[issue43475] Worst-case behaviour of hash collision with float NaN
realead added the comment: @mark.dickinson > ...my expectation was that there would be few cases of breakage, and that for > those few cases it shouldn't be difficult to fix the breakage. This expectation is probably correct. My issue is somewhat only partly on-topic here: If one wants to have all NaNs in one equivalency class (e.g. if used as a key-value for example in pandas) it is almost impossible to do so in a consistent way without taking a performance hit. It seems to be possible builtin-types (even if frozenset won't be pretty), but already something like class A: def __init__(self, a): self.a=a def __hash__(self): return hash(self.a) def __eq__(self, other): return self.a == other.a is not easy to handle. A special comparator for containers would be an ultimative solution, but I see how this could be too much hassle for a corner case. -- ___ Python tracker <https://bugs.python.org/issue43475> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue43475] Worst-case behaviour of hash collision with float NaN
realead added the comment:
This change makes life harder for people trying to get sane behavior with sets
for float-, complex- and tuple-objects containing NaNs. With "sane" meaning,
that
set([nan, nan, nan]) => {nan}
Until now, one has only to override the equal-comparison for these types but
now also hash-function needs to be overriden (which is more complicated e.g.
for tuple).
On a more abstract level: there are multiple possible equivalence relations.
The one used right now for floats is Py_EQ and results in
float("nan)!=float("nan") due to IEEE 754.
In hash-set/dict we would like to use an equivalence relation for which all
NaNs would be in the same equivalence class. Maybe a new comparator is called
for (like PY_EQ_FOR_HASH_COLLECTION), which would yield float("nan") equivalent
to float("nan") and which should be used in hash-set/dict?
--
nosy: +realead
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