An object is an instance (or not)?

Mario Figueiredo marfig at gmail.com
Tue Jan 27 21:12:24 CET 2015


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.



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