Pretty simple to do, but regardless here is a copy-and-paste'able set of commands that should work on OS X/Linux (if you have the Apple Developer Tools installed, or the build-essential package on Ubuntu):
cd /tmp/ curl -O http://www.python.org/ftp/python/2.6/Python-2.6b3.tgz gunzip Python-2.6b3.tgz tar -xf Python-2.6b3.tar cd Python-2.6b3/ ./configure --prefix=/usr/local/python2.6 # lots of lines ending in "... yes" hopefully make # will take a while, no errors sudo make install # enter your password, as it requires root access to put files in /usr/local/ # lots of stuff flies by.. sudo ln -s /usr/local/python2.6/bin/python2.6 /usr/local/bin/python2.6 sudo ln -s /usr/local/python2.6/bin/2to3 /usr/local/bin/2to3 # this puts python2.6 into your $PATH</pre class='code'>
That will install python2.6 into /usr/local/python2.6/ and symlink the commands python2.6 and 2to3 into /usr/local/bin/
Then you just run python2.6 /path/to/myscript.py
To see warnings about things that will break in Python3, run python2.6 -r /path/to/myscript.py
