python instances and type

Hello,

Sorry for the long email, but thanks in advance.

I am not quite sure what is happening, so I have not been
able to
adequately seek a solution via Google, so if this answer is
fairly
easy to find on Google, let me know.  I am not sure how to
produce an
example of my problem without using the python code and
classes that I
have, so I will try to compose an effective  theoretical
example
below.  If there are any clarifications to be made please
let me know.

Basically the problem I am having is obtaining the proper
type
identification from a Python object.  So going through the
motions,
lets say I have 2 types:

class foo_1:
   data = ""
class foo_2:
  foo_1s={}

Time goes by and some assignments are performed and foo_2 is
populated
with some foo_1 objects.  Now, I am using the the function
below,
get_class_resolution to get the type of foo_1 and foo_2, and
it works
well when I don't perform any black magic voodoo and simply
use after
the object is created.

It would return the string "foo_1" for the example
below:

x = foo_1()
x.data = "boring"
print type(x), type(x).mro()
<class 'foo_1'> [<class 'foo_1'>]
print get_class_resolution(x)
foo_1


But I need to do black magic voodoo, and when I perform an
operation
similar to below
on a foo_2 object the get_class_resolution returns
"instance".  In
fact I am expecting it to return foo_1.

foo_2o = foo_2()
foo_2o.foo_1s["boring"] = x
bar = foo_2o.foo_1s.values()[0]
print type(bar), type(bar).mro()
<type 'instance'> [<type 'instance'>, <type
'object'>]
print get_class_resolution(bar)
instance

I am not sure what the difference between the instance vs. 
a class,
but I never thought the type of the object was allowed to
change. Any
help would be appreciated.


Thanks, Adam

def get_class_resolution(obj):
    '''
        get the first class resolution string for a
particular object

        type obj: Mixed/ Any
        param obj: this is the object to get the name of. 
Good
                    for aiding in the comparison of two
objects
                    by name and heirarchy

        rtype: string
        return: class name resolution of the object
    '''
    # typical output is "[<class
'class.resolution'> <type 'object'>]"
    print str(type(obj).mro())
    print str(type(obj))
    name = str(type(obj).mro()).split("'")[1]
    if len(name.split(".")) > 1:
        return
".".join(name.split(".")[:-1])
    return name
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Sorry, I cannot replicate your described behaviour:

andreasandi-lap:~> cat /tmp/t.py
def get_class_resolution(obj):
    '''
        get the first class resolution string for a
particular object

        type obj: Mixed/ Any
        param obj: this is the object to get the name of. 
Good
                    for aiding in the comparison of two
objects
                    by name and heirarchy

        rtype: string
        return: class name resolution of the object
    '''
    # typical output is "[<class
'class.resolution'> <type 'object'>]"
    print str(type(obj).mro())
    print str(type(obj))
    name = str(type(obj).mro()).split("'")[1]
    if len(name.split(".")) > 1:
        return
".".join(name.split(".")[:-1])
    return name

class foo_1:
   data = ""
class foo_2:
  foo_1s={}
 
x = foo_1()
x.data = "boring"
print type(x), type(x).mro()
print get_class_resolution(x)
foo_1
foo_2o = foo_2()
foo_2o.foo_1s["boring"] = x
bar = foo_2o.foo_1s.values()[0]
print type(bar), type(bar).mro()
print get_class_resolution(bar)

andreasandi-lap:~> python2.4 /tmp/t.py
<type 'instance'> [<type 'instance'>, <type
'object'>]
[<type 'instance'>, <type 'object'>]
<type 'instance'>
instance
<type 'instance'> [<type 'instance'>, <type
'object'>]
[<type 'instance'>, <type 'object'>]
<type 'instance'>
instance
andreasandi-lap:~> exit

