[Python-Dev] Definition of equality check behavior

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()

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

2. 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 

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 

3. 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 "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __eq__

Hope this helps.


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