How can I use a different version of Python for the Save & Run button?

The Save & Run button defaults to the version of Python that was the most recent one that we supported when your account was created -- which for all but the oldest accounts will be a recent 3.x. If you want to use a different version, you can do so by inserting a special line at the top of your file, called a hashbang.

For example, you could use 2.7 like this:

#!/usr/bin/python2.7
print "hello from python 2.7"

Or you could use 3.4 like this:

#!/usr/bin/python3.4
print("hello from python 3.4")

You can also use a hashbang to run your script in a virtualenv that you've defined by pointing it at the python executable in the virtualenv.

#!/home/myusername/.virtualenvs/myvenv/bin/python
print("hello from python in a virtualenv")

NB the hashbang has to be the very first line in the file.