Question

In the app we’re developing using Django, in some cases we need to automatically assign permissions to users for some models, that has owners (there is no rule for field’s name, it can be “user”, “owner”, “coach” etc., also there can by more than one field.) My solution is to create a decorator containing those fields names, that will be put before model definition, like this (not using django-specific code in samples):

@auto_assign_perms('owner', 'user')
class Test(Base):
    pass

Let’s assume that Base is an abstract class deriving after Django’s Model class, where I add functionality to assign permissions after object is saved. For now I only print a list of users assigned to the class. Below you can find code for the decorator and Base class:

class auto_assign_perms(object):
    def __init__(self, *users):
        self.users = users

    def __call__(self, cls):
        cls.owners.update(self.users)
        return cls


class Base(object):
    owners = set()

    def save(self, *args, **kwargs):
        for owner in self.owners:
            print owner,
        print

And my models could look like this:

@auto_assign_perms('owner', 'user')
class Test(Base):
    pass

@auto_assign_perms('coach')
class Test2(Base):
    pass

The problem is that both child classes contains all three fields ('owner', 'user', 'coach'), altough print self.__class__.__name__ in Base.save() method properly shows “Test” or “Test2”. I tried to add classmethod get_owners() in Base class and then iterating over its results, but it doesn’t helps.
How can I solve this? Maybe I should use metaclasses (I don’t get them yet)? Thanks in advance.

Answer 1

You need to set the list of owners, not update:

class auto_assign_perms(object):
    def __init__(self, *users):
        self.users = users

    def __call__(self, cls):
        cls.owners = set(self.users) # <- here
        return cls

#some tests
@auto_assign_perms('owner', 'user')
class Test(Base):
    pass

@auto_assign_perms('coach')
class Test2(Base):
    pass


t = Test()
t.save()
t = Test2()
t.save()

>>> 
owner user
coach