I have subclassed dict to add an extra method (so no overriding).
Now, I try to compare two of those subclasses, and I get something weird :
>>> d1.items() == d2.items()
True
>>> d1.values() == d2.values()
True
>>> d1.keys() == d2.keys()
True
>>> d1 == d2
False
EDIT
That's damn weird ... I don't understand at all ! Anybody with an insight on how the dict.eq is implemented ?
Following is all the code :
# ------ Bellow is my dict subclass (with no overriding) :
class ClassSetDict(dict):
def subsetget(self, klass, default=None):
class_sets = set(filter(lambda cs: klass <= cs, self))
# Eliminate supersets
for cs1 in class_sets.copy():
for cs2 in class_sets.copy():
if cs1 <= cs2 and not cs1 is cs2:
class_sets.discard(cs2)
try:
best_match = list(class_sets)[0]
except IndexError:
return default
return self[best_match]
# ------ Then an implementation of class sets
class ClassSet(object):
# Set of classes, allowing to easily calculate inclusions
# with comparison operators : `a < B` <=> "A strictly included in B"
def __init__(self, klass):
self.klass = klass
def __ne__(self, other):
return not self == other
def __gt__(self, other):
other = self._default_to_singleton(other)
return not self == other and other < self
def __le__(self, other):
return self < other or self == other
def __ge__(self, other):
return self > other or self == other
def _default_to_singleton(self, klass):
if not isinstance(klass, ClassSet):
return Singleton(klass)
else:
return klass
class Singleton(ClassSet):
def __eq__(self, other):
other = self._default_to_singleton(other)
return self.klass == other.klass
def __lt__(self, other):
if isinstance(other, AllSubSetsOf):
return issubclass(self.klass, other.klass)
else:
return False
class AllSubSetsOf(ClassSet):
def __eq__(self, other):
if isinstance(other, AllSubSetsOf):
return self.klass == other.klass
else:
return False
def __lt__(self, other):
if isinstance(other, AllSubSetsOf):
return issubclass(self.klass, other.klass) and not other == self
else:
return False
# ------ and finally the 2 dicts that don't want to be equal !!!
d1 = ClassSetDict({AllSubSetsOf(object): (int,)})
d2 = ClassSetDict({AllSubSetsOf(object): (int,)})
the problem you're seing has nothing at all to do with subclassing dict
. in fact this behavior can be seen using a regular dict. The problem is how you have defined the keys you're using. A simple class like:
>>> class Foo(object):
... def __init__(self, value):
... self.value = value
...
... def __eq__(self, other):
... return self.value == other.value
...
Is enough to demonstrate the problem:
>>> f1 = Foo(5)
>>> f2 = Foo(5)
>>> f1 == f2
True
>>> d1 = {f1: 6}
>>> d2 = {f2: 6}
>>> d1.items() == d2.items()
True
>>> d1 == d2
False
What's missing is that you forgot to define __hash__
. Every time you change the equality semantics of a class, you should make sure that the __hash__
method agrees with it: when two objects are equal, they must have equal hashes. dict
behavior depends strongly on the hash value of keys.
When you inherit from object
, you automatically get both __eq__
and __hash__
, the former compares object identity, and the latter returns the address of the object (so they agree), but when you change __eq__
, you're still seeing the old __hash__
, which no longer agrees and dict
gets lost.
Simply provide a __hash__
method that in a stable way combines the hash values of its attributes.
>>> class Bar(object):
... def __init__(self, value):
... self.value = value
...
... def __eq__(self, other):
... return self.value == other.value
...
... def __hash__(self):
... return hash((Bar, self.value))
...
>>> b1 = Bar(5)
>>> b2 = Bar(5)
>>> {b1: 6} == {b2: 6}
True
>>>
When using __hash__
in this way, it's also a good idea to make sure that the attributes do not (or better, cannot) change after the object is created. If the hash value changes while collected in a dict, the key will be "lost", and all sorts of weird things can happen (even weirder than the issue you initially asked about)
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