How can I tell if an Expando subclass has a property defined?

Question

I'm creating an app that I want to have an expandable set of properties (each a RatingProperty) I also want to validate that any dynamic properties are of the RatingProperty type.

In the Expando documentation it says:

Tip: If you want to validate a dynamic property value using a Property class, you can instantiate the Property class and call its validate() method on the value.

So if I want to validate a dynamic property I need to know what the class's non-dynamic properties are. How can I ask my class what it's defined properties are?

I've considered creating a class method that takes a string and returns true if that string is in a list of property names that I create and maintain, but it seems like a hack. I've searched the Google for tips, but haven't had any luck.

Thanks, Pat

Answer 1

After a bit more research (damn you lazyweb!) I've found a solution that I think is acceptable:

A dynamic property can't be of a db subclassed property type. Thus, there are two distinct steps that must be taken. First you need to create an instance of your property class and validate your value:

test = db.RatingProperty()
if test.validate(valueToSave):
    #do your thing

Next you need to check if the property you want to save is a declared property:

if valueToSaveKey not in myObject.properties():
    #if not save it as desired
    myObject.valueToSaveKey = valueToSave

The down side here is that the value you save isn't stored as the property type you want.

Answer 2

I may be misunderstanding your question, but if you have a list of properties you expect to find, why not just use a standard db.Model, instead of an Expando? You can add additional properties to a Model class, as long as you either provide a default or don't make them required.