Raymond Hettinger <[email protected]> added the comment:
We should consider biting the bullet and revising the default NaN sort order.
It has been a perpetual irritant.
1) Have NaNs always compare to less than any other float value.
2) When comparing two distinct NaNs, use the NaN payload
and fall back to object id if necessary.
3) When comparing two identical NaNs, always return False.
That preserves the IEEE-754 guarantee that NaNs never compare equal to one
another -- which is one of the few properties that people rely on.
IEEE-754 doesn't concern itself at all with the notion of object identity, so
giving two NaNs a deterministic but unequal ordering isn't at odds with the
standard.
IMHO, making min(), max(), and sorted() both deterministic and commutative is a
big enough win to warrant foregoing one of the less important properties of
NaN. User care about determinism and commutativity more than they care that
float('NaN') > 10 and float('NaN') < 10 both return False.
“Logic clearly dictates that the needs of the many outweigh the needs of the
few.” -- Spock, The Wrath of Khan ;-)
Python is practical language, so we should consider making a choice based on
pragmatism rather than strict standard compliance. Bug reports like this are
the tip of the iceberg. Given the prevalence of NaNs being used as a
placeholder for missing data, it is likely that people struggle with this every
day.
The original design of NaNs was to let invalid intermediate results flow
through a calculation and not require error testing at every step. But in
practice, NaNs are not commonly used this way. Mainly, they are used as the
float equivalent of None in an array of floats. We should support this
reality. IMO it is a small disaster that NaNs completely wreck sorting and do
so silently. From user POV, an arcane property of NaNs is an inadequate
justification for the misbehavior.
----------
nosy: +rhettinger
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<https://bugs.python.org/issue44370>
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