How should I deal with a circular import in Google App Engine?

Question

If I have "a.py"

from google.appengine.ext import db
class A(db.Model):
    db.ReferenceProperty(b.B)
    ...other stuff

and another file "b.py"

from google.appengine.ext import db
class B(db.Model):
    db.ReferenceProperty(a.A)
    ...other stuff

It would appear that Python simply does not allow circular dependencies. Normally I guess you would alter the code such that the two classes actually can resolve themselves without importing one another directly. Perhaps by consolidating their reference to one another through a third intermediary? But I can't just use a normal intermediary class, as all classes would ultimately need to be persisted to the database? Is there any correct solution to structuring the above code such that it works?

I have a feeling that I am going to get a lot of "bad smelling code", "decouple", "bad design", etc comments. So I ask that if you say that, please illustrate what you would do with an actual example. Are there any solutions that would involve leaving the references, classes, and modules as they stand?

Thank you.

Answer 1

What happens if you define both models in the same module? e.g. a_b.py

Answer 2

According to the documentation:

ReferenceProperty has another handy feature: back-references. When a model has a ReferenceProperty to another model, each referenced entity gets a property whose value is a Query that returns all of the entities of the first model that refer to it.

So you should probably be able to use the automatically added back-reference.

Answer 3

The workaround is to have a ReferenceProperty in at least one of the models that doesn't restrict itself to a particular class, and then enforce only referencing that class in your own code.

e.g.,

class A(db.Model):
  b = db.ReferenceProperty()

class B(db.Model):
  a = db.ReferenceProperty(A)

You'll be able to assign any model instance to the b variable; just make sure you only assign actual Bs.