Kwankyu Lee wrote:
> Do we have such cases in Python proper? I mean a case that disobeys
> (1).

I can't think of any example for base types(*). But this is explicitly 
allowed by PEP207:

|    3 The == and != operators are not assumed to be each other's
|      complement (e.g. IEEE 754 floating point numbers do not satisfy
|      this).  It is up to the type to implement this if desired.

and I guess there are a number of other libraries that use this option.

(*) What I said about NaNs earlier in the thread was incorrect: I think 
the rule is actually that NaN < a, NaN > a, NaN == a are all false for 
all a (even when a is [the same] NaN), but NaN != a is true for all a. 
And that's, of course, assuming a particular mapping of the language's 
comparison operators to IEEE754 comparison predicates, which is the job 
of the language specification. And hence I don't understand what the 
remark about IEEE754 in the above quotation refers to.

-- 
Marc

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