When you say:

... classes in the file which are an instance of ...

i think you mean "...classes in the file which are subclass of...". If so:

classes = [cls for cls in dir(module) if issubbclass(cls, models.Model)] 

When you say:

... classes in the file which are an instance of ...

i think you mean "...classes in the file which are subclass of...". If so:

classes = [cls for cls in dir(module)  if issubbclass(getattr(module, cls), models.Model)]