[Python-Dev] == on object tests identity in 3.x (original) (raw)

Benjamin Peterson benjamin at python.org
Tue Jul 8 01:49:40 CEST 2014

On Mon, Jul 7, 2014, at 16:36, Andreas Maier wrote:

Am 2014-07-07 19:43, schrieb Ethan Furman: > 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. > Well, one could argue that using the address of an object for its value equality test is pretty close to guessing, considering that given a sensible definition of value equality, objects of different identity can very well be equal but will always be considered unequal based on the address.

Probably the best argument for the behavior is that "x is y" should imply "x == y", which preludes raising an exception. No such invariant is desired for ordering, so default implementations of < and > are not provided in Python 3.

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