Register a post_load hook after Schema defintion?

Question

I'm trying to implement a Schema that deserializes into an object, where the object class is not known at Schema-definition time. I would have assumed that I can register a post_load function at runtime, but it appears that post_load works only for class methods.

It seems like I could make it work by either:

• Updating Schema._hooks manually or by
• Somehow creating a bound method at runtime and registering that.

Since both of these options are somewhat hack-ish, is there an official way to achieve the same result?

Answer 1

I don't think you need a metaclass.

Define a base schema with a post-load method that just needs the class.

class CustomSchema(Schema):

    @post_load
    def make_obj(self, data):
        return self.OBJ_CLS(**data)

If the class is known at import time (not your use case), this allows you to factorize the instantiation by just providing the class. Nice already.

class PetSchema(CustomSchema):

    OBJ_CLS = Pet

If the class is not known at import time, then it can be provided afterwards.

class PetSchema(CustomSchema):
    pass


PetSchema.OBJ_CLS = Pet

If you need some more processing before instantiating, then you can override make_obj in any class, as you show in your answer.

class PetSchema(CustomSchema):
    def make_obj(self, data):
        data = my_func(data)
        return Pet(**data)

---

More generally, this mechanism allows you to define hooks in a base schema. This is a good way to overcome a current limitation in marshmallow: the fact that multiple post_load methods can be executed in any order. Define a single post_load method in a base class with a hook for each processing step. (This contrived example doesn't really illustrate the point.)

class CustomSchema(Schema):

    @post_load
    def post_load_steps(self, data):
        data = self.post_load_step_1(data)
        data = self.post_load_step_2(data)
        data = self.post_load_step_3(data)
        return data

    def post_load_step_1(self, data):
        return data

    def post_load_step_2(self, data):
        return data

    def post_load_step_3(self, data):
        return data