5.2. OOP Equality

5.2.1. Default Behavior

By default, all objects are not equal

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname

>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')

>>> a == b
False

5.2.2. Equality Method

Both object are not equal

Mind, that the following implementation of __eq__ method is erroneous and should not be used.

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
...
...     def __eq__(self, other):
...         return self.firstname == other.firstname \
...            and self.lastname == other.lastname

>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')

>>> a == b
True

5.2.3. Equality Problem

Both object are not equal

Mind, that the following implementation of __eq__ method is erroneous and should not be used:

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
...
...     def __eq__(self, other):
...         return self.firstname == other.firstname \
...            and self.lastname == other.lastname
>>>
>>>
>>> class Admin:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname

>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')
>>> c = Admin('Mark', 'Watney')

>>> a == b
True

>>> a == c
True

5.2.4. Equality Solution

Both object are not equal

The following implementation of __eq__ method is correct and can be used:

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
...
...     def __eq__(self, other):
...         return self.__class__ is other.__class__ \
...            and self.firstname == other.firstname \
...            and self.lastname == other.lastname
>>>
>>>
>>> class Admin:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname

>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')
>>> c = Admin('Mark', 'Watney')

>>> a == b
True

>>> a == c
False

5.2.5. Dataclasses

By default eq=True (if not specified)
Objects of the same type are equal
Objects of different types are not equal

SetUp:

>>> from dataclasses import dataclass

By default instances of the same dataclass are equal:

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
>>>
>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')
>>>
>>> a == b
True

You can turn this behavior off by setting eq=False:

>>> @dataclass(eq=False)
... class User:
...     firstname: str
...     lastname: str
>>>
>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')
>>>
>>> a == b
False