1.编辑：vi ~/.bash_profile，在末尾加上如下内容：

PATH="/Library/Frameworks/Python.framework/Versions/2.7/bin:${PATH}"
export PATH
# Setting PATH for Python 3.9
PATH="/Library/Frameworks/Python.framework/Versions/3.9/bin:${PATH}"

2、编辑：vi ~/.bashrc，添加如下内容：(⚠️python不能加System)

alias python2='/System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7'
alias python3='/Library/Frameworks/Python.framework/Versions/3.9/bin/python3.9'
alias python=python3

3、让配置生效

source ~/.bash_profile

source ~/.bashrc

4、终端输入python/python3:
输入python2: