[Tutor] Overriding equality tests in Python

[Mitya Sirenef]

On 03/23/2013 12:08 AM, Robert Sjoblom wrote:
> Hi list. I'll preface this by  saying that I am very grateful for all
 > of you, and thank you in advance to anyone that answers.
 >
 > I'm currently working on a roulette simulator, because it seemed like
 > fun. I found out I needed a way to compare two different outcomes, and
 > it was suggested to me that I should override the __eq__ and __ne__
 > methods. Said and done, I did:
 > class Outcome():
 > def __init__(self, name): #Ignoring odds for now
 > self.name = name
 >
 > def __eq__(self, other):
 > '''returns True if Outcome.name matches other.name''':
 > if self.name == other.name: return True
 > def __ne__(self, other):
 > '''returns True if Outcome.name does not match other.name'''
 > if self.name != other.name: return True
 >
 > Now, this works, as far as this is concerned:
 >>>> a = Outcome('Bob')
 >>>> b = Outcome('Ray')
 >>>> c = Outcome('Bob')
 >>>> a == b
 >>>> a == c
 > True
 >>>> a != b
 > True
 >>>>
 > However, if I were to create a class without the __eq__ and __ne__
 > definitions, what is to prevent me from doing: a.name == b.name ? Or
 > am I missing something in my implementation of the overrides? Is there
 > a reason why I shouldn't do .name comparisons?
 >


Firstly, in __eq__, you can do: return self.name == other.name
Second, you are right that you can also compare attributes directly. But
the meaning is different, if I'm reading your program, a == b tells me
"object a is equal to b for the purpose of this program", a.name ==
b.name tells me, name attrs are equal. a == b is also a shorter and more
readable expression. __eq__ is also used in constructs like:

if a in mylist: ...

__eq__ is probably used for other things, too; the basic idea is that it
tells Python that objects are equal, and Python can use this information
in various circumstances.

  -m



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