Develop Reference

Python equality of class instances [duplicate]

When writing custom classes it is often important to allow equivalence by means of the == and != operators. In Python, this is made possible by implementing the __eq__ and __ne__ special methods, respectively. The easiest way I've found to do this is the following method:
class Foo:
def __init__(self, item):
self.item = item
def __eq__(self, other):
if isinstance(other, self.__class__):
return self.__dict__ == other.__dict__
else:
return False
def __ne__(self, other):
return not self.__eq__(other)
Do you know of more elegant means of doing this? Do you know of any particular disadvantages to using the above method of comparing __dict__s?
Note: A bit of clarification--when __eq__ and __ne__ are undefined, you'll find this behavior:
>>> a = Foo(1)
>>> b = Foo(1)
>>> a is b
False
>>> a == b
False
That is, a == b evaluates to False because it really runs a is b, a test of identity (i.e., "Is a the same object as b?").
When __eq__ and __ne__ are defined, you'll find this behavior (which is the one we're after):
>>> a = Foo(1)
>>> b = Foo(1)
>>> a is b
False
>>> a == b
True

Consider this simple problem:
class Number:
def __init__(self, number):
self.number = number
n1 = Number(1)
n2 = Number(1)
n1 == n2 # False -- oops
So, Python by default uses the object identifiers for comparison operations:
id(n1) # 140400634555856
id(n2) # 140400634555920
Overriding the __eq__ function seems to solve the problem:
def __eq__(self, other):
"""Overrides the default implementation"""
if isinstance(other, Number):
return self.number == other.number
return False
n1 == n2 # True
n1 != n2 # True in Python 2 -- oops, False in Python 3
In Python 2, always remember to override the __ne__ function as well, as the documentation states:
There are no implied relationships among the comparison operators. The
truth of x==y does not imply that x!=y is false. Accordingly, when
defining __eq__(), one should also define __ne__() so that the
operators will behave as expected.
def __ne__(self, other):
"""Overrides the default implementation (unnecessary in Python 3)"""
return not self.__eq__(other)
n1 == n2 # True
n1 != n2 # False
In Python 3, this is no longer necessary, as the documentation states:
By default, __ne__() delegates to __eq__() and inverts the result
unless it is NotImplemented. There are no other implied
relationships among the comparison operators, for example, the truth
of (x<y or x==y) does not imply x<=y.
But that does not solve all our problems. Let’s add a subclass:
class SubNumber(Number):
pass
n3 = SubNumber(1)
n1 == n3 # False for classic-style classes -- oops, True for new-style classes
n3 == n1 # True
n1 != n3 # True for classic-style classes -- oops, False for new-style classes
n3 != n1 # False
Note: Python 2 has two kinds of classes:
classic-style (or old-style) classes, that do not inherit from object and that are declared as class A:, class A(): or class A(B): where B is a classic-style class;
new-style classes, that do inherit from object and that are declared as class A(object) or class A(B): where B is a new-style class. Python 3 has only new-style classes that are declared as class A:, class A(object): or class A(B):.
For classic-style classes, a comparison operation always calls the method of the first operand, while for new-style classes, it always calls the method of the subclass operand, regardless of the order of the operands.
So here, if Number is a classic-style class:
n1 == n3 calls n1.__eq__;
n3 == n1 calls n3.__eq__;
n1 != n3 calls n1.__ne__;
n3 != n1 calls n3.__ne__.
And if Number is a new-style class:
both n1 == n3 and n3 == n1 call n3.__eq__;
both n1 != n3 and n3 != n1 call n3.__ne__.
To fix the non-commutativity issue of the == and != operators for Python 2 classic-style classes, the __eq__ and __ne__ methods should return the NotImplemented value when an operand type is not supported. The documentation defines the NotImplemented value as:
Numeric methods and rich comparison methods may return this value if
they do not implement the operation for the operands provided. (The
interpreter will then try the reflected operation, or some other
fallback, depending on the operator.) Its truth value is true.
In this case the operator delegates the comparison operation to the reflected method of the other operand. The documentation defines reflected methods as:
There are no swapped-argument versions of these methods (to be used
when the left argument does not support the operation but the right
argument does); rather, __lt__() and __gt__() are each other’s
reflection, __le__() and __ge__() are each other’s reflection, and
__eq__() and __ne__() are their own reflection.
The result looks like this:
def __eq__(self, other):
"""Overrides the default implementation"""
if isinstance(other, Number):
return self.number == other.number
return NotImplemented
def __ne__(self, other):
"""Overrides the default implementation (unnecessary in Python 3)"""
x = self.__eq__(other)
if x is NotImplemented:
return NotImplemented
return not x
Returning the NotImplemented value instead of False is the right thing to do even for new-style classes if commutativity of the == and != operators is desired when the operands are of unrelated types (no inheritance).
Are we there yet? Not quite. How many unique numbers do we have?
len(set([n1, n2, n3])) # 3 -- oops
Sets use the hashes of objects, and by default Python returns the hash of the identifier of the object. Let’s try to override it:
def __hash__(self):
"""Overrides the default implementation"""
return hash(tuple(sorted(self.__dict__.items())))
len(set([n1, n2, n3])) # 1
The end result looks like this (I added some assertions at the end for validation):
class Number:
def __init__(self, number):
self.number = number
def __eq__(self, other):
"""Overrides the default implementation"""
if isinstance(other, Number):
return self.number == other.number
return NotImplemented
def __ne__(self, other):
"""Overrides the default implementation (unnecessary in Python 3)"""
x = self.__eq__(other)
if x is not NotImplemented:
return not x
return NotImplemented
def __hash__(self):
"""Overrides the default implementation"""
return hash(tuple(sorted(self.__dict__.items())))
class SubNumber(Number):
pass
n1 = Number(1)
n2 = Number(1)
n3 = SubNumber(1)
n4 = SubNumber(4)
assert n1 == n2
assert n2 == n1
assert not n1 != n2
assert not n2 != n1
assert n1 == n3
assert n3 == n1
assert not n1 != n3
assert not n3 != n1
assert not n1 == n4
assert not n4 == n1
assert n1 != n4
assert n4 != n1
assert len(set([n1, n2, n3, ])) == 1
assert len(set([n1, n2, n3, n4])) == 2

