python 2 instead of python 3 as the (temporary) default python?

You can use virtualenv

# Use this to create your temporary python "install"
# (Assuming that is the correct path to the python interpreter you want to use.)
virtualenv -p /usr/bin/python2.7 --distribute temp-python

# Type this command when you want to use your temporary python.
# While you are using your temporary python you will also have access to a temporary pip,
# which will keep all packages installed with it separate from your main python install.
# A shorter version of this command would be ". temp-python/bin/activate"
source temp-python/bin/activate

# When you no longer wish to use you temporary python type
deactivate

Enjoy!

mkdir ~/bin
PATH=~/bin:$PATH
ln -s /usr/bin/python2 ~/bin/python

To stop using python2, exit or rm ~/bin/python.

Just call the script using something like python2.7 or python2 instead of just python.

So:

python2 myscript.py

instead of:

python myscript.py

What you could alternatively do is to replace the symbolic link "python" in /usr/bin which currently links to python3 with a link to the required python2/2.x executable. Then you could just call it as you would with python 3.