macos - How to link home brew python version and set it as default

Question

I just switched from MacPorts to HomeBrew. After installing all the required XCode versions and other software I tried installing python using homebrew: I think it successfully installed, but when I do which python it still shows me 2.7.3 which I think is the version shipped with Mountain Lion.

which python
/usr/local/bin/python

python --version
Python 2.7.3

so I tried to install again

brew install python --framework --universal
Warning: python-2.7.5 already installed, it's just not linked

But it says python 2.7.5 already install and not linked, I tried to do brew link python

That led me to following message so, I have no idea what I should be doing:

Linking /usr/local/Cellar/python/2.7.5... Warning: Could not link python. Unlinking...

Error: Could not symlink file: /usr/local/Cellar/python/2.7.5/bin/smtpd2.py
Target /usr/local/bin/smtpd2.py already exists. You may need to delete it.
To force the link and overwrite all other conflicting files, do:
  brew link --overwrite formula_name

To list all files that would be deleted:
  brew link --overwrite --dry-run formula_name

Answer 1

After installing python3 with brew install python3 I was getting the error:

Error: An unexpected error occurred during the `brew link` step
The formula built, but is not symlinked into /usr/local
Permission denied @ dir_s_mkdir - /usr/local/Frameworks
Error: Permission denied @ dir_s_mkdir - /usr/local/Frameworks

After typing brew link python3 the error was:

Linking /usr/local/Cellar/python/3.6.4_3... Error: Permission denied @ dir_s_mkdir - /usr/local/Frameworks

To solve the problem:

sudo mkdir -p /usr/local/Frameworks
sudo chown -R $(whoami) /usr/local/*
brew link python3

After this, I could open python3 by typing python3 ??

(From https://github.com/Homebrew/homebrew-core/issues/20985)