Django Model Validation

I'm really excited about model validation, in Django circa 1.2. It's going to save me from the disastrous hack of ModelForms I've used so far. One wants something to validate data before saving it, since validating it by saving it is apparently a Bad Idea, since you may have already made changes to the database by the time the error is thrown.

I wanted this done automatically, and Django only had form validation until 1.2, so I hacked forms into some sort of all-purpose wrapper for models. I've since learned to program Django like an adult, and model validation came around just in time anyway. But there's a problem I have with it.

First I should point out that there's a few different things that get validated, but the two I'll point out are A) checking that required fields are set and B) whatever I put into the clean() function. Judging by some errors I've gotten while debugging/testing, Django seems to be checking B before A.

Now, when I use a ModelForm, sometimes I want to use the commit=False option when I save. This returns a model that hasn't been committed to the database. Sometimes there's extra data I want to add to the model that the form didn't supply. Sometimes that data is in fact necessary for the model to be valid. So clearly Django shouldn't check A, and it doesn't. Here's the funny thing though: it does check B. Why would it do that? I can understand checking when I call is_valid(), I can control when that's called, making sure I added everything first.

So far the consequences of checking B early have been trying to access members that haven't yet been set, in my clean() function. So in clean() I just check for those items, and just let it pass if they don't exist. I figure, yeah, it'll pass certain tests when it shouldn't. But if it's really running through validation (not just B as in the save (commit=False) case) that means it'll catch the missing members on the A pass anyway.

Maybe I got something wrong, but I thought it was a weird design decision.

Significant Whitespace in Python Data Structures

I recently wrote a program in Python for parsing files. I'm pretty naive still when it comes to functional programming, but I'm still excited about it, so I wanted it to be more functional in style. It had a complicated data structure representing the file structure, instead of a loop with bunch of if-thens. By Python standards I may have gone a bit overboard. Guido probably would not have approved of my code (not to mention what follows in this blog post).

So as a result, most of the program became whitespace irrelevant. Huge dicts of lists of tuples, etc. It made me think that relevant whitespace might become handy for data too. And while talking on IRC about it this morning I realized I could sortof hack it using decorators and generators. So here's what it looks like:

http://gist.github.com/611646

So I have two examples:

• example.py - This shows how you can define a more complicated structure with whitespace instead of a bunch of ){(}[].
  

• inlinefunc.py - This demonstrates a sort of side-effect benefit. You can have multi-line functions inline in a list (or tuple or dict). Usually you're stuck with lambdas, and of course that starts to look confusing too.

I'd like to clean up the syntax. Obviously having to have a decorator before and a yield after isn't great. I'll have to think about how I could do that. Maybe make an even dirtier hack by doing introspection.

Any thoughts?