10.4. Operator Comparison

• == - eq

• != - ne

• < - lt

• <= - le

• > - gt

• >= - ge

10.4.1. About

Table 10.3. Comparison Operator Overload

Operator	Method
obj == other	obj.__eq__(other)
obj != other	obj.__ne__(other)
obj < other	obj.__lt__(other)
obj <= other	obj.__le__(other)
obj > other	obj.__gt__(other)
obj >= other	obj.__ge__(other)

10.4.2. Object Equality

>>> class Astronaut:
...     firstname: str
...     lastname: str
...
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> a = Astronaut('Mark', 'Watney')
>>> b = Astronaut('Mark', 'Watney')
>>>
>>> a == b
False
>>>
>>> hex(id(a))  
'0x11b9706a0'
>>> hex(id(b))  
'0x11b970700'
>>> id(a) == id(b)
False

>>> class Astronaut:
...     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 = Astronaut('Mark', 'Watney')
>>> b = Astronaut('Mark', 'Watney')
>>>
>>> print(a == b)
True
>>>
>>> hex(id(a))  
'0x11b970c70'
>>> hex(id(b))  
'0x11b9704c0'
>>> id(a) == id(b)
False

10.4.3. Problem

• When you compare objects with the same fields from two different classes

>>> class Astronaut:
...     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 Cosmonaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> a = Astronaut('Mark', 'Watney')
>>> c = Cosmonaut('Mark', 'Watney')
>>>
>>> print(a == c)
True

10.4.4. Solution

• Always remember to compare classes

• This way you avoid bug, when both has the same fields and values

>>> class Astronaut:
...     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 Cosmonaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> a = Astronaut('Mark', 'Watney')
>>> c = Cosmonaut('Mark', 'Watney')
>>>
>>> print(a == c)
False

10.4.5. Eq Works at Both Sides

>>> class Astronaut:
...     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 Cosmonaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> a = Astronaut('Mark', 'Watney')
>>> c = Cosmonaut('Mark', 'Watney')
>>>
>>> print(a == c)
True

>>> class Astronaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> class Cosmonaut:
...     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 = Astronaut('Mark', 'Watney')
>>> c = Cosmonaut('Mark', 'Watney')
>>>
>>> print(a == c)
True

10.4.6. Use Case - 0x01

• Game

>>> hero < Damage(20)  
>>> hero > Damage(20)  

10.4.7. Assignments

Code 10.28. Solution

"""
* Assignment: Operator Comparison Equals
* Complexity: easy
* Lines of code: 3 lines
* Time: 5 min

English:
    1. Override operator for code to work correctly
    2. Do not use `dataclasses`
    3. Run doctests - all must succeed

Polish:
    1. Nadpisz operator aby poniższy kod zadziałał poprawnie
    2. Nie używaj `dataclasses`
    3. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> Mission(2035, 'Ares 3') == Mission(2035, 'Ares 3')
    True
    >>> Mission(2035, 'Ares 3') == Mission(1973, 'Apollo 18')
    False
    >>> Mission(2035, 'Ares 3') == Mission(2035, 'Apollo 18')
    False
    >>> Mission(2035, 'Ares 3') == Mission(1973, 'Ares 3')
    False
"""

class Mission:
    def __init__(self, year, name):
        self.year = year
        self.name = name