Python 2.6 / 3.0: Determining if a method is inherited

**MRAB** · Oct 6 '08, 12:15 AM

Re: Python 2.6 / 3.0: Determining if a method is inherited

Fuzzyman wrote:

Hello all,
>
I may well be being dumb (it has happened before), but I'm struggling
to fix some code breakage with Python 2.6.
>
I have some code that looks for the '__lt__' method on a class:
>
if hasattr(clr, '__lt__'):
>
However - in Python 2.6 object has grown a default implementation of
'__lt__', so this test always returns True.
>

>class X(object): pass

...

>X.__lt__

<method-wrapper '__lt__' of type object at 0xa15cf0>

>X.__lt__ == object.__lt__

False
>
So how do I tell if the X.__lt__ is inherited from object? I can look
in the '__dict__' of the class - but that doesn't tell me if X
inherits '__lt__' from a base class other than object. (Looking inside
the method wrapper repr with a regex is not an acceptable answer...)
>
Some things I have tried:
>

>X.__lt__.__sel f__

>dir(X.__lt__ )

['__call__', '__class__', '__cmp__', '__delattr__', '__doc__',
'__format__', '__getattribute __', '__hash__', '__init__', '__name__',
'__new__', '__objclass__', '__reduce__', '__reduce_ex__' , '__repr__',
'__self__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook __']

>X.__lt__.__fun c__

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'method-wrapper' object has no attribute '__func__'
>
>
Hmmm... I can get this working with Python 2.6 with:
>
if '__lt__' in dir(cls):
>
The default implementation of '__lt__' doesn't appear in the dir of
classes. However this fails with Python 3 where the default
implementation *does* appear in the output of 'dir'. Any suggestions?
>

Methods are objects. How do you know if two references refer to the
same object? You use "is":

X.__lt__ is object.__lt__

**Aaron \Castironpi\ Brady** · Oct 6 '08, 02:05 AM

Re: Python 2.6 / 3.0: Determining if a method is inherited

On Oct 5, 7:13 pm, MRAB <goo...@mrabarn ett.plus.comwro te:

Fuzzyman wrote:

Hello all,

>

I may well be being dumb (it has happened before), but I'm struggling
to fix some code breakage with Python 2.6.

>

I have some code that looks for the '__lt__' method on a class:

>

if hasattr(clr, '__lt__'):

>

However - in Python 2.6 object has grown a default implementation of
'__lt__', so this test always returns True.

>

Hmmm... I can get this working with Python 2.6 with:

>
Methods are objects. How do you know if two references refer to the
same object? You use "is":
>
X.__lt__ is object.__lt__

That doesn't work for me.

>>class A( object ):

.... pass
....

>>class B( A ):

.... def __lt__( self, other ):
.... return self
....

>>a= A()
>>b= B()
>>B.__lt__ is object.__lt__

False

>>A.__lt__ is object.__lt__

False

>>>

Further, it's been noted before that

A().meth is not A().meth

**Fuzzyman** · Oct 6 '08, 09:35 AM

Re: Python 2.6 / 3.0: Determining if a method is inherited

On Oct 6, 1:13 am, MRAB <goo...@mrabarn ett.plus.comwro te:

Fuzzyman wrote:

Hello all,

>

I may well be being dumb (it has happened before), but I'm struggling
to fix some code breakage with Python 2.6.

>

I have some code that looks for the '__lt__' method on a class:

>

if hasattr(clr, '__lt__'):

>

However - in Python 2.6 object has grown a default implementation of
'__lt__', so this test always returns True.

>

>>class X(object): pass

...

>>X.__lt__

<method-wrapper '__lt__' of type object at 0xa15cf0>

>>X.__lt__ == object.__lt__

False

>

So how do I tell if the X.__lt__ is inherited from object? I can look
in the '__dict__' of the class - but that doesn't tell me if X
inherits '__lt__' from a base class other than object. (Looking inside
the method wrapper repr with a regex is not an acceptable answer...)

>

Some things I have tried:

>

>>X.__lt__.__se lf__

>>dir(X.__lt_ _)

['__call__', '__class__', '__cmp__', '__delattr__', '__doc__',
'__format__', '__getattribute __', '__hash__', '__init__', '__name__',
'__new__', '__objclass__', '__reduce__', '__reduce_ex__' , '__repr__',
'__self__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook __']

>>X.__lt__.__fu nc__

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'method-wrapper' object has no attribute '__func__'

>

Hmmm... I can get this working with Python 2.6 with:

>

if '__lt__' in dir(cls):

>

The default implementation of '__lt__' doesn't appear in the dir of
classes. However this fails with Python 3 where the default
implementation *does* appear in the output of 'dir'. Any suggestions?

>
Methods are objects. How do you know if two references refer to the
same object? You use "is":
>
X.__lt__ is object.__lt__

Didn't you see that even an equality test fails - so they are not the
same (that being the problem)...

They are unbound method objects - in Python 3 the unbound method has
gone away, so the problem is with Python 2.6.

Michael
--

Home - LAWYER ACTION GROUP

http://www.ironpythoninaction.com/

Pedestrian accidents can happen in the blink of an eye, changing lives forever. When you're out for a stroll or crossing the street, an unexpected collision

**Aaron \Castironpi\ Brady** · Oct 6 '08, 06:05 PM

Re: Python 2.6 / 3.0: Determining if a method is inherited

On Oct 6, 4:30 am, Fuzzyman <fuzzy...@gmail .comwrote:

On Oct 6, 1:13 am, MRAB <goo...@mrabarn ett.plus.comwro te:
>
>
>

Fuzzyman wrote:

Hello all,

>

I may well be being dumb (it has happened before), but I'm struggling
to fix some code breakage with Python 2.6.

>

I have some code that looks for the '__lt__' method on a class:

>

if hasattr(clr, '__lt__'):

>

However - in Python 2.6 object has grown a default implementation of
'__lt__', so this test always returns True.

>

>class X(object): pass
...
>X.__lt__
<method-wrapper '__lt__' of type object at 0xa15cf0>
>X.__lt__ == object.__lt__
False

>

So how do I tell if the X.__lt__ is inherited from object? I can look
in the '__dict__' of the class - but that doesn't tell me if X
inherits '__lt__' from a base class other than object. (Looking inside
the method wrapper repr with a regex is not an acceptable answer...)

>

Some things I have tried:

>

>X.__lt__.__sel f__
<class '__main__.X'>
>dir(X.__lt__ )
['__call__', '__class__', '__cmp__', '__delattr__', '__doc__',
'__format__', '__getattribute __', '__hash__', '__init__', '__name__',
'__new__', '__objclass__', '__reduce__', '__reduce_ex__' , '__repr__',
'__self__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook __']
>X.__lt__.__fun c__
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'method-wrapper' object has no attribute '__func__'

>

Hmmm... I can get this working with Python 2.6 with:

>

if '__lt__' in dir(cls):

>

The default implementation of '__lt__' doesn't appear in the dir of
classes. However this fails with Python 3 where the default
implementation *does* appear in the output of 'dir'. Any suggestions?

>

Methods are objects. How do you know if two references refer to the
same object? You use "is":

>

X.__lt__ is object.__lt__

>
Didn't you see that even an equality test fails - so they are not the
same (that being the problem)...
>
They are unbound method objects - in Python 3 the unbound method has
gone away, so the problem is with Python 2.6.
>
Michael
--http://www.ironpythoni naction.com/

Not tested extensively.

class NoLTException( Exception ): pass

class NoLT( object ):
def __lt__( self, other ):
raise NoLTException()

class A( NoLT ):
pass

class B( A ):
def __lt__( self, other ):
return self

def test_lt( obj ):
try:
obj.__lt__( None )
except NoLTException:
return False
except:
pass
return True

>>a= A()
>>b= B()
>>test_lt( a )

False

>>test_lt( b )

True

>>>

This method won't work for arbitrary classes, only ones that you
control, that inherit from 'NoLT'. The 'test_lt' function works by
trying to call '__lt__' on its argument. The parameter to it doesn't
matter because of what happens next. If '__lt__' raises a
NoLTException, you know it was inherited from NoLT. Otherwise, even
if another exception occurs, the object you know has '__lt__'.

It's a very object oriented solution. Essentially you're inheriting
all the classes that you want to fail, from a class that does.

