How to structure Python code to support multiple releases of MY project (i.e. not multiple versions of Python)
There are a couple of tricks - they basically reply on discipline on the development teams part:

Publish an API and stick to it all V2 functions of the same name must accept the same parameters and return compatible results when so called.
Have a general interface say fred that just wraps your V1 functions 
Make use of named and defaulted parameters rather than positional parameters, then if your version 2 code supports some additional parameters but the defaults result in the version 1 behaviour you will be golden.
Use Namespaces
If you define your functions in the fn(*arg, **agv) format you can have a wrapper in your general interface that works out if the V1 or V2 functions are intended in a given call.
You can have an optional parameter ifversion=1 in all your functions that might be ambiguous and use it when you specifically need the version 2 stuff.

There is another really simple method which is for the V2 front end to connect to a different port or use a specific flag in the html requrest.
I'm working on the back end of a web-based system.  My code will receive calls from our web site and perform the actions requested by the user.  We would like to support multiple versions of our front end simultaneously.  So, for example, I might receive a request from V1 of our front end or from V2 of it.  I need to respond to either of these calls.
As you might expect, a lot of my code will be the same across versions.  For example, my function *get_list_access_params()*  will probably appear in both V1 and V2 (although there may be some changes to the code in it).  My listener should grab the request, figure out which version of our system the call came from and, then, call the right version of *get_list_access_params()*.
My hope is to not have to duplicate and rename the function as v1_get... and v2_get... but, rather, to duplicate the function in two code files, a v1 file and a v2 file.
This must be a common need but I can't figure out where to look for the answer.  Does anyone have a quick answer or can you direct me to a simple place to find it (I am a Python novice, BTW)?  Thank you!