Difference between type and class

[Nikolaus Rath]

Hello,

Can someone explain to me the difference between a type and a class?
After reading http://www.cafepy.com/article/python_types_and_objects/
it seems to me that classes and types are actually the same thing:

- both are instances of a metaclass, and the same metaclass ('type')
can instantiate both classes and types.
- both can be instantiated and yield an "ordinary" object
- I can even inherit from a type and get a class

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 hope I have managed to get across the point of my confusion...
Thanks in advance,

-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C

[Nikolaus Rath]

Maric Michaud <ma***@aristote .infowrites:

Le Thursday 31 July 2008 16:46:28 Nikolaus Rath, vous avez écritÂ*:

>Maric Michaud <ma***@aristote .infowrites:

Can someone explain to me the difference between a type and a class?

If your confusion is of a more general nature I suggest reading the
introduction of `Design Patterns' (ISBN-10: 0201633612), under
`Specifying Object Interfaces'.

In short: A type denotes a certain interface, i.e. a set of signatures,
whereas a class tells us how an object is implemented (like a
blueprint). A class can have many types if it implements all their
interfaces, and different classes can have the same type if they share a
common interface. The following example should clarify matters:

Of course, this is what a type means in certain literature about OO
(java-ish), but this absolutely not what type means in Python. Types
are a family of object with a certain status, and they're type is
"type", conventionnaly named a metatype in standard OO.

[...]

Hmm. Now you have said a lot about Python objects and their type, but
you still haven't said what a type actually is (in Python) and in what
way it is different from a class. Or did I miss something?

This paragraph ?

"""
- types, or classes, are all instance of type 'type' (or a subclass of it),

Well, I couldn't quite make sense of '..are instance of type...'
without knowing what you actually meant with "type* in this context,
but...

Maybe it's still unclear that "types" and "classes" *are* synonyms
in Python.

...in this case it becomes clear. Then my question reduces to the one
in the other post (why do 'int' and 'myint' have different __repr__
results).
Already thanks for your help,
-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C

[Maric Michaud]

Le Thursday 31 July 2008 17:00:51 Nikolaus Rath, vous avez écritÂ*:

There are some confusion about the terms here.

Classes are instances of type 'type',

Could you please clarify what you mean with 'instance of type X'? I
guess you mean that 'y is an instance of type X' iif y is constructed
by instantiating X. Is that correct?

Correct, you can verify this with the isinstance builtin :

>>>[1]: isinstance(int( ), int)

...[1]: True

>>>[2]: isinstance(int, object)

...[2]: True

>>>[3]: isinstance(int, type)

...[3]: True

>>>[4]: class A(object) : pass

...:

>>>[5]: isinstance(A, type)

...[5]: True

What the <type intmeans 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.

But 'int' is an instance of 'type' (the metaclass):

>int.__class_ _

<type 'type'>

so it should also return '<class int>' if that's the default behavior
of the 'type' metaclass.

The fact that a class is an instance of type, which it is always true, doesn't
mean its metaclass is "type", it could be any subclass of type :

>>>[6]: class A(object) :

...: class __metaclass__(t ype) :
...: def __repr__(self) : return "<type A>"
...:
...:

>>>[7]: isinstance(A, type)

...[7]: True

>>>[8]: A.__class__

...[8]: <class '__main__.__met aclass__'>

>>>[9]: issubclass(A.__ class__, type)

...[9]: True

I think that to get '<type int>' one would have to define a new
metaclass like this:

def type_meta(type) :
Â* Â* def __repr__(self)
Â* Â* Â* Â* Â*return "<type %s>" % self.__name__

and then one should have int.__class__ == type_meta. But obviously
that's not the case. Why?

Moreover:

>class myint(int):

... Â* Â*pass
...

>myint.__class_ _ == int.__class__

True

*is* comparaison fits better here.

>int

<type 'int'>

>myint

<class '__main__.myint '>

despite int and myint having the same metaclass. So if the
representation is really defined in the 'type' metaclass, then
type.__repr__ has to make some kind of distinction between int and
myint, so they cannot be on absolute equal footing.

