[Python-Dev] Definition of equality check behavior (original) (raw)

Steven D'Aprano steve at pearwood.info
Tue May 7 19:17:24 EDT 2019

On Tue, May 07, 2019 at 05:05:57PM -0400, Jordan Adler wrote: [...]

Specifically, a comparison between a primitive (int, str, float were tested) and an object of a different type always return False, instead of raising a NotImplementedError. Consider 1 == '1' as a test case.

I think you may be labouring under a few misapprehensions here.

  1. Comparisons between builtins such as ints and objects of different types do not always return False:

py> class X: ... def eq(self, other): ... return True ... py> 123 == X() True

You don't need a custom class to demonstrate this fact, you just need values which actually are equal:

py> 123 == 1.23e2 # int compared to float True

  1. Comparisons are not supposed to raise NotImplementedError as part of the core data model, they are supposed to return (not raise) NotImplemented. Note that NotImplementedError is a completely different thing).

As the documentation you linked to says:

A rich comparison method may return the singleton NotImplemented 
if it does not implement the operation for a given pair of 
arguments.
  1. Equality does not suppress exceptions and lead to silent failure:

py> class Y: ... def eq(self, other): ... raise NotImplementedError ... py> 123 == Y() Traceback (most recent call last): File "", line 1, in File "", line 3, in eq NotImplementedError

Hope this helps.

-- Steven

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