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Groups > comp.lang.python > #84665 > unrolled thread

An object is an instance (or not)?

Started byMario Figueiredo <marfig@gmail.com>
First post2015-01-27 21:12 +0100
Last post2015-01-27 18:11 -0800
Articles 3 on this page of 63 — 17 participants

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  An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-27 21:12 +0100
    Re: An object is an instance (or not)? Ethan Furman <ethan@stoneleaf.us> - 2015-01-27 12:36 -0800
    Re: An object is an instance (or not)? André Roberge <andre.roberge@gmail.com> - 2015-01-27 12:50 -0800
      Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-27 22:06 +0100
        Re: An object is an instance (or not)? André Roberge <andre.roberge@gmail.com> - 2015-01-27 13:38 -0800
          Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-27 22:43 +0100
            Re: An object is an instance (or not)? André Roberge <andre.roberge@gmail.com> - 2015-01-27 13:49 -0800
              Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-27 22:58 +0100
                Re: An object is an instance (or not)? Ben Finney <ben+python@benfinney.id.au> - 2015-01-28 09:32 +1100
                  Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 18:55 +1100
                    Re: An object is an instance (or not)? Ben Finney <ben+python@benfinney.id.au> - 2015-01-28 22:20 +1100
                Re: An object is an instance (or not)? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-01-27 23:12 +0000
                  Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-28 01:52 +0100
                    Re: An object is an instance (or not)? Marko Rauhamaa <marko@pacujo.net> - 2015-01-28 07:25 +0200
                      Re: An object is an instance (or not)? Rustom Mody <rustompmody@gmail.com> - 2015-01-27 21:53 -0800
                    Re: An object is an instance (or not)? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-01-28 05:57 +0000
                      Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-28 11:48 +0100
        Re: An object is an instance (or not)? random832@fastmail.us - 2015-01-28 00:37 -0500
          Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 19:45 +1100
        Re: An object is an instance (or not)? Devin Jeanpierre <jeanpierreda@gmail.com> - 2015-01-27 21:43 -0800
        Re: An object is an instance (or not)? random832@fastmail.us - 2015-01-28 01:01 -0500
          Re: An object is an instance (or not)? Rustom Mody <rustompmody@gmail.com> - 2015-01-27 22:22 -0800
            Re: An object is an instance (or not)? Ben Finney <ben+python@benfinney.id.au> - 2015-01-28 18:03 +1100
          Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 19:48 +1100
        Re: An object is an instance (or not)? Ben Finney <ben+python@benfinney.id.au> - 2015-01-28 17:59 +1100
        Re: An object is an instance (or not)? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-01-28 20:21 +1300
        Re: An object is an instance (or not)? Ben Finney <ben+python@benfinney.id.au> - 2015-01-28 18:48 +1100
        Re: An object is an instance (or not)? random832@fastmail.us - 2015-01-28 12:08 -0500
          Re: An object is an instance (or not)? Rustom Mody <rustompmody@gmail.com> - 2015-01-28 18:29 -0800
    Re: An object is an instance (or not)? Ned Batchelder <ned@nedbatchelder.com> - 2015-01-27 18:22 -0500
      Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-28 01:17 +0100
        Re: An object is an instance (or not)? Chris Angelico <rosuav@gmail.com> - 2015-01-28 11:30 +1100
          Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-28 01:35 +0100
            Re: An object is an instance (or not)? alex23 <wuwei23@gmail.com> - 2015-01-28 14:44 +1000
            Re: An object is an instance (or not)? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-01-28 23:33 +1300
              Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 22:52 +1100
                Re: An object is an instance (or not)? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-01-29 19:22 +1300
                  Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-29 18:16 +1100
                    Re: An object is an instance (or not)? Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-29 08:48 -0700
                    Re: An object is an instance (or not)? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-01-30 22:07 +1300
                      Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-31 10:45 +1100
        Re: An object is an instance (or not)? Ned Batchelder <ned@nedbatchelder.com> - 2015-01-27 21:21 -0500
          Re: An object is an instance (or not)? Rustom Mody <rustompmody@gmail.com> - 2015-01-27 18:52 -0800
          Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 20:16 +1100
            Re: An object is an instance (or not)? Chris Angelico <rosuav@gmail.com> - 2015-01-28 21:01 +1100
              Re: An object is an instance (or not)? Rick Johnson <rantingrickjohnson@gmail.com> - 2015-01-29 14:22 -0800
        Re: An object is an instance (or not)? Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-27 19:15 -0700
        Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 20:28 +1100
        Re: An object is an instance (or not)? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-01-28 23:33 +1300
      Re: An object is an instance (or not)? Grant Edwards <invalid@invalid.invalid> - 2015-01-28 00:23 +0000
    Re: An object is an instance (or not)? Ben Finney <ben+python@benfinney.id.au> - 2015-01-28 10:39 +1100
      Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-28 01:24 +0100
        Re: An object is an instance (or not)? Ben Finney <ben+python@benfinney.id.au> - 2015-01-28 12:00 +1100
          Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-28 02:14 +0100
        Re: An object is an instance (or not)? alex23 <wuwei23@gmail.com> - 2015-01-28 14:47 +1000
          Re: An object is an instance (or not)? Rustom Mody <rustompmody@gmail.com> - 2015-01-27 21:23 -0800
          Re: An object is an instance (or not)? random832@fastmail.us - 2015-01-28 00:44 -0500
            Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 20:21 +1100
        Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 20:17 +1100
      Re: An object is an instance (or not)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-28 20:09 +1100
    Re: An object is an instance (or not)? Chris Angelico <rosuav@gmail.com> - 2015-01-28 10:42 +1100
      Re: An object is an instance (or not)? Mario Figueiredo <marfig@gmail.com> - 2015-01-28 01:31 +0100
    Re: An object is an instance (or not)? Rustom Mody <rustompmody@gmail.com> - 2015-01-27 18:11 -0800

