I was struggling with an issue today on a template that was blowing up due to missing keys in a template data hash. At first I tried to write some conditional code in the template. That ended up being quite ugly so then I was trying to find a way to condition the template on whether the key was present.

Then it struck me that the issue was really the missing key. As the hash is prepared in Python code the original implementation frugally avoided creating the entry if the underlying data wasn’t present. While this is clever and minimal it actually just pushes the complexity out of the Presenter logic and into the template where it is much worse because you can only interact with the Presentation’s abstraction of the original data.

The problem here is that dynamic languages sometimes allow you to be too clever. In a declared structure system you have to declare all your data and provide sensible defaults. This makes writing the templating solution at lot easier as you never have a missing data problem to deal with (you still have headaches with duff defaults but that is a different post).

So I went back to the presenter and declared an empty list under the key that had been causing me grief. Bingo! My failure went away and the default behaviour of not generating any content for the empty list kicked off, reducing my template back to the unconditional single-line I had originally.