On Thursday, November 5, 2015 at 2:30:08 AM UTC-8, Simon King wrote:
> Certainly, if "A is B" then "A==B" will be True. Actually I am not sure
> if A.__eq__ would be invoked at all if you compare to identical objects
> (i.e., I don't know what Python does).
>
This is famous in Python:
sage: NAN = float("NaN")
sage: NAN == NAN
False
Dictionary lookup shortcuts equality testing on identical keys, leading to
the surprising:
sage: D={NAN: 1}
sage: D[NAN]
1
sage: D[float("NaN")]
KeyError: nan
sage: { NAN, NAN, float("NaN")}
{nan, nan}
(i.e., even for a key that is not equal to itself, it is still possible to
retrieve the value under that key using *exactly* that key)
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