You're right, type(int) is type, the way it renders differently is a detailof
its implementation, you can do things with builtin types (written in C) you
coudn't do in pure python, exactly as you couldn't write recursive types
like 'object' and 'type'.
--
_____________

Maric Michaud

[Nikolaus Rath]

Maric Michaud <ma***@aristote .infowrites:

>>What the <type intmeans 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.

But 'int' is an instance of 'type' (the metaclass):

>>int.__class __

<type 'type'>

so it should also return '<class int>' if that's the default behavior
of the 'type' metaclass.

The fact that a class is an instance of type, which it is always true, doesn't
mean its metaclass is "type", it could be any subclass of type :

Yes, that is true. But it's not what I said above (and below).
'obj.__class__' gives the class of 'obj', so if 'int.__class__ is
type' then 'type' is the class of 'int' and 'int' is *not* an instance
of some metaclass derived from 'type'.

>I think that to get '<type int>' one would have to define a new
metaclass like this:

def type_meta(type) :
Â* Â* def __repr__(self)
Â* Â* Â* Â* Â*return "<type %s>" % self.__name__

and then one should have int.__class__ is type_meta. But obviously
that's not the case. Why?

Moreover:

>>class myint(int):

... Â* Â*pass
...

>>myint.__class __ is int.__class__

True

>>int

<type 'int'>

>>myint

<class '__main__.myint '>

despite int and myint having the same metaclass. So if the
representati on is really defined in the 'type' metaclass, then
type.__repr_ _ has to make some kind of distinction between int and
myint, so they cannot be on absolute equal footing.

You're right, type(int) is type, the way it renders differently is a
detail of its implementation, you can do things with builtin types
(written in C) you coudn't do in pure python, exactly as you
couldn't write recursive types like 'object' and 'type'.

If it is just a matter of different rendering, what's the reason for
doing it like that? Wouldn't it be more consistent and straightforward
to denote builtin types as classes as well?

And where exactly is this different rendering implemented? Could I
write my own type (in C, of course) and make it behave like e.g.
'int'? I.e. its rendering should be different and not inherited to
subclasses:

>>my_type

<strange_thin g 'my_type'>

>>a = my_type(42)
a.__class__

<strange_thin g 'my_type'>

>>class derived(my_type ):
pass

<class 'derived>

or would I have to change the implemention of 'type' for this (since
it contains the __repr__ function that renders the type)?
This is of course purely theoretical and probably without any
practical relevance. I'm if I just can't stop drilling, but I think
this is really interesting.
Best,
-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C

[Steven D'Aprano]

On Thu, 31 Jul 2008 13:32:39 +0200, Thomas Troeger wrote:

>Can someone explain to me the difference between a type and a class?

If your confusion is of a more general nature I suggest reading the
introduction of `Design Patterns' (ISBN-10: 0201633612), under
`Specifying Object Interfaces'.

In short: A type denotes a certain interface, i.e. a set of signatures,
whereas a class tells us how an object is implemented (like a
blueprint). A class can have many types if it implements all their
interfaces, and different classes can have the same type if they share a
common interface.

I fear you're introducing a rather complicated answer for a simple
question. In Python, the simple answer is that built-in objects like int,
float, str etc. are referred to as "types", and custom objects created
using the class keyword are referred to as "classes". This is a
historical distinction that will disappear in time.

We can see that types and classes already have the same type in Python:

>>class Parrot(object):

.... pass
....

>>type(Parrot )

<type 'type'>

>>type(str)

<type 'type'>

The following example should clarify matters:

class A:
def bar(self):
print "A"

Alas, you've chosen the worst-possible example to "clarify" matters,
because old-style classic classes are *not* unified with types, and will
disappear in the future:

>>class Old: # don't inherit from object

.... pass
....

