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To: python-list@python.org
From: Ned Batchelder <ned@nedbatchelder.com>
Subject: Re: An object is an instance (or not)?
Date: Tue, 27 Jan 2015 18:22:00 -0500
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On 1/27/15 3:12 PM, Mario Figueiredo wrote:
> This is a follow up from a previous discussion in which it is argued
> that the following code produces the correct error message terminology,
> considering that in Python an object is also an instance.

I don't know what the difference is between "object" and "instance".  An 
object is an instance of a class.  The two words are interchangeable as 
far as I know.

> An instance IS an object. On that we can agree. After all, everything in
> Python is an object. Even classes are. We can even pass them as function
> arguments:
>
>      >>> class Sub:
> 	    pass
>
>      >>> def show(aClass):
> 	    print(type(aClass))
> 	
>      >>> show(Sub)
>      <class 'type'>
>
> The problem is that an object isn't always an instance. The word
> instance in OOP has a very formal meaning.

A common mistake is to believe that "OOP" is a well-defined term.  It's 
not it's a collection of ideas that are expressed slightly differently 
in each language.

> In programming languages in
> which the classes aren't fully realized objects, it is ok to speak of
> 'instance' and 'object' interchangeably.

I don't know what a "not fully realized object" is.

> But even then, sometimes the
> term 'object instance' is preferred, as a way to separate these
> 'instances' from other variable instances that may not be created from
> class definitions (e.g. C++ built-in types).
>
> The fact that in Python classes are objects, should not just eliminate
> this distinction. The OOP terminology should exist beyond the language
> implementing it. It facilitates discourse and helps transmiting concepts
> when describing your ideas to another programmer. And because in python,
> classes are of the type 'type' and they exist as fully realized objects,
> is no excuse to make a distinction between them and their own fully
> realized instances. The language implementation details should not exist
> as a way for us to freely reformulate long standing terms.
>
> Because, from my own study of Python, I've came to realize that the
> distinction between objects and instances of objects actually exists. In
> Python, class objects cannot participate in OOP, only their instances.

What does "participate in OOP" mean?

> This is why I say that even in Python, where a class is an object, an
> object is not always an instance. The language itself forces that
> distinction.

-- 
Ned Batchelder, http://nedbatchelder.com