On OS X High Sierra, I had to do this:
sudo install -d -o $(whoami) -g admin /usr/local/Frameworks
brew uninstall --ignore-dependencies python
brew install python
python --version # should work, returns 2.7, which is a Python thing (it's weird, but ok)
credit to https://gist.github.com/irazasyed/7732946#gistcomment-2235469
I think it's better than recursively chowning the /usr/local dir, but that may solve other problems ;)