ordering with duck typing in 3.1

[andrew cooke]

hi,

please, what am i doing wrong here? the docs say http://docs.python.org/release/3.1.3/library/stdtypes.html#comparisons "in general, __lt__() and __eq__() are sufficient, if you want the conventional meanings of the comparison operators" but i am seeing

assert 2 < three

E TypeError: unorderable types: int() < IntVar()

with this test:


class IntVar(object):

def __init__(self, value=None):
if value is not None: value = int(value)
self.value = value

def setter(self):
def wrapper(stream_in, thunk):
self.value = thunk()
return self.value
return wrapper

def __int__(self):
return self.value

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

def __eq__(self, other):
return self.value == other

def __hash__(self):
return hash(self.value)


class DynamicTest(TestCase):

def test_lt(self):
three = IntVar(3)
assert three < 4
assert 2 < three
assert 3 == three

so what am i missing?

thanks,
andrew

 

[Thomas Rachel]

Am 07.04.2012 14:23 schrieb andrew cooke:

class IntVar(object):

def __init__(self, value=None):
if value is not None: value = int(value)
self.value = value

def setter(self):
def wrapper(stream_in, thunk):
self.value = thunk()
return self.value
return wrapper

def __int__(self):
return self.value

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

def __eq__(self, other):
return self.value == other

def __hash__(self):
return hash(self.value)

so what am i missing?

If I don't confuse things, I think you are missing a __gt__() in your
IntVar() class.

This is because first, a '2 < three' is tried with 2.__lt__(three). As
this fails due to the used types, it is reversed: 'three > 2' is
equivalent. As your three doesn't have a __gt__(), three.__gt__(2) fails
as well.


Thomas

 

[mwilson]

Am 07.04.2012 14:23 schrieb andrew cooke:



If I don't confuse things, I think you are missing a __gt__() in your
IntVar() class.

This is because first, a '2 < three' is tried with 2.__lt__(three). As
this fails due to the used types, it is reversed: 'three > 2' is
equivalent. As your three doesn't have a __gt__(), three.__gt__(2) fails
as well.

Practically, yes. Just that that's not what the documentation says. Looks
like Python no longer tries to cobble together missing relations based on
the "usual" properties of ordering.

Mel.

 

[Jon Clements]

Any reason you can't derive from int instead of object? You may also want to check out functions.total_ordering on 2.7+

 

[Neil Cerutti]

Any reason you can't derive from int instead of object? You may
also want to check out functions.total_ordering on 2.7+

functools.total_ordering

I was temporarily tripped up by the aforementioned documentation,
myself.

 

[Jon Clements]

functools.total_ordering

I was temporarily tripped up by the aforementioned documentation,
myself.

Oops. I sent it from a mobile tablet device - I got auto-corrected. But yes, it is functools.total_ordering - TY you Neil.

Jon.