Python 3.3 has been released on 29 September 2012, several months after Ubuntu 12.04 was released. It is included in Ubuntu 12.10 though as python3.3 package
If you want to install Python 3.3 on Ubuntu version which does not have it in its repositories, you have the following options:
Use a PPA
There's a PPA containing Old and New Python versions maintained by Felix Krull. See Luper Rouch's answer for installation instructions.
Compile Python from source
This is very easy and allows you to have multiple Python versions without messing with system python interpreter (which is used by a lot of Ubuntu own programs). On my dev machine I have literally dozens of different Python versions from 2.4 to 3.2 living happily in /opt.
we need C compiler and other stuff to compile Python
sudo apt-get install build-essential
SQLite libs need to be installed in order for Python to have SQLite support.
sudo apt-get install libsqlite3-dev
sudo apt-get install sqlite3 # for the command-line client
sudo apt-get install bzip2 libbz2-dev
Download and compile Python:
wget http://www.python.org/ftp/python/3.3.5/Python-3.3.5.tar.xz
tar xJf ./Python-3.3.5.tar.xz
cd ./Python-3.3.5
./configure --prefix=/opt/python3.3
make && sudo make install
Some nice touches to install a py command by creating a symlink:
mkdir ~/bin
ln -s /opt/python3.3/bin/python3.3 ~/bin/py
Alternatively, you can install a bash alias named py instead:
echo 'alias py="/opt/python3.3/bin/python3.3"' >> .bashrc
And this is it. Now you can have any Python version, even an alpha, or, say, to have a few copies of Python 3.3 compiled with different settings... not that many people need that though :)
Use pyenv
There's a software called pyenv which may help you to automate the procedure - what it essentially does is compile Python from source, installing it in your home directory. Its goal is to help you manage multiple Python versions.
Best Answer
The project is now hosted on Github :