1.在Mac下用Homebrew安装Python3.5(Mac 自带的Python是2.7)
brew install python3
2.安装完成之后发现出错了
Error: An unexpected error occurred during the `brew link` step
The formula built, but is not symlinked into /usr/local
Permission denied - /usr/local/Frameworks
Error: Permission denied - /usr/local/Frameworks
3.刚刚开始以为权限不够，用sudo重新安装也是不行，再次重新安装(不用sudo)出现这样的错误：
brew install python3
Warning: python3-3.5.0 already installed, it's just not linked
4.没办法，只要用brew doctor检查看看错误在哪里(省略了一部分不重要的提示)
**Please note that these warnings are just used to help the Homebrew maintainers**
**with debugging if you file an issue. If everything you use Homebrew for is**
**working fine: please don't worry and just ignore them. Thanks!**
Warning: The /usr/local is not writable.
You should probably change the ownership and permissions of /usr/local
back to your user account.
sudo chown -R $(whoami) /usr/local
Warning: The /usr/local directory is not writable.
Even if this directory was writable when you installed Homebrew, other
software may change permissions on this directory. Some versions of the
"InstantOn" component of Airfoil are known to do this.
You should probably change the ownership and permissions of /usr/local
back to your user account.
sudo chown -R $(whoami):admin /usr/local
5.根据提示输入sudo chown -R $(whoami):admin /usr/local，然后再brew link python3即可