**Fuzzyman** · Oct 6 '08, 06:25 PM

Re: Python 2.6 / 3.0: Determining if a method is inherited

On Oct 6, 7:01 pm, "Aaron \"Castironpi \" Brady" <castiro...@gma il.com>
wrote:

On Oct 6, 4:30 am, Fuzzyman <fuzzy...@gmail .comwrote:
>
>
>

On Oct 6, 1:13 am, MRAB <goo...@mrabarn ett.plus.comwro te:

>

Fuzzyman wrote:
Hello all,

>

I may well be being dumb (it has happened before), but I'm struggling
to fix some code breakage with Python 2.6.

>

I have some code that looks for the '__lt__' method on a class:

>

if hasattr(clr, '__lt__'):

>

However - in Python 2.6 object has grown a default implementation of
'__lt__', so this test always returns True.

>

>>class X(object): pass
...
>>X.__lt__
<method-wrapper '__lt__' of type object at 0xa15cf0>
>>X.__lt__ == object.__lt__
False

>

So how do I tell if the X.__lt__ is inherited from object? I can look
in the '__dict__' of the class - but that doesn't tell me if X
inherits '__lt__' from a base class other than object. (Looking inside
the method wrapper repr with a regex is not an acceptable answer...)

>

Some things I have tried:

>

>>X.__lt__.__se lf__
<class '__main__.X'>
>>dir(X.__lt_ _)
['__call__', '__class__', '__cmp__', '__delattr__', '__doc__',
'__format__', '__getattribute __', '__hash__', '__init__', '__name__',
'__new__', '__objclass__', '__reduce__', '__reduce_ex__' , '__repr__',
'__self__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook __']
>>X.__lt__.__fu nc__
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'method-wrapper' object has no attribute '__func__'

>

Hmmm... I can get this working with Python 2.6 with:

>

if '__lt__' in dir(cls):

>

The default implementation of '__lt__' doesn't appear in the dir of
classes. However this fails with Python 3 where the default
implementation *does* appear in the output of 'dir'. Any suggestions?

>

Methods are objects. How do you know if two references refer to the
same object? You use "is":

>

X.__lt__ is object.__lt__

>

Didn't you see that even an equality test fails - so they are not the
same (that being the problem)...

>

They are unbound method objects - in Python 3 the unbound method has
gone away, so the problem is with Python 2.6.

>

Michael
--http://www.ironpythoni naction.com/

>
Not tested extensively.
>
class NoLTException( Exception ): pass
>
class NoLT( object ):
def __lt__( self, other ):
raise NoLTException()
>
class A( NoLT ):
pass
>
class B( A ):
def __lt__( self, other ):
return self
>
def test_lt( obj ):
try:
obj.__lt__( None )
except NoLTException:
return False
except:
pass
return True
>
>
>

>a= A()
>b= B()
>test_lt( a )

False

>test_lt( b )

True
>
This method won't work for arbitrary classes, only ones that you
control, that inherit from 'NoLT'. The 'test_lt' function works by
trying to call '__lt__' on its argument. The parameter to it doesn't
matter because of what happens next. If '__lt__' raises a
NoLTException, you know it was inherited from NoLT. Otherwise, even
if another exception occurs, the object you know has '__lt__'.
>
It's a very object oriented solution. Essentially you're inheriting
all the classes that you want to fail, from a class that does.

But not a very good solution to the problem...

The specific problem is to determine if an arbitrary class implements
a specified comparison method. The general problem (that gives rise to
the specific problem) is to write a class decorator that can implement
all comparison methods from a class that implements only one.

See: http://code.activestate.com/recipes/576529/

Michael
--

Home - LAWYER ACTION GROUP

http://www.ironpythoninaction.com/

Pedestrian accidents can happen in the blink of an eye, changing lives forever. When you're out for a stroll or crossing the street, an unexpected collision

**Aaron \Castironpi\ Brady** · Oct 6 '08, 06:25 PM

Re: Python 2.6 / 3.0: Determining if a method is inherited

On Oct 6, 1:17 pm, Fuzzyman <fuzzy...@gmail .comwrote:

On Oct 6, 7:01 pm, "Aaron \"Castironpi \" Brady" <castiro...@gma il.com>
wrote:

It's a very object oriented solution. Essentially you're inheriting
all the classes that you want to fail, from a class that does.

