Re: Difference between type and class

Le Thursday 31 July 2008 14:30:19 Nikolaus Rath, vous avez écrit :
oj <ojeeves@xxxxxxxxx> writes:
On Jul 31, 11:37 am, Nikolaus Rath <Nikol...@xxxxxxxx> wrote:
So why does Python distinguish between e.g. the type 'int' and the
class 'myclass'? Why can't I say that 'int' is a class and 'myclass'
is a type?

I might be wrong here, but I think the point is that there is no
distinction. A class (lets call it SomeClass for this example) is an
object of type 'type', and an instance of a class is an object of type
'SomeClass'.

But there seems to be a distinction:
class int_class(object):

... pass
...

int_class

<class '__main__.int_class'>

int

<type 'int'>

why doesn't this print

class int_class(object):

... pass
...

int_class

<type '__main__.int_class'>

int

<type 'int'>

or

class int_class(object):

... pass
...

int_class

<class '__main__.int_class'>

int

<class 'int'>

If there is no distinction, how does the Python interpreter know when
to print 'class' and when to print 'type'?


There are some confusion about the terms here.

Classes are instances of type 'type', but types are both instances and
subclasses of 'type'.
This recursivity is the base of the object model.

An instance of 'type' is a class (or a new type), but instances of a classes
are not. 'type' is a metatype in term of OO.

What the <type int> means is that int is not a user type but a builtin type,
instances of int are not types (or classes) but common objects, so its nature
is the same as any classes.

The way it prints doesn't matter, it's just the __repr__ of any instance, and
the default behavior for instances of type is to return '<class XX>', but it
can be easily customized.

[1]: class A(object) :
...: class __metaclass__(type) :
...: def __repr__(self) : return "<type A>"
...:
...:

[2]: A
...[2]: <type A>

[3]: type('toto', (object,), {})
...[3]: <class '__main__.toto'>



--
_____________

Maric Michaud
.

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