>>type(Old)

<type 'classobj'>
So, to the Original Poster:

In Python, new-style classes and types are the same, but it is
traditional to refer to customer objects as "class" and built-in objects
as "types". Old-style classes are different, but you are discouraged from
using old-style classes unless you have a specific reason for needing
them (e.g. backwards compatibility).
--
Steven

[Miles]

On Thu, Jul 31, 2008 at 1:59 PM, Nikolaus Rath wrote:

If it is just a matter of different rendering, what's the reason for
doing it like that? Wouldn't it be more consistent and straightforward
to denote builtin types as classes as well?

Yes, and in Python 3, it will be so:

>>class myint(int): pass

....

>>int

<class 'int'>

>>myint

<class '__main__.myint '>

The reason the distinction is made currently is that before Python
2.2, "types" were built-in (or C extension) classes, and "classes"
were Python classes; it was not possible to subclass built-in types.
That "classic" style of classes are still supported in Python 2.2 and
above (but not in Python 3), by not inheriting from object or any
other built-in. However, for new-style classes, the only distinction
is in the repr.

>>class classic: pass

....

>>class newstyle(object ): pass

....

>>type(classi c)

<type 'classobj'>

>>type(classic( ))

<type 'instance'>

>>classic.__cla ss__

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: class classic has no attribute '__class__'

>>classic().__c lass__

<class __main__.classi c at 0x64e70>

>>>
type(newstyle )

<type 'type'>

>>type(newstyle ())

<class '__main__.newst yle'>

Further reading:

http://www.python.org/download/relea....3/descrintro/
http://svn.python.org/view?rev=23331&view=rev
http://bugs.python.org/issue2565

-Miles

[Nikolaus Rath]

Miles <se*********@gm ail.comwrites:

On Thu, Jul 31, 2008 at 1:59 PM, Nikolaus Rath wrote:

>If it is just a matter of different rendering, what's the reason for
doing it like that? Wouldn't it be more consistent and straightforward
to denote builtin types as classes as well?

Yes, and in Python 3, it will be so:

[..]

http://svn.python.org/view?rev=23331&view=rev

That makes matters absolutely clear. Thanks a lot. No more questions
from my side ;-).
Best,

-Nikolaus
--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C

[Nikolaus Rath]

Steven D'Aprano <st***@REMOVE-THIS-cybersource.com .auwrites:

So, to the Original Poster:

In Python, new-style classes and types are the same, but it is
traditional to refer to customer objects as "class" and built-in
objects as "types". Old-style classes are different, but you are
discouraged from using old-style classes unless you have a specific
reason for needing them (e.g. backwards compatibility).

I see. Thanks a lot for the explanation.
Best,

-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C

[Thomas Troeger]

Steven D'Aprano wrote:

>class A:
def bar(self):
print "A"

Alas, you've chosen the worst-possible example to "clarify" matters,
because old-style classic classes are *not* unified with types, and will
disappear in the future:

Of course I wanted to write `class A(object)', but I keep forgetting
this one because I'm still used to the old ways...

Will this disappear in Python 3.0., i.e. can you again simply write

class A:

and inherit from object automagically? That would be nice since that's
what most people probably want.

[Matthew Woodcraft]

Thomas Troeger <th************ ****@siemens.co mwrote:

Will this disappear in Python 3.0., i.e. can you again simply write

class A:

and inherit from object automagically?

Short answer: yes.

-M-

[Terry Reedy]

Thomas Troeger wrote:

Steven D'Aprano wrote:

>>class A:
def bar(self):
print "A"

Alas, you've chosen the worst-possible example to "clarify" matters,
because old-style classic classes are *not* unified with types, and
will disappear in the future:

Of course I wanted to write `class A(object)', but I keep forgetting
this one because I'm still used to the old ways...

Will this disappear in Python 3.0., i.e. can you again simply write
class A:
and inherit from object automagically?

Yes.
IDLE 3.0b2

>>class a: pass

>>a

<class '__main__.a'>

>>a.__bases__

(<class 'object'>,)

3.0 is really a nicer version. Once the final release is out, the main
reason to stick with 2.x for new code will be if it depends on
third-party code (including your own ;-) that has not been upgraded.