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#84686

FromChris Angelico <rosuav@gmail.com>
Date2015-01-28 10:42 +1100
Message-ID<mailman.18193.1422402146.18130.python-list@python.org>
In reply to#84665
On Wed, Jan 28, 2015 at 10:22 AM, Ned Batchelder <ned@nedbatchelder.com> wrote:
> 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.

My understanding is that "instance" is meaningless unless followed by
"of". That is to say, 123.45 is an object, and it is an instance *of*
the 'float' class. Everything in Python is an instance *of something*,
so in a sense, you can say that everything is an instance, but that's
like saying that everything has a color. Sure it does, but you need to
be more specific.

ChrisA

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#84692

FromMario Figueiredo <marfig@gmail.com>
Date2015-01-28 01:31 +0100
Message-ID<MPG.2f324c19ac737fdc9896ab@nntp.aioe.org>
In reply to#84686
In article <mailman.18193.1422402146.18130.python-list@python.org>, 
rosuav@gmail.com says...
> 
> On Wed, Jan 28, 2015 at 10:22 AM, Ned Batchelder <ned@nedbatchelder.com> wrote:
> > 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.
> 
> My understanding is that "instance" is meaningless unless followed by
> "of". That is to say, 123.45 is an object, and it is an instance *of*
> the 'float' class. Everything in Python is an instance *of something*,
> so in a sense, you can say that everything is an instance, but that's
> like saying that everything has a color. Sure it does, but you need to
> be more specific.
> 

In programming languages in which class definitions aren't first-class 
objects, the terms are in fact used interchangeably. And rightly so, 
because an object is in fact always an instance of some class.

Python and a few other languages implement class definitions as first-
class objects. In this case, the distinction between an object and an 
instance is actually an implementation detail and comes with its own 
semantics. This is why I object to the notion that in Python object = 
instance.

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#84704

FromRustom Mody <rustompmody@gmail.com>
Date2015-01-27 18:11 -0800
Message-ID<9a470405-d22d-448b-83ff-93fccd92bea6@googlegroups.com>
In reply to#84665
On Wednesday, January 28, 2015 at 1:42:47 AM UTC+5:30, 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.
> 
>     >>> class Sub:
>     >>>     pass
>     
>     >>> foo = Sub()
>     >>> foo.__bases__
>     [...]
>     AttributeError: 'Sub' object has no attribute '__bases__'
> 
> I'm making this into a new thread, because the particular discussion of 
> whether an object is an instance in Python seems more interesting than 
> discussing whether that error message should be changed or not.
> 
> Here's another example where the terminology keeps indicating that in 
> Python an object is an instance:
> 
>     >>> class Sub:
> 	    pass
> 
>     >>> x = Sub()
>     >>> x
>     <__main__.Sub object at 0x02631690>
> 
> The problem is that I personally cannot agree with this terminology and 
> I would like to hear arguments that could convince me to adopt the 
> Python way. But before your replies, here's my argumentation:
> 
> 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. In programming languages in 
> which the classes aren't fully realized objects, it is ok to speak of 
> 'instance' and 'object' interchangeably. 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. 
> 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.
> 
> ...
> 
> I don't think that any of this is reason enough to rewrite error 
> messages in Python. Seems unnecessary. What I'm arguing thought is that 
> error messages in Python cannot become the source of new terminology. 
> The language itself implements a very clear distinction between class 
> objects and their instances. And it is thus wrong of us to say that 
> Object = Instance. At least from an OOP perspective.

The issue (as I see it) is primarily with English - in particular the word 
'is', technically called copula:
https://en.wikipedia.org/wiki/Copula_%28linguistics%29

Is is(!) used in 3 ways that can get mutually incompatible:
Predication: The sky is blue
Identity: Two plus three is five [Notice how you've used '=' above]
Belonging: (OOP isa relation): Cat is a mammal

Some people have found this (mis|over)use of 'is' so problematic that they have 
suggested to doctor English to not use is as far as possible:
https://en.wikipedia.org/wiki/E-Prime

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