You need to be careful with inheritance:
>>> class Foo:
def __eq__(self, other):
if isinstance(other, self.__class__):
return self.__dict__ == other.__dict__
else:
return False
>>> class Bar(Foo):pass
>>> b = Bar()
>>> f = Foo()
>>> f == b
True
>>> b == f
False
Check types more strictly, like this:
def __eq__(self, other):
if type(other) is type(self):
return self.__dict__ == other.__dict__
return False
Besides that, your approach will work fine, that's what special methods are there for.

The way you describe is the way I've always done it. Since it's totally generic, you can always break that functionality out into a mixin class and inherit it in classes where you want that functionality.
class CommonEqualityMixin(object):
def __eq__(self, other):
return (isinstance(other, self.__class__)
and self.__dict__ == other.__dict__)
def __ne__(self, other):
return not self.__eq__(other)
class Foo(CommonEqualityMixin):
def __init__(self, item):
self.item = item

Not a direct answer but seemed relevant enough to be tacked on as it saves a bit of verbose tedium on occasion. Cut straight from the docs...
functools.total_ordering(cls)
Given a class defining one or more rich comparison ordering methods, this class decorator supplies the rest. This simplifies the effort involved in specifying all of the possible rich comparison operations:
The class must define one of __lt__(), __le__(), __gt__(), or __ge__(). In addition, the class should supply an __eq__() method.
New in version 2.7
#total_ordering
class Student:
def __eq__(self, other):
return ((self.lastname.lower(), self.firstname.lower()) ==
(other.lastname.lower(), other.firstname.lower()))
def __lt__(self, other):
return ((self.lastname.lower(), self.firstname.lower()) <
(other.lastname.lower(), other.firstname.lower()))

You don't have to override both __eq__ and __ne__ you can override only __cmp__ but this will make an implication on the result of ==, !==, < , > and so on.
is tests for object identity. This means a is b will be True in the case when a and b both hold the reference to the same object. In python you always hold a reference to an object in a variable not the actual object, so essentially for a is b to be true the objects in them should be located in the same memory location. How and most importantly why would you go about overriding this behaviour?
Edit: I didn't know __cmp__ was removed from python 3 so avoid it.

From this answer: https://stackoverflow.com/a/30676267/541136 I have demonstrated that, while it's correct to define __ne__ in terms __eq__ - instead of
def __ne__(self, other):
return not self.__eq__(other)
you should use:
def __ne__(self, other):
return not self == other

I think that the two terms you're looking for are equality (==) and identity (is). For example:
>>> a = [1,2,3]
>>> b = [1,2,3]
>>> a == b
True <-- a and b have values which are equal
>>> a is b
False <-- a and b are not the same list object

The 'is' test will test for identity using the builtin 'id()' function which essentially returns the memory address of the object and therefore isn't overloadable.
However in the case of testing the equality of a class you probably want to be a little bit more strict about your tests and only compare the data attributes in your class:
import types
class ComparesNicely(object):
def __eq__(self, other):
for key, value in self.__dict__.iteritems():
if (isinstance(value, types.FunctionType) or
key.startswith("__")):
continue
if key not in other.__dict__:
return False
if other.__dict__[key] != value:
return False
return True
This code will only compare non function data members of your class as well as skipping anything private which is generally what you want. In the case of Plain Old Python Objects I have a base class which implements __init__, __str__, __repr__ and __eq__ so my POPO objects don't carry the burden of all that extra (and in most cases identical) logic.

