExec: Unrar not installed? (rarfile.UNRAR_TOOL='unrar')

下载UnRar.exe
链接：http://pan.baidu.com/s/1o8ObqCa 密码：dsru

放在当前文件夹，运行main.py。
目录树变为：

C:.
│  1_STL_算法简介.wmv
│  2_STL_算法_填充新值.wmv
│  main.py
│  UnRAR.exe
│
├─1_STL_算法简介
│      1_STL_算法简介.wmv
│      STL_算法简介.rar
│
└─2_STL_算法_填充新值
        2_STL_算法_填充新值.wmv
        STL_算法_填充新值.rar