An object is an instance (or not)?

Gregory Ewing greg.ewing at canterbury.ac.nz
Thu Jan 29 07:22:31 CET 2015


Steven D'Aprano wrote:

> In fairness, "inherit" is standard terminology for the way instances get
> their behaviour from their class.

I'm not sure that's true, but even if it is, it's
not the same kind of inheritance relationship as
exists between a class and a base class, which was
my point.

> Also, you can override methods *on the instance*:

I wouldn't call that a method -- it's just an instance
attribute that happens to be a function. You can tell
it's not a method because it doesn't get a 'self' argument.

> In Python, obj.talk performs the following (grossly simplified)
> process:
> 
> * search the instance for an attribute "talk"
> * search the class
> * search all the base classes
> * fail
> 
> (simplified because I have ignored the roles of __getattr__ and
> __getattribute__, of metaclasses, and the descriptor protocol)

By ignoring the descriptor protocol, you're simplifying
away something very important. It's the main thing that
makes the instance-of relation different from the
subclass-of relation.

> The normal way of giving a class methods that are callable from the class is
> to define them on the class with the classmethod or staticmethod
> decorators. Using a metaclass is usually overkill :-)

True, but I was trying to illustrate the symmetry
between classes and instances, how the classic OOP
ideas of Smalltalk et al are manifest in Python,
and to show that classes *can* "participate fully
in OOP" just like any other objects if you want them
to. Python's "class methods" are strange beasts that
don't have an equivalent in the classic OOP model,
so they would only have confused matters.

And there are cases where you *do* need a metaclass,
such as giving a __dunder__ method to a class, so it's
useful to know how to do it just in case.

> An early essay on Python metaclasses was subtitled "The Killer-Joke".

Quick, translate this post into German before anyone
sees too much of it!

-- 
Greg



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