On Oct 6, 4:30 am, Fuzzyman <[EMAIL PROTECTED]> wrote:
> On Oct 6, 1:13 am, MRAB <[EMAIL PROTECTED]> wrote:
>
>
>
> > 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
>>> 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.
--
http://mail.python.org/mailman/listinfo/python-list