Instead of using subclassing/mixins, I like to use a generic class decorator
def comparable(cls):
""" Class decorator providing generic comparison functionality """
def __eq__(self, other):
return isinstance(other, self.__class__) and self.__dict__ == other.__dict__
def __ne__(self, other):
return not self.__eq__(other)
cls.__eq__ = __eq__
cls.__ne__ = __ne__
return cls
Usage:
#comparable
class Number(object):
def __init__(self, x):
self.x = x
a = Number(1)
b = Number(1)
assert a == b

This incorporates the comments on Algorias' answer, and compares objects by a single attribute because I don't care about the whole dict. hasattr(other, "id") must be true, but I know it is because I set it in the constructor.
def __eq__(self, other):
if other is self:
return True
if type(other) is not type(self):
# delegate to superclass
return NotImplemented
return other.id == self.id

I wrote a custom base with a default implementation of __ne__ that simply negates __eq__:
class HasEq(object):
"""
Mixin that provides a default implementation of ``object.__neq__`` using the subclass's implementation of ``object.__eq__``.
This overcomes Python's deficiency of ``==`` and ``!=`` not being symmetric when overloading comparison operators
(i.e. ``not x == y`` *does not* imply that ``x != y``), so whenever you implement
`object.__eq__ <https://docs.python.org/2/reference/datamodel.html#object.__eq__>`_, it is expected that you
also implement `object.__ne__ <https://docs.python.org/2/reference/datamodel.html#object.__ne__>`_
NOTE: in Python 3+ this is no longer necessary (see https://docs.python.org/3/reference/datamodel.html#object.__ne__)
"""
def __ne__(self, other):
"""
Default implementation of ``object.__ne__(self, other)``, delegating to ``self.__eq__(self, other)``.
When overriding ``object.__eq__`` in Python, one should also override ``object.__ne__`` to ensure that
``not x == y`` is the same as ``x != y``
(see `object.__eq__ <https://docs.python.org/2/reference/datamodel.html#object.__eq__>`_ spec)
:return: ``NotImplemented`` if ``self.__eq__(other)`` returns ``NotImplemented``, otherwise ``not self.__eq__(other)``
"""
equal = self.__eq__(other)
# the above result could be either True, False, or NotImplemented
if equal is NotImplemented:
return NotImplemented
return not equal
If you inherit from this base class, you only have to implement __eq__ and the base.
In retrospect, a better approach might have been to implement it as a decorator instead. Something like #functools.total_ordering

re-use pytest assertion logic for complex objects

How can I reuse the python "smart" reporting for objects?
E.g.:
class Foo(object):
def __init__(self, text, sequence):
self.text = text
self.sequence = sequence
def __eq__(self, other):
return type(self) == type(other) and self.text == other.text and self.sequence == other.sequence
def __ne__(self, other):
return not self.__eq__(other)
What I want is to get the assertion report, just as I would assert the primitive fields, e.g. (of course makes sense to not print attributes that satisfy the comparison operator - and concatenate together the failing attribute reports). Something like:
def test_magic():
> assert Foo(text='a', sequence=[1]) == Foo(text='a', sequence=[1, 2])
E assert Foo(text='a', sequence=[1]) == Foo(text='a', sequence=[1, 2])
E attribute `sequence`:
E [1] == [1, 2]
E Right contains more items, first extra item: 2
E Use -v to get the full diff
Can I do this by writing some plugin, etc and calling in that the internal comparator? Or what is the recommended solution?

Overloading __eq__ in a class

I'm trying to overload the == operator in a class, and this is the init method:
class Point:
def __init__(self, a, b, c, d):
self.a = a
self.b = b
self.c = c
self.d = d
self._fields = ['a','b','c','d']
And I'm trying to overload the == operator, and here is my code for that:
def __eq__(self,right):
if type(right) == type(self):
for i in self._fields:
print(self._fields.index(i))
else:
return False
return True
For == to be true, all the values in init should be the same. So if I have test=Point(1,2,3), and then I have test2 = Point(1,2,3), then test==test2 should return True. However, I have test=Point(1,2,3), and test2=Point(1,1,3) and this is returning True. Can anybody figure out why this is?

You're testing whether self['a'] == right['a'] when what you want is self.a == right.a. You should use the getattr function to do what you want to do.

