I\'m running Mountain Lion and the basic default Python version is 2.7. I downloaded Python 3.3 and want to set it as default.
Currently:
$ python
Well... It's kinda old. But still deserves a good answer.
And the good one is You Don't Wanna Touch The Default Python On Mac.
Install any Python version you need via Homebrew or whatever and use it in virtualenv. Virtualenv is often considered to be something crap-like, but it's still way, wayyyy better than changing python version system-wide (macOS is likely to protect itself from such actions) or user-wide, bash-wide... whatever. Just forget about the default Python. Using playgrounds like venv is what your OS will be most, very most grateful for.
The case is, for example, many modern Linux distributions get rid of Python2 installed out-of-the-box, leaving only Python3 in the system. But everytime you try to install something old with python2 as a dependency... hope you understand what I mean. A good developer doesn't care. Good developers create clean playgrounds with python version they desire.