Everything is an object in python - object class and type class

[Steven D'Aprano]

On Tue, 2 Jun 2015 08:36 pm, Eddilbert Macharia wrote:

> you guys are just confusing me, you are going in loops, and still i have
> understood ,what makes everything in python an object. hey is where i'm at
> : *** type in python refers to data types e.g. int, str, boolean e.t.c.
> right ?

Yes. Also classes you create with the "class" keyword:

class K(object):
    ...

K is now a "type", just like int, str, list, object, etc.


> *** The interpreter creates two classes type and object when setting up a
> python environment. right ?

Many more than just two: it also creates list, str, dict, etc. But *first*
it has to create type and object. So you are correct.


> *** The creator (metaclass) of all data types (i.e. int,str) in python is
> the class type. right ?

Correct.

[Aside: I'm only talking about Python 3 here. In Python 2 there is also a
second hierarchy of classes, called "classic classes" or "old-style
classes", which are *not* subclasses of type. But let's just ignore them,
because they are gone in the most recent versions of Python.]


>>>> isinstance(int,type)
> True
> 
> *** The instance of class type is a data type an instance of class type.
> right ?
>>>> type(type)
> <class 'type'>

type has many instances, not just one.

Instances of int are individual ints, like 0, 1, 2, 3, ...

Instances of type are individual types, like int, dict, str, list, ...

But one of those many instances of type is, yes, type itself! So *one* of
the instances of type is type, which is also an instance of itself:

>>>> isinstance(type,type)
> True

Correct. This makes type special. Most types are *not* instances of
themselves:

py> isinstance(int, int)
False


> *** Class type gets some of its behavior from class object through
> inheritance.right ?
> 
>>>> issubclass(type,object)
> True

Correct.


> *** instance of class object is type, in the sense it created using class
> type which inherits from class object.right ?
> 
>>>> isinstance(object,type)
> True
>>>> isinstance(object,object)
> True
> 
> ****so is it right to say, everything in python is an object because they
> are instance of the class type which inherits from class object ?

No! That's not what we mean when we say "everything is an object".

Eddilbert, have you programmed in any other languages? It would help you
understand if you have.

"Object" has a general meaning in computer science and programming, it is a
compound data structure that is explicitly linked to a type which provides
functionality that operates on that data structure.

In the programming language C, *no* values are objects. C has types (int16,
uint32, bool, and many more) but no objects.

In the programming language Java, *some* values are objects, and some values
are not objects.

In the programming language Python, *all* values are objects, in the general
sense. That is what we mean by "Everything is an object".

Let's go back in time about 15 years or so. We're now using Python 1.5. In
Python 1.5, there is no built-in object, and type is just a function, not a
class:

>>> import sys
>>> print sys.version
1.5.2 (#1, Aug 27 2012, 09:09:18)  [GCC 4.1.2 20080704 (Red Hat 4.1.2-52)]
>>> object
Traceback (innermost last):
  File "<stdin>", line 1, in ?
NameError: object
>>> type
<built-in function type>


In Python 1.5, classes you create do not inherit from object, because object
does not exist! BUT even in Python 1.5, it is true that everything is an
object.

Remember, "object" can refer to two things:

- *specifically* the class called "object";

- the *general concept* of objects, from object oriented programming.


In Python 1.5:

- everything is an object [the general concept]

- nothing is an instance of the class called "object"


In Python 2:

- everything is an object [the general concept]

- some things, but not all things, are instances of the class
  called "object"


In Python 3:

- everything is an object [the general concept]

- everything is an instance of the class called "object"




-- 
Steven