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 21  5196 
On Jul 31, 11:37*am, Nikolaus Rath <Nikol...@rath. orgwrote:
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'.
So int is a type, but if you have an int variable, its type is int.
Same for your classes.
This is, ignoring old style classes. Make sure all your classes
inherit from object to get new style classes.
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:
class A:
def bar(self):
print "A"
class B:
def bar(self):
print "B"
class C:
def bla(self):
print "C"
def foo(x):
x.bar()
you can call foo with instances of both A and B, because both classes
share a common type, namely the type that has a `bar' method), but not
with an instance of C because it has no method `bar'. Btw, this example
shows the use of duck typing ( http://en.wikipedia.org/wiki/Duck_typing).
HTH,
Thomas.
Thomas Troeger <th************ ****@siemens.co mwrites:
>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:
class A:
def bar(self):
print "A"
class B:
def bar(self):
print "B"
class C:
def bla(self):
print "C"
def foo(x):
x.bar()
you can call foo with instances of both A and B, because both classes
share a common type, namely the type that has a `bar' method), but not
with an instance of C because it has no method `bar'. Btw, this
example shows the use of duck typing
(http://en.wikipedia.org/wiki/Duck_typing).
That would imply that I cannot create instances of a type, only of
a class that implements the type, wouldn't it?
But Python denotes 'int' as a type *and* I can instantiate it.
Still confused,
-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
oj <oj*****@gmail. comwrites:
On Jul 31, 11:37Â*am, Nikolaus Rath <Nikol...@rath. orgwrote:
>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(objec t):
.... pass
....
>>int_class
<class '__main__.int_c lass'>
>>int
<type 'int'>
why doesn't this print
>>class int_class(objec t):
.... pass
....
>>int_class
<type '__main__.int_c lass'>
>>int
<type 'int'>
or
>>class int_class(objec t):
.... pass
....
>>int_class
<class '__main__.int_c lass'>
>>int
<class 'int'>
If there is no distinction, how does the Python interpreter know when
to print 'class' and when to print 'type'?
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
Le Thursday 31 July 2008 14:30:19 Nikolaus Rath, vous avez écritÂ*:
oj <oj*****@gmail. comwrites:
On Jul 31, 11:37Â*am, Nikolaus Rath <Nikol...@rath. orgwrote:
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(objec t):
... pass
...
>int_class
<class '__main__.int_c lass'>
>int
<type 'int'>
why doesn't this print
>class int_class(objec t):
... pass
...
>int_class
<type '__main__.int_c lass'>
>int
<type 'int'>
or
>class int_class(objec t):
... pass
...
>int_class
<class '__main__.int_c lass'>
>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 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.
>>>[1]: class A(object) :
...: class __metaclass__(t ype) :
...: def __repr__(self) : return "<type A>"
...:
...:
>>>[2]: A
...[2]: <type A>
>>>[3]: type('toto', (object,), {})
...[3]: <class '__main__.toto' >
--
_____________
Maric Michaud
That would imply that I cannot create instances of a type, only of
a class that implements the type, wouldn't it?
But Python denotes 'int' as a type *and* I can instantiate it.
Now I start getting confused also ;-)
>>a=5
a.__class__
<type 'int'>
>>a.__class__._ _class__
<type 'type'>
>>dir(a)
['__abs__', '__add__', '__and__', '__class__', '__cmp__', '__coerce__',
'__delattr__', '__div__', '__divmod__', '__doc__', '__float__',
'__floordiv__', '__getattribute __', '__getnewargs__ ', '__hash__',
'__hex__', '__index__', '__init__', '__int__', '__invert__', '__long__',
'__lshift__', '__mod__', '__mul__', '__neg__', '__new__', '__nonzero__',
'__oct__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__',
'__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__' , '__repr__',
'__rfloordiv__' , '__rlshift__', '__rmod__', '__rmul__', '__ror__',
'__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__',
'__rxor__', '__setattr__', '__str__', '__sub__', '__truediv__', '__xor__']
I think in Python everything is implemented as a class which makes
sense. AFAIK you can implement a certain interface in a custom class and
it behaves like, for example, a builtin integer. But I guess one of the
gurus can clarify this matter with an appropriate URL ;-)
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?
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
Maric Michaud <ma***@aristote .infowrites:
Le Thursday 31 July 2008 14:30:19 Nikolaus Rath, vous avez écritÂ*:
>oj <oj*****@gmail. comwrites:
On Jul 31, 11:37Â*am, Nikolaus Rath <Nikol...@rath. orgwrote:
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(objec t):
... pass
...
>>int_class
<class '__main__.int_c lass'>
>>int
<type 'int'>
why doesn't this print
>>class int_class(objec t):
... pass
...
>>int_class
<type '__main__.int_c lass'>
>>int
<type 'int'>
or
>>class int_class(objec t):
... pass
...
>>int_class
<class '__main__.int_c lass'>
>>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',
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?
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.
I think that to get '<type int>' one would have to define a new
metaclass like this:
def type_meta(type) :
def __repr__(self)
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):
.... pass
....
>>myint.__class __ == int.__class__
True
>>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.
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
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 sharea
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),
they can be instantiated and they produce objects (ordinary object in
general) with theirslef as a type.
"""
Maybe it's still unclear that "types" and "classes" *are* synonyms in Python.
--
_____________
Maric Michaud This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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