Running python program using earlier version of python

Question 1

Either directly use the Python 2.4 interpreter to run it, or modify the programs she-bang line to point to the interpreter you wish to use.

Note that there's many things in common use in recent python (any/all, the 1 if 2 else 3 syntax, as well as major stdlib and language changes) that may cause your program to experience difficulties.

It's also worth noting that a lot of the common 3rd party modules require at least 2.5 and some of those are even dropping that and only guaranteeing 2.6+ compatibility.

Question 2

You can install Python 2.4 as well. Any of the minor versions, 2.4, 2.5, 2.6, etc. can live side by side.

Code you write for 2.4 will also run on Python 2.7, albeit that you may hit some deprecation warnings. If you are using the 2.7 interpreter to write 2.4 code, you'll need to be careful that you don't use syntax and modules that have been added in newer Python versions.

To see what has been added, look at the What's new documentation; there is a document for each minor version:

You specifically want to look for syntax changes and for new modules to avoid.

Question 3

There are a few things that can bite you. Some syntactic changes have happened since 2.4, so you may get syntax errors. The standard library is bigger in 2.7, so you may have some things missing. The docs generally lists the version of python when things were added, and can be great help in making sure things will run on different python versions. Generally, syntax and libraries are forward compatible, so if you have to support 2.4, I would write using 2.4 and things should work with 2.7. The same is not true in reverse.