[Python-Dev] == on object tests identity in 3.x

Ethan Furman ethan at stoneleaf.us
Mon Jul 7 19:43:34 CEST 2014

On 07/07/2014 09:56 AM, Andreas Maier wrote:
> Am 07.07.2014 17:55, schrieb Ethan Furman:
>> On 07/07/2014 04:22 AM, Andreas Maier wrote:
>>> Where is the discrepancy between the documentation of == and its
>>> default implementation on object documented?
>> There's seems to be no discrepancy (at least, you have not shown it),
> The documentation states consistently that == tests the equality of the value of an object. The default implementation
> of == in both 2.x and 3.x tests the object identity. Is that not a discrepancy?

One could say that the value of an object is the object itself.  Since different objects are different, then they are 
not equal.

>> but to answer the question about why the default equals operation is an
>> identity test:
>>    - all objects should be equal to themselves (there is only one that
>> isn't, and it's weird)
> I agree. But that is not a reason to conclude that different objects (as per their identity) should be unequal. Which is
> what the default implementation does.

Python cannot know which values are important in an equality test, and which are not.  So it refuses to guess.

Think of a chess board, for example.  Are any two black pawns equal?  All 16 pawns came from the same Pawn class, the 
only differences would be in the color and position, but the movement type is the same for all.

So equality for a pawn might mean the same color, or it might mean color and position, or it might mean can move to the 
same position... it's up to the programmer to decide which of the possibilities is the correct one.  Quite frankly, have 
equality mean identity in this case also makes a lot of sense.

>>    - equality tests should not, as a general rule, raise exceptions --
>> they should return True or False
> Why not? Ordering tests also raise exceptions if ordering is not implemented.

Besides the pawn example, this is probably a matter of practicality over purity -- equality tests are used extensively 
through-out Python, and having exceptions raised at possibly any moment would not be a fun nor productive environment.

Ordering is much less frequent, and since we already tried always ordering things, falling back to type name if 
necessary, we have discovered that that is not a good trade-off.  So now if one tries to order things without specifying 
how it should be done, one gets an exception.


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