How do I memoize expensive calculations on Django model objects?

Question

I have several TextField columns on my UserProfile object which contain JSON objects. I've also defined a setter/getter property for each column which encapsulates the logic for serializing and deserializing the JSON into python datastructures.

The nature of this data ensures that it will be accessed many times by view and template logic within a single Request. To save on deserialization costs, I would like to memoize the python datastructures on read, invalidating on direct write to the property or save signal from the model object.

Where/How do I store the memo? I'm nervous about using instance variables, as I don't understand the magic behind how any particular UserProfile is instantiated by a query. Is __init__ safe to use, or do I need to check the existence of the memo attribute via hasattr() at each read?

Here's an example of my current implementation:

class UserProfile(Model):
    text_json = models.TextField(default=text_defaults)

    @property
    def text(self):
        if not hasattr(self, "text_memo"):
            self.text_memo = None
        self.text_memo = self.text_memo or simplejson.loads(self.text_json)
        return self.text_memo
    @text.setter
    def text(self, value=None):
        self.text_memo = None
        self.text_json = simplejson.dumps(value)

Answer 1

You may be interested in a built-in django decorator django.utils.functional.memoize.

Django uses this to cache expensive operation like url resolving.