Python 2.6: 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.

>>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__'

Michael Foord
--
http://www.ironpythoninaction.com/

[Valentino Volonghi aka Dialtone]

Fuzzyman <fu******@gmail .comwrote:

So how do I tell if the X.__lt__ is inherited from object? I can look

I don't have python 2.6 installed so I can't try but what I think could
work is:

>>class Foo(object):

.... def hello(self):
.... pass
....

>>class Bla(Foo):

.... def hello(self):
.... pass
....

>>class Baz(Foo):

.... pass
....

>>Baz.hello.im_ func

<function hello at 0x2b1630>

>>Bla.hello.im_ func

<function hello at 0x2b1670>

>>Foo.hello.im_ func

<function hello at 0x2b1630>

>>Foo.hello.im_ func is Baz.hello.im_fu nc

True

>>Foo.hello.im_ func is Bla.hello.im_fu nc

False

Which to me also makes sense. If it's inherited I expect it to be the
very same function object of one of the bases (which you can get with
inspect.getmro( Clazz)). On the other end if you override it it's not the
same function object so it won't return True when applied to 'is'.

HTH

--
Valentino Volonghi aka Dialtone
http://stacktrace.it -- Aperiodico di resistenza informatica
Blog: http://www.twisted.it/
Public Beta: http://www.adroll.com/

[Aaron \Castironpi\ Brady]

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...)

They're of different types. I'm not sure how much you could use that,
or how reliable it is.

>>class A( object ):

.... pass
....

>>class B( object ):

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

>>hasattr( A, '__lt__' )

True

>>hasattr( B, '__lt__' )

True

>>A.__lt__

<method-wrapper '__lt__' of type object at 0x00B81D48>

>>B.__lt__

<unbound method B.__lt__>

>>>

[Fuzzyman]

On Oct 5, 8:15*pm, dialUAZ###UZ#$A At...@gWARAmail .com (Valentino
Volonghi aka Dialtone) wrote:

Fuzzyman <fuzzy...@gmail .comwrote:

So how do I tell if the X.__lt__ is inherited from object? I can look

I don't have python 2.6 installed so I can't try but what I think could
work is:

>class Foo(object):

... * * def hello(self):
... * * * * pass
...>>class Bla(Foo):

... * * def hello(self):
... * * * * pass
...>>class Baz(Foo):

... * * pass
...>>Baz.hello. im_func

<function hello at 0x2b1630>>>Bla. hello.im_func

<function hello at 0x2b1670>>>Foo. hello.im_func

<function hello at 0x2b1630>>>Foo. hello.im_func is Baz.hello.im_fu nc
True

>Foo.hello.im_f unc is Bla.hello.im_fu nc

False

Nope:

Python 2.6 (trunk:66714:66 715M, Oct 1 2008, 18:36:04)
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright" , "credits" or "license" for more information.

>>class X(object): pass

....

>>X.__lt__.im_f unc

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

>>>

What I went with in the end:

import sys as _sys

if _sys.version_in fo[0] == 3:
def _has_method(cls , name):
for B in cls.__mro__:
if B is object:
continue
if name in B.__dict__:
return True
return False
else:
def _has_method(cls , name):
for B in cls.mro():
if B is object:
continue
if name in B.__dict__:
return True
return False

See this page for why I needed it:

http://www.voidspace.org.uk/python/w...04.shtml#e1018

Michael

Which to me also makes sense. If it's inherited I expect it to be the
very same function object of one of the bases (which you can get with
inspect.getmro( Clazz)). On the other end if you override it it's not the
same function object so it won't return True when applied to 'is'.

HTH

--
Valentino Volonghi aka Dialtonehttp://stacktrace.it-- Aperiodico di resistenza informatica
Blog:http://www.twisted.it/
Public Beta:http://www.adroll.com/

[Terry Reedy]

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

In 3.0, the test returns true because function attributes only get
wrapped when bound. In the meanwhile, " 'object' in repr(X.__lt__)"
should do it for you.

tjr

[Christian Heimes]

Terry Reedy wrote:

In 3.0, the test returns true because function attributes only get
wrapped when bound. In the meanwhile, " 'object' in repr(X.__lt__)"
should do it for you.

