when to use == and when to use is

[Ned Batchelder]

On 3/10/14 2:09 PM, George Trojan wrote:
> I know this question has been answered:
> http://stackoverflow.com/questions/6570371/when-to-use-and-when-to-use-is ,
> but I still have doubts. Consider the following code:
>
> class A:
>      def __init__(self, a):
>          self._a = a
>      #def __eq__(self, other):
>      #    return self._a != other._a
>
> obj_0 = A(0)
> obj_1 = A(1)
> obj_2 = A(2)
>
> obj = obj_1
>
> if obj == obj_0:
>      print(0)
> elif obj == obj_1:
>      print(1)
> elif obj == obj_2:
>      print(2)
>
> if obj is obj_0:
>      print(0)
> elif obj is obj_1:
>      print(1)
> elif obj is obj_2:
>      print(2)
>
> Both if statements work, of course. Which is more efficient? My use-case
> scenario are matplotlib objects, the __eq__ operator might involve a bit
> of work. The "if" statement is a selector in a callback. I know that obj
> is one of obj_0, ..., or none of them. I do not care if obj_1 is equal
> to obj_2.
>

The last sentence seems telling to me: if you don't care if objects are 
equal, then don't use ==.

Of course, a long change of if's to figure out which object you have 
seems odd to me...

--Ned.

> George


-- 
Ned Batchelder, http://nedbatchelder.com