Metaclass/abc hackery

[Marco Buttu]

On 10/12/2013 04:47 AM, Demian Brecht wrote:
>
> Working on this though brought up a question: Is there anything in the
> data model that acts like "__setattr__" but when operating on a class
> definition instead of an instance? I'd be able to get rid of the
> late_bind function if something like that's available... Not likely
> something that would be used very often, but would likely sometimes be
> useful.
>
> Thanks,

I am not sure about your question, but I try to explain a bit some 
possibilities. If you define a __setattr__ method in the metaclass, then 
you can intercept the attribute assignments only after class creation:

 >>> class MetaFoo(type):
...     def __setattr__(cls, name, value):
...         print("in __setattr__(): ", name, value)
...         super().__setattr__(name, value)
...
 >>>
 >>> class Foo(metaclass=MetaFoo):
...     a = 33

As you can see, the above code does not print the message. But after 
class creation it does:

 >>> Foo.a = 33
in __setattr__():  a 33

This because during the class creation there is any class yet, so it is 
not possible to intercept argument assignment by __setattr__.
If you want to intercept the assignments during class creation too, you 
can intercept the class attribute dictionary assignment. In this case 
you can just write a dictionary object that overrides __setitem__, and 
then by overriding the __prepare__ metaclass method in order to return 
this dictionary:

 >>> class Namespace(dict):
...     def __setitem__(self, name, value):
...         print('In Namespace.__setitem__():', name, value)
...         super().__setitem__(name, value)
...
 >>> class MetaFoo(type):
...     def __prepare__(clsname, bases):
...         return Namespace()
...     def __setattr__(cls, name, value):
...         print("In MetaFoo.__setattr__(): ", name, value)
...         super().__setattr__(name, value)
...
 >>> class Foo(metaclass=MetaFoo):
...     a = 33
...
In Namespace.__setitem__(): __module__ __main__
In Namespace.__setitem__(): __qualname__ Foo
In Namespace.__setitem__(): a 33
 >>> Foo.a = 33
In MetaFoo.__setattr__():  a 33

Of course, it is not a so good solution, because if you need to manage 
in the same way either the attributes before or after the class 
creation, you have to do it by writing some code outside the methods:

 >>> def manage(name, value):
...     print('do something with', name, value)
...
 >>> class Namespace(dict):
...     def __setitem__(self, name, value):
...         print('In Namespace.__setitem__():', name, value)
...         manage(name, value)
...         super().__setitem__(name, value)
...
 >>> class MetaFoo(type):
...     def __prepare__(clsname, bases):
...         return Namespace()
...     def __setattr__(cls, name, value):
...         print("In MetaFoo.__setattr__(): ", name, value)
...         manage(name, value)
...         super().__setattr__(name, value)
...
 >>> class Foo(metaclass=MetaFoo):
...     a = 33
...
In Namespace.__setitem__(): __module__ __main__
do something with __module__ __main__
In Namespace.__setitem__(): __qualname__ Foo
do something with __qualname__ Foo
In Namespace.__setitem__(): a 33
do something with a 33
 >>> Foo.a = 33
In MetaFoo.__setattr__():  a 33
do something with a 33

-- 
Marco Buttu