From
https://www.python.org/ftp/python/3.8.2/python-3.8.2-amd64.exe
or
http://npm.taobao.org/mirrors/python/3.8.2/python-3.8.2-amd64.exe

you can:
Change the installation directory and chose some modules you like.

But, If you change de default installation directory, the following step 2.1 must be done.

After the installation finished, you can check it by run command : python -V

D:\Python38>python -V
Python 3.8.2

If there is something wrong with the PATH, you should set the PATH. or you can create a bat file with these commands.

PATH = D:\Python38;D:\Python38\Scripts

Everytime you open a new command console, run this bat file first.

To check configuration, use command python -m site

D:\Python38>python -m site
sys.path = [
  'D:\\Python38',
  'D:\\Python38\\python38.zip',
  'D:\\Python38\\DLLs',
  'D:\\Python38\\lib',
  'D:\\Python38\\lib\\site-packages',
]
USER_BASE: 'C:\\Users\\xxx\\AppData\\Roaming\\Python' (doesn't exist)
USER_SITE: 'C:\\Users\\xxx\\AppData\\Roaming\\Python\\Python38\\site-packages' (doesn't exist)
ENABLE_USER_SITE: True

It shows that USER_BASE and USER_SITE are error. modify file D:\Python38\Lib\site.py, change these two parameter to

USER_SITE = "D:\Python38\Lib\site-packages"
USER_BASE = "D:\Python38\Scripts"

check again: python -m site

python -m pip install --upgrade pip --force-reinstall

check pip by :

D:\Python38>pip -V
pip 20.0.2 from D:\Python38\Lib\site-packages\pip (python 3.8)     

去掉#import site 前面的 “#” 注释

• 下载get-pip.py https://bootstrap.pypa.io/get-pip.py
• 运行get-pip.py完成安装 (python get-pip.py)

just install, revise the site.py and upgrede pip with:

python -m pip install --upgrade pip


-i https://pypi.tuna.tsinghua.edu.cn/simple