Simon Willison’s Weblog

Backporting from Python 2.3 to Python 2.2

We have a home-grown templating system at work, which I intend to dedicate an entry to some time in the future. We originally wrote it in Python 2.2, but upgraded to Python 2.3 a while ago and have since been evolving our code in that environment. Today I found a need to load the most recent version of our templating system on to a small, long neglected application that had been running the original version ever since it had enough features to be usable.

Unfortunately, this application was running on a server that only had Python 2.2. Installing Python 2.3 would have been somewhat more painful here than on other servers we run for reasons I won’t go in to, so I decided to have a go at getting our current code to run under the older Python version.

In the end, I only had to make three minor changes, all at the top of the file in question.

1. I added from __future__ import generators as the very first line of the file. We use generators (with the yield statement) in a few places—this feature was only properly added in Python 2.3, but was made available in Python 2.2 as a “future enhancement” through the aforementioned obscure import.

2. I added True, False = 1, 0 on the next line down. Surprisingly, Python 2.2 had no support for a boolean type and instead used a test for non-zero. The above line defines constants that behave enough like Python 2.3’s True and False to avoid any problems.

3. I defined an enumerate function, which was introduced for real in Python 2.3. Here’s the code I used:

   
   def enumerate(obj):
       for i, item in zip(range(len(obj)), obj):
           yield i, item 
   

All in all it only took around ten minutes to put the above together, after which the script worked just fine. It was interesting to see how our code had grown to rely on Python 2.3 features without us realising it.

Update: Check this entry’s comments for improvements to the above code snippets.