Python 2.6 / 3.0: Determining if a method is inherited

[Fuzzyman]

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.
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:
['__call__', '__class__', '__cmp__', '__delattr__', '__doc__',
'__format__', '__getattribute__', '__hash__', '__init__', '__name__',
'__new__', '__objclass__', '__reduce__', '__reduce_ex__', '__repr__',
'__self__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__']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?

Michael Foord

 

[MRAB]

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.
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:
['__call__', '__class__', '__cmp__', '__delattr__', '__doc__',
'__format__', '__getattribute__', '__hash__', '__init__', '__name__',
'__new__', '__objclass__', '__reduce__', '__reduce_ex__', '__repr__',
'__self__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__']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]

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.
.... pass
........ def __lt__( self, other ):
.... return self
....
Further, it's been noted before that

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

 

[Fuzzyman]

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.


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__.__self__

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__.__func__

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

 

[Aaron \Castironpi\ Brady]

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__.__self__
<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__.__func__
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.ironpythoninaction.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

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]

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__.__self__
<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__.__func__
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.ironpythoninaction.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


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

 

[Aaron \Castironpi\ Brady]

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.ironpythoninaction.com/

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

 

[Fuzzyman]

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

Thankfully that page I pointed you to has the solution I came up with
- walk the method resolution order of the class checking in the
classes' '__dict__' to see what they explicitly implement.

Given that you can get hold of X.__lt__ I was surprised by how hard it
was to tell whether that was an inherited implementation or not.

Michael

 

[Pekka Laukkanen]

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.

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.ismethod or by checking methods' im_class attribute
directly:
.... pass
........ def __str__(self):
.... return ''
........ pass
....Traceback (most recent call last):
<class '__main__.E'>


Cheers,
.peke

 

[Pekka Laukkanen]

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.ismethod or by checking methods' im_class attribute
directly:

... pass
...
... def __str__(self):
... return ''
...
... pass
...
Traceback (most recent call last):

<class '__main__.E'>

Ooops, didn't notice this was suggested already. One more attempt,
hopefully this is unique. =)
Traceback (most recent call last):
<type 'str'>

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

Cheers,
.peke