[Python-ideas] Fwd: Define a method or function attribute outside of a class with the dot operator

[Steve Dower]

On 10Feb2017 1400, Stephan Hoyer wrote:
> An important note is that ideally, we would still have way of indicating
> that Spam.func should exists in on the Spam class itself, even if it
> doesn't define the implementation. I suppose an abstractmethod
> overwritten by the later definition might do the trick, e.g.,
>
> class Spam(metaclass=ABCMeta):
>     @abstractmethod
>     def func(self):
>         pass
>
> def Spam.func(self):
>     return __class__

An abstractfunction should not become a concrete function on the 
abstract class - the right way to do this is to use a subclass.

class SpamBase(metaclass=ABCMeta):
     @abstractmethod
     def func(self):
         pass

class Spam(SpamBase):
     def func(self):
         return __class__


If you want to define parts of the class in separate modules, use mixins:

from myarray.transforms import MyArrayTransformMixin
from myarray.arithmetic import MyArrayArithmeticMixin
from myarray.constructors import MyArrayConstructorsMixin

class MyArray(MyArrayConstructorsMixin, MyArrayArithmeticMixin, 
MyArrayTransformMixin):
     pass


The big different between these approaches and the proposal is that the 
proposal does not require both parties to agree on the approach. This is 
actually a terrible idea, as subclassing or mixing in a class that 
wasn't meant for it leads to all sorts of trouble unless the end user is 
very careful. Providing first-class syntax or methods for this 
discourages carefulness. (Another way of saying it is that directly 
overriding class members should feel a bit dirty because it *is* a bit 
dirty.)

As Paul said in an earlier email, the best use of non-direct assignment 
in function definitions is putting it into a dispatch dictionary, and in 
this case making a decorator is likely cleaner than adding new syntax.

But by all means, let's have a PEP. It will simplify the discussion when 
it comes up in six months again (or whenever the last time this came up 
was - less than a year, I'm sure).

Cheers,
Steve