Currently, all your code does while iterating over the fields is print out their indexes. It only will ever return False for objects of different types. Instead, you should use getattr to get the actual attribute values that correspond to the names in _fields:
def __eq__(self, other):
return (self._fields == other._fields and
all(getattr(self, attr) == getattr(other, attr) for attr in self._fields)
Note that I've changed the test for the two objects having the same type for one that checks that they have the same field (this is a kind of duck-typing). If you want to stick with a type check, I'd make _fields a class attribute, so you will know that every instance has the same value for it.
Alternately, you could do away with the _fields attribute completely and just hard code the attribute names:
def __eq__(self, other):
return (type(self) == type(other) and
self.a == other.a and self.b == other.b and
self.c == other.c and self.d == other.d)

overidding Pythons __eq__ method , isistance & eq mothods return false

I'm new to Python from the Java world.
I have written a Python class called "Instance" with 3 properties(attribute, value, and class). I want to override the "eq" method & also the "hash" method, I'm using the "attribute" & "value" properties used for object comparison. I instantiated two objects with the same values, however they return as not equal.
Code is below , Class Instance:
'''Class of type Instance'''
class Instance(object):
__attribute = None;
__value = None;
__classification = None;
#constructor
def __init__(self,attribute,value,classification):
self.attribute = attribute;
self.value = value;
self.classification = classification;
#setters & getters
def setAttribute(self,attribute):
self.attribute = attribute
def setValue(self,value):
self.value = value
def setClassification(self,classification):
self.classification = classification
def getAttribute(self):
return self.Attribute;
def getValue(self):
return self.Value
def getClassification(self):
return self.Classification
def __eq__(self, other):
#if self & other are the same instance & attribute & value equal
return isinstance(self,other) and (self.attribute == other.attribute) and (self.value == other.value)
def __hash__(self):
return hash(self.attribute, self.value)
I'm instantiating in , another Python module called Testing:
if __name__ == '__main__':
pass
from Instance import *
instance1 = Instance('sameValue', 1,'Iris-setosa')
instance2 = Instance('sameValue', 1,'Iris-setosa')
if (instance1 is instance2):
print "equals"
else:
print "not equals"
The program returns: not equals.

Your first problem is isinstance(self, other) isn't asking whether self and other are both instances of compatible types, or whether they're the same instance (as your comment says), it's asking whether self is an instance of the type other. Since other isn't even a type, the answer is always false.
You probably wanted isinstance(self, type(other)). Or maybe something more complicated, like isinstance(self, type(other)) or isinstance(other, type(self)).
Or maybe you don't really want this at all; even for equality testing, duck typing is often a good idea. If other has the same attributes as self, and also hashes to the same value, is that good enough? The answer may be no… but you definitely should ask the question.
Your second problem is a misunderstanding of is:
if (instance1 is instance2):
print "equals"
else:
print "not equals"
The whole point of is is that it's asking whether these are the same object, not whether these two (possibly distinct) objects are equal to each other. For example:
>>> a = []
>>> b = []
>>> a == b
True
>>> a is b
False
They're both empty lists, so they're equal to each other, but they're two different empty lists, which is why you can do this:
>>> a.append(0)
>>> b
[]
And the same is true with your class. Each Instance that you create is going to be a different, separate instance—even if they're all equal.
The __eq__ method that you define customized the == operator. There is no way to customize the is operator.

Unexpected behavior for python set.__contains__

Borrowing the documentation from the __contains__ documentation
print set.__contains__.__doc__
x.__contains__(y) <==> y in x.
This seems to work fine for primitive objects such as int, basestring, etc. But for user-defined objects that define the __ne__ and __eq__ methods, I get unexpected behavior. Here is a sample code:
class CA(object):
def __init__(self,name):
self.name = name
def __eq__(self,other):
if self.name == other.name:
return True
return False
def __ne__(self,other):
return not self.__eq__(other)
obj1 = CA('hello')
obj2 = CA('hello')
theList = [obj1,]
theSet = set(theList)
# Test 1: list
print (obj2 in theList) # return True
# Test 2: set weird
print (obj2 in theSet) # return False unexpected
# Test 3: iterating over the set
found = False
for x in theSet:
if x == obj2:
found = True
print found # return True
# Test 4: Typcasting the set to a list
print (obj2 in list(theSet)) # return True
So is this a bug or a feature?

For sets and dicts, you need to define __hash__. Any two objects that are equal should hash the same in order to get consistent / expected behavior in sets and dicts.
I would reccomend using a _key method, and then just referencing that anywhere you need the part of the item to compare, just as you call __eq__ from __ne__ instead of reimplementing it:
class CA(object):
def __init__(self,name):
self.name = name
def _key(self):
return type(self), self.name
def __hash__(self):
return hash(self._key())
def __eq__(self,other):
if self._key() == other._key():
return True
return False
def __ne__(self,other):
return not self.__eq__(other)

This is because CA doesn't implement __hash__
A sensible implementation would be:
def __hash__(self):
return hash(self.name)

A set hashes it's elements to allow a fast lookup. You have to overwrite the __hash__ method so that a element can be found:
class CA(object):
def __hash__(self):
return hash(self.name)
Lists don't use hashing, but compare each element like your for loop does.