In an inherited class, "embedded" classes is referenced?

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Christian Joergensen

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Dec 19, 2007, 4:23:42 PM12/19/07
to
Hello

I stumpled upon this "feature" during my work tonight, and found it
a bit confusing:

>>> class A(object):
... class C:
... foobar = 42
...
>>> class B(A): pass
...
>>> A.C
<class __main__.C at 0xb7cf735c>
>>> B.C
<class __main__.C at 0xb7cf735c>
>>> B.C.foobar = 60
>>> A.C.foobar
60

When I inherit B from A, I would expect that A.C and B.C would be two
different classes? But apparently not.

Can anyone give me an explanation? Or a better workaround than
something along the line of:

>>> B.C = type("C", (object,), {'foobar': 60})

Instead of:

>>> B.C.foobar = 60

Thanks,

--
Christian Joergensen | Linux, programming or web consultancy
http://www.razor.dk | Visit us at: http://www.gmta.info

Carl Banks

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Dec 19, 2007, 5:13:57 PM12/19/07
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On Dec 19, 4:23 pm, Christian Joergensen <m...@razor.dk> wrote:
> Hello
>
> I stumpled upon this "feature" during my work tonight, and found it
> a bit confusing:
>
> >>> class A(object):
>
> ... class C:
> ... foobar = 42
> ...>>> class B(A): pass
> ...
> >>> A.C
>
> <class __main__.C at 0xb7cf735c>>>> B.C
>
> <class __main__.C at 0xb7cf735c>>>> B.C.foobar = 60
> >>> A.C.foobar
>
> 60
>
> When I inherit B from A, I would expect that A.C and B.C would be two
> different classes? But apparently not.

No, when a class inherits a class member from a subclass, both classes
reference the same object. This is true of any object: classes,
lists, sets, etc. For instance, if you were to do this,

class A(object):
class C(object): pass
d = [1,2,3,4]
e = set(("a","b","c","d"))

class B(A):
pass


Then you would find that

A.C is B.C
A.d is B.d
A.e is B.e

They are all the same object.

Perhaps you are misled by the example methods? Even them, the same
function object is referenced by both classes. The difference is,
when accessing a method, a class doesn't return the function object
itself, but a method object instead (which is a binding between a
function and a class, used to set the value of "self" when calling
it).

But only functions do something like that, not classes.


> Can anyone give me an explanation? Or a better workaround than
> something along the line of:
>
> >>> B.C = type("C", (object,), {'foobar': 60})

Well you could do this and not bother with the type call (but you'd
still have to do it by hand).

class B(A):
class C(A.C):
foobar = 60


Metaclass programming, or at least some clever properties, would be
required to do it automatically. You could try something like this
(untested) to automatically subclass any class variables that are
instances of type:


class AutoSubclassMetaclass(type):
def __new__(cls,name,bases,clsdict):
for key,value in clsdict.iteritems():
if isinstance(value,type):
clsdict[key] = type(value.__name__,(value,),{})
type.__new__(cls,name,bases,clsdict)


class A(object):
__metaclasS__ = AutoSubclassMetaclass
class C(object):
foobar = 40

class B(A):
pass


Carl Banks

Diez B. Roggisch

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Dec 19, 2007, 5:30:03 PM12/19/07
to
Christian Joergensen schrieb:

> Hello
>
> I stumpled upon this "feature" during my work tonight, and found it
> a bit confusing:
>
>>>> class A(object):
> ... class C:
> ... foobar = 42
> ...
>>>> class B(A): pass
> ...
>>>> A.C
> <class __main__.C at 0xb7cf735c>
>>>> B.C
> <class __main__.C at 0xb7cf735c>
>>>> B.C.foobar = 60
>>>> A.C.foobar
> 60
>
> When I inherit B from A, I would expect that A.C and B.C would be two
> different classes? But apparently not.
>
> Can anyone give me an explanation? Or a better workaround than
> something along the line of:

Why should they be different? The class-statment of A is only exectuted
once, as is the nested class' C. Which makes A.C just a "normal"
class-variable. That is of course shared amongst subclasses. As are
methods, properties and every other thing living in the A.__dict__ due
to the MRO in python.

The more important question is: what do you need C for? Do you have by
any chance a Java-background and think of C as inner/nested class as in
Java? This feature doesn't exist in Python.

Your workaround might be implementable using a metaclass in a more
conveinient way, but I'm not sure-footed enough with metaclasses to
provide a solution out of my head now.

Diez

Christian Joergensen

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Dec 21, 2007, 6:10:40 AM12/21/07
to
Carl Banks <pavlove...@gmail.com> writes:

[...]

> No, when a class inherits a class member from a subclass, both classes
> reference the same object. This is true of any object: classes,
> lists, sets, etc. For instance, if you were to do this,
>
> class A(object):
> class C(object): pass
> d = [1,2,3,4]
> e = set(("a","b","c","d"))
>
> class B(A):
> pass
>
>
> Then you would find that
>
> A.C is B.C
> A.d is B.d
> A.e is B.e
>
> They are all the same object.

I see.

> Perhaps you are misled by the example methods? Even them, the same
> function object is referenced by both classes. The difference is,
> when accessing a method, a class doesn't return the function object
> itself, but a method object instead (which is a binding between a
> function and a class, used to set the value of "self" when calling
> it).
>
> But only functions do something like that, not classes.

Great explanation. This makes sense. I didn't think of it that way.

[...]

> Metaclass programming, or at least some clever properties, would be
> required to do it automatically. You could try something like this
> (untested) to automatically subclass any class variables that are
> instances of type:
>
>
> class AutoSubclassMetaclass(type):
> def __new__(cls,name,bases,clsdict):
> for key,value in clsdict.iteritems():
> if isinstance(value,type):
> clsdict[key] = type(value.__name__,(value,),{})
> type.__new__(cls,name,bases,clsdict)
>
>
> class A(object):
> __metaclasS__ = AutoSubclassMetaclass
> class C(object):
> foobar = 40
>
> class B(A):
> pass

Intriguing :-)

Thank you for your timed,

Christian Joergensen

unread,
Dec 21, 2007, 6:16:34 AM12/21/07
to
"Diez B. Roggisch" <de...@nospam.web.de> writes:

[...]

> The more important question is: what do you need C for? Do you have by
> any chance a Java-background and think of C as inner/nested class as
> in Java? This feature doesn't exist in Python.

I was working on some framework code where each class would have a
nested class containing meta information about the outer class
(because all properties would be treated in a special way by the
framework).

I needed to inherit two classes and got confused when they didn't
inherit their meta information - but referenced it :)

> Your workaround might be implementable using a metaclass in a more
> conveinient way, but I'm not sure-footed enough with metaclasses to
> provide a solution out of my head now.

That would be fun ;-)

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