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

Steven D'Aprano steve at pearwood.info
Mon Jun 1 06:09:10 CEST 2015


On Mon, 1 Jun 2015 10:29 am, Eddilbert Macharia wrote:

> 
> So what im getting from above reponse, everything in python is an object
> because the are instance of the metaclass type and also because they are
> subclasses of the class object ?

No.

Everything in Python is an object because Python has no "primitive"
or "unboxed" types, no machine values. This has nothing to do with
metaclasses. Even if Python had no metaclasses, it could still be true
that "everything is an object".

To summarise:

(1) In Python, "everything is an object" (that is, all values are objects)
is because Python does not include any primitive unboxed values. Anything
which can be used as a value (int, str, float, functions, modules, etc) are
implemented as objects.

(2) Python *also* has metaclasses. This does not follow from the above --
they are independent statements about the language. We could, if we want,
invent new languages:

- one where everything is an object, just like Python, but lacking
metaclasses;

- one which has metaclasses, just like Python, but not everything is an
object;

- and one which *neither* has metaclasses, *nor* everything is an object.

(Java is an example of the third one -- it has something kinda-sorta like
metaclasses, but it's just a tool for reflection, not the real thing.)


If that's enough, you can stop reading. But if you want more detail:


We can characterise various kinds of things in a programming language:

(1) Simple machine types, like bytes, integers, floats. These have no
internal structure, apart from individual bits. These are sometimes
called "machine primitives", and are the sorts of things you work with in
assembly language.

(2) Compound or structured machine types. These are made up of one or more
simple machine type, and include things like C structs and arrays.

(3) Objects, which are a more complex kind of compound type. More
importantly, objects involve a way of thinking about programming: some form
of inheritance, for example. Physically, an object is just a struct. But
it's a struct with a specific kind of meaning to the programming language,
which allows the language to provide inheritance, runtime types, and
various other goodies involved in Object Oriented Programming.

A language can choose to allow any or these, or all of these:

- assembly languages typically only support the most primitive, simple
machine types;

- languages like C and Pascal supports only simple and compound machine
types;

- some languages include both simple machine primitives and objects, such as
Java and Javascript;

- other languages don't offer any direct access to machine primitives, so
all values are objects, for example Python and Ruby.


To give a concrete example, consider a language with an "integer" type. In
C, an integer will be a machine primitive, a single, unstructured chunk of
memory with typically 32 bits, and that's all there is to it. In Python, an
integer will be an object, a large block of *structured* memory, typically
over 100 bits long, and different parts of that structure are used
internally to the interpreter.

A separate issue is whether or not classes are values. In some languages,
classes are *not* values: you cannot pass a class as argument to a
function, or assign it to a variable. In languages like these, classes are
not things (values), so "everything is an object" remains true even though
classes are not objects. Java is an example of that.

In Python, classes *are* variables. So we could have:

"In Python, everything is an object, or a class";

or

"In Python, everything is an object, including classes".


Which is true? The second is true: classes themselves are objects.

Finally, we have one last feature: does the language provide metaclasses?
Python does. Ruby, which is otherwise very similar to Python, does not:

In Python, everything is an object, and we have metaclasses.

In Ruby, everything is an object, but we don't have metaclasses.

http://ruby.dzone.com/news/ruby-doesnt-have-meta-classes



-- 
Steven



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