Amdreas
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2007/4/16, Adam Pridgen <atpridgenmail.utexas.edu>:
> Hello,
>
> Sorry for the long email, but thanks in advance.
>
> I am not quite sure what is happening, so I have not
been able to
> adequately seek a solution via Google, so if this
answer is fairly
> easy to find on Google, let me know.  I am not sure how
to produce an
> example of my problem without using the python code and
classes that I
> have, so I will try to compose an effective 
theoretical example
> below.  If there are any clarifications to be made
please let me know.
>
> Basically the problem I am having is obtaining the
proper type
> identification from a Python object.  So going through
the motions,
> lets say I have 2 types:
> class foo_1:
>    data = ""
> class foo_2:
>   foo_1s={}
>
> Time goes by and some assignments are performed and
foo_2 is populated
> with some foo_1 objects.  Now, I am using the the
function below,
> get_class_resolution to get the type of foo_1 and
foo_2, and it works
> well when I don't perform any black magic voodoo and
simply use after
> the object is created.
>
> It would return the string "foo_1" for the
example below:
>
> x = foo_1()
> x.data = "boring"
> print type(x), type(x).mro()
> <class 'foo_1'> [<class 'foo_1'>]
> print get_class_resolution(x)
> foo_1

Would it? When I try it out, I get:

>>> class foo_1:
  data = ""

>>> x = foo_1()
>>> x.data = "boring"
>>> print type(x), type(x).mro()
<type 'instance'> [<type 'instance'>, <type
'object'>]
>>> get_class_resolution(x)
[<type 'instance'>, <type 'object'>]
<type 'instance'>
'instance'


To get your desired behaviour, you need something like:

>>> class foo_1(object):
        data = ""


>>> x = foo_1()
>>> x.data = "boring"
>>> print type(x), type(x).mro()
<class '__main__.foo_1'> [<class
'__main__.foo_1'>, <type 'object'>]
>>> get_class_resolution(x)
[<class '__main__.foo_1'>, <type 'object'>]
<class '__main__.foo_1'>
'__main__'
>>>
--
Andre Engels, andreengelsgmail.com
ICQ: 6260644  --  Skype: a_engels
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Adam Pridgen wrote:

> x = foo_1()
> x.data = "boring"
> print type(x), type(x).mro()
> <class 'foo_1'> [<class 'foo_1'>]

> print type(bar), type(bar).mro()
> <type 'instance'> [<type 'instance'>,
<type 'object'>]

As Andre has pointed out, this is a difference between new-
and 
old-style classes, not an artifact of the way you are using
the classes.

The type and the class of an instance of an old-style class
are not the 
same. To find out the class of an old-style class instance,
you should 
use the __class__ attribute:
In [1]: class foo_1: pass   # old-style class
    ...:
In [2]: class foo_2(object): pass   # new-style class
    ...:
In [3]: f1 = foo_1()
In [4]: type(f1)
Out[4]: <type 'instance'>
In [5]: f1.__class__
Out[5]: <class __main__.foo_1 at 0x1201090>

For instances of new-style classes, it's class and type are
the same and 
__class__ and type() give the same result:
In [14]: f2=foo_2()
In [15]: f2.__class__
Out[15]: <class '__main__.foo_2'>
In [16]: type(f2)
Out[16]: <class '__main__.foo_2'>

If you want to know the base classes that will be searched
for attribute 
access of an old-style class, use inspect.getmro(). This
will work for 
new-style classes also.

In [19]: import inspect
In [22]: class foo_3(foo_1): pass
    ....:
In [23]: f3=foo_3()
In [24]: type(f3).mro()
Out[24]: [<type 'instance'>, <type 'object'>]
In [25]: inspect.getmro(f3.__class__)
Out[25]: (<class __main__.foo_3 at 0x1272c90>,
<class __main__.foo_1 at 
0x1201090>)

Kent

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Kent Johnson wrote:
> The type and the class of an instance of an old-style
class are not the 
> same. 

For a little more explanation, see
http://docs.py
thon.org/ref/node33.html

For a lot more explanation, see
http://www.py
thon.org/doc/newstyle.html

Kent
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