Elegant ways to support equivalence ("equality") in Python classes
Question:
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
Answers:
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.
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 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
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 ‘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.
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…
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()))
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
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
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
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
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.
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 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
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 ‘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.
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…
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()))
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 ofx==y
does not imply thatx!=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 isNotImplemented
. There are no other implied
relationships among the comparison operators, for example, the truth
of(x<y or x==y)
does not implyx<=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 asclass A:
,class A():
orclass A(B):
whereB
is a classic-style class; -
new-style classes, that do inherit from
object
and that are declared asclass A(object)
orclass A(B):
whereB
is a new-style class. Python 3 has only new-style classes that are declared asclass A:
,class A(object):
orclass 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
callsn1.__eq__
;n3 == n1
callsn3.__eq__
;n1 != n3
callsn1.__ne__
;n3 != n1
callsn3.__ne__
.
And if Number
is a new-style class:
- both
n1 == n3
andn3 == n1
calln3.__eq__
; - both
n1 != n3
andn3 != n1
calln3.__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
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
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