how to pass in dynamic data to decorators

Question

Hi,

I am trying to write a base crud controller class that does the following:

class BaseCrudController:
    model = ""
    field_validation = {}
    template_dir = ""

    @expose(self.template_dir)
    def new(self, *args, **kwargs)
        ....

    @validate(self.field_validation, error_handler=new)
    @expose()
    def  post(self, *args, **kwargs):
        ...

My intent is to have my controllers extend this base class, set the model, field_validation, and template locations, and am ready to go.

Unfortunately, decorators (to my understanding), are interpreted when the function is defined. Hence it won't have access to instance's value. Is there a way to pass in dynamic data or values from the sub class?

For example:

class AddressController(BaseCrudController):
    model = Address
    template_dir = "addressbook.templates.addresses"

When I try to load AddressController, it says "self is not defined". I am assuming that the base class is evaluating the decorator before the sub class is initialized.

Thanks, Steve

Answer 1

Perhaps using a factory to create the class would be better than subclassing:

def CrudControllerFactory(model, field_validation, template_dir):
    class BaseCrudController:
        @expose(template_dir)
        def new(self, *args, **kwargs)
            ....

        @validate(field_validation, error_handler=new)
        @expose()
        def  post(self, *args, **kwargs):
            ....

    return BaseCrudController

Answer 2

Unfortunately, decorators (to my understanding), are interpreted when the function is defined. Hence it won't have access to instance's value. Is there a way to pass in dynamic data or values from the sub class?

The template needs to be called with the name of the relevant attribute; the wrapper can then get that attribute's value dynamically. For example:

import functools

def expose(attname=None):
  if attname:
    def makewrapper(f):
      @functools.wraps(f)
      def wrapper(self, *a, **k):
        attvalue = getattr(self, attname, None)
        ...use attvalue as needed...
      return wrapper
    return makewrapper
  else:
    ...same but without the getattr...

Note that the complication is only because, judging from the code snippets in your Q, you want to allow the expose decorator to be used both with and without an argument (you could move the if attname guard to live within wrapper, but then you'd uselessly repeat the check at each call -- the code within wrapper may also need to be pretty different in the two cases, I imagine -- so, shoehorning two different control flows into one wrapper may be even more complicated). BTW, this is a dubious design decision, IMHO. But, it's quite separate from your actual Q about "dynamic data".

The point is, by using the attribute name as the argument, you empower your decorator to fetch the value dynamically "just in time" when it's needed. Think of it as "an extra level of indirection", that well-known panacea for all difficulties in programming!-)