This session should give you some hints how to archive your goal :)
Have fun!

>>import types
class A(object):

.... def __lt__(self):
.... pass
....

>>isinstance(A. __lt__, types.MethodTyp e)

True

>>isinstance(A. __gt__, types.MethodTyp e)

False

>>type(A.__lt__ )

<type 'instancemethod '>

>>type(A.__gt__ )

<type 'method-wrapper'>

>>type(A.__str_ _)

<type 'wrapper_descri ptor'>

>>type(A.__new_ _)

<type 'builtin_functi on_or_method'>

>>A.__lt__.im_f unc

<function __lt__ at 0x7f03f9298500>

>>A.__lt__.im_f unc == A.__lt__.im_fun c

True

>>A.__gt__.im_f unc

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

[Fuzzyman]

On Oct 5, 11:54 pm, Terry Reedy <tjre...@udel.e duwrote:

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

In 3.0, the test returns true because function attributes only get
wrapped when bound. In the meanwhile, " 'object' in repr(X.__lt__)"
should do it for you.

So long as its never used on a class with 'object' in the name.
Doesn't sound like a particularly *good* solution to me. :-)

Michael

tjr

--
http://www.ironpythoninaction.com/

[Fuzzyman]

On Oct 6, 7:16 am, Christian Heimes <li...@cheimes. dewrote:

Terry Reedy wrote:

In 3.0, the test returns true because function attributes only get
wrapped when bound. In the meanwhile, " 'object' in repr(X.__lt__)"
should do it for you.

This session should give you some hints how to archive your goal :)
Have fun!

>>import types
>>class A(object):

... def __lt__(self):
... pass
...

>>isinstance(A. __lt__, types.MethodTyp e)

True

>>isinstance(A. __gt__, types.MethodTyp e)

False

>>type(A.__lt__ )

<type 'instancemethod '>

>>type(A.__gt__ )

<type 'method-wrapper'>

>>type(A.__str_ _)

<type 'wrapper_descri ptor'>

>>type(A.__new_ _)

<type 'builtin_functi on_or_method'>

>>A.__lt__.im_f unc

<function __lt__ at 0x7f03f9298500>

>>A.__lt__.im_f unc == A.__lt__.im_fun c

True

>>A.__gt__.im_f unc

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

However - I need to detect whether a method is inherited from
*object*. All those differences you have shown behave identically if
the class inherits from 'int'.

I am trying to detect explicit implementation of comparison methods -
so there is a big difference between inheriting from int and
inheriting from object (meaningful comparison methods existing on one
and not the other).

What I went for in the end:

import sys as _sys

if _sys.version_in fo[0] == 3:
def _has_method(cls , name):
for B in cls.__mro__:
if B is object:
continue
if name in B.__dict__:
return True
return False
else:
def _has_method(cls , name):
for B in cls.mro():
if B is object:
continue
if name in B.__dict__:
return True
return False

See this page for why I needed it:

http://www.voidspace.org.uk/python/w...04.shtml#e1018

Michael

--
http://www.ironpythoninaction.com/

[Terry Reedy]

Fuzzyman wrote:

On Oct 5, 11:54 pm, Terry Reedy <tjre...@udel.e duwrote:

>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

In 3.0, the test returns true because function attributes only get
wrapped when bound. In the meanwhile, " 'object' in repr(X.__lt__)"
should do it for you.

So long as its never used on a class with 'object' in the name.
Doesn't sound like a particularly *good* solution to me. :-)

From what you posted, 'type object at' should work.

[Fuzzyman]

On Oct 6, 7:02*pm, Terry Reedy <tjre...@udel.e duwrote:

Fuzzyman wrote:

On Oct 5, 11:54 pm, Terry Reedy <tjre...@udel.e duwrote:

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
In 3.0, the test returns true because function attributes only get
wrapped when bound. *In the meanwhile, " 'object' in repr(X.__lt__)"
should do it for you.

So long as its never used on a class with 'object' in the name.
Doesn't sound like a particularly *good* solution to me. :-)

*From what you posted, 'type object at' should work.

It's still a hack...