系统默认的Python环境：
/System/Library/Frameworks/Python.framework/Versions自己安装的python环境在：
/Library/Frameworks/Python.framework/Versions# Homebrew
export PATH=/usr/local/bin:$PATH
# Setting PATH for Python 3.6
# The original version is saved in .bash_profile.pysave
# PATH="/Library/Frameworks/Python.framework/Versions/3.6/bin:${PATH}"
PATH="/System/Library/Frameworks/Python.framework/Versions/2.7/bin:${PATH}"
export PATH更改 ～/.bash_profile 文件的PATH 指向 python 版本的
source .bash_profile