>
But not a very good solution to the problem...
>
The specific problem is to determine if an arbitrary class implements
a specified comparison method. The general problem (that gives rise to
the specific problem) is to write a class decorator that can implement
all comparison methods from a class that implements only one.
>
See:http://code.activestate.com/recipes/576529/
>
Michael
--http://www.ironpythoni naction.com/

Nope, I'm out of ideas, I'm afraid.

**Pekka Laukkanen** · Oct 6 '08, 11:35 PM

Re: Python 2.6 / 3.0: Determining if a method is inherited

2008/10/5 Fuzzyman <fuzzyman@gmail .com>:

I may well be being dumb (it has happened before), but I'm struggling
to fix some code breakage with Python 2.6.
>
I have some code that looks for the '__lt__' method on a class:
>
if hasattr(clr, '__lt__'):
>
However - in Python 2.6 object has grown a default implementation of
'__lt__', so this test always returns True.
>

>>>class X(object): pass

...

>>>X.__lt__

<method-wrapper '__lt__' of type object at 0xa15cf0>

>>>X.__lt__ == object.__lt__

False
>
So how do I tell if the X.__lt__ is inherited from object? I can look
in the '__dict__' of the class - but that doesn't tell me if X
inherits '__lt__' from a base class other than object. (Looking inside
the method wrapper repr with a regex is not an acceptable answer...)

I don't have Python 2.6 available, but if __lt__ on it works similarly
as __str__ on Python 2.5, you might be able to achieve this either
with inspect.ismetho d or by checking methods' im_class attribute
directly:

>>class C(object):

.... pass
....

>>class D(object):

.... def __str__(self):
.... return ''
....

>>class E(D):

.... pass
....

>>import inspect
>>inspect.ismet hod(C().__str__ )

False

>>inspect.ismet hod(D().__str__ )

True

>>inspect.ismet hod(E().__str__ )

True

>>>
>>C().__str__.i m_class

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'method-wrapper' object has no attribute 'im_class'

>>D().__str__.i m_class

>>E().__str__.i m_class

<class '__main__.E'>

Cheers,
.peke

**Pekka Laukkanen** · Oct 6 '08, 11:55 PM

Re: Python 2.6 / 3.0: Determining if a method is inherited

2008/10/7 Pekka Laukkanen <peke@iki.fi> :

2008/10/5 Fuzzyman <fuzzyman@gmail .com>:

>I may well be being dumb (it has happened before), but I'm struggling
>to fix some code breakage with Python 2.6.
>>
>I have some code that looks for the '__lt__' method on a class:
>>
>if hasattr(clr, '__lt__'):
>>
>However - in Python 2.6 object has grown a default implementation of
>'__lt__', so this test always returns True.
>>

>>>>class X(object): pass

>...

>>>>X.__lt__

><method-wrapper '__lt__' of type object at 0xa15cf0>

>>>>X.__lt__ == object.__lt__

>False
>>
>So how do I tell if the X.__lt__ is inherited from object? I can look
>in the '__dict__' of the class - but that doesn't tell me if X
>inherits '__lt__' from a base class other than object. (Looking inside
>the method wrapper repr with a regex is not an acceptable answer...)

>
I don't have Python 2.6 available, but if __lt__ on it works similarly
as __str__ on Python 2.5, you might be able to achieve this either
with inspect.ismetho d or by checking methods' im_class attribute
directly:
>

>>>class C(object):

... pass
...

>>>class D(object):

... def __str__(self):
... return ''
...

>>>class E(D):

... pass
...

>>>import inspect
>>>inspect.isme thod(C().__str_ _)

False

>>>inspect.isme thod(D().__str_ _)

True

>>>inspect.isme thod(E().__str_ _)

True

>>>>
>>>C().__str__. im_class

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'method-wrapper' object has no attribute 'im_class'

>>>D().__str__. im_class

>>>E().__str__. im_class

Ooops, didn't notice this was suggested already. One more attempt,
hopefully this is unique. =)

>>C().__str__._ _objclass__

>>D().__str__._ _objclass__

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'function' object has no attribute '__objclass__'

>>'spam'.__str_ _.__objclass__

<type 'str'>

Someone who actually knows what __objclas__ does can probably comment
does this make any sense in your case.

Cheers,
.peke

Python 2.6 / 3.0: Determining if a method is inherited

Python 2.6 / 3.0: Determining if a method is inherited

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