[Python-Dev] decimal.py: == and != comparisons involving NaNs (original) (raw)

Stefan Krah stefan-usenet at bytereef.org
Mon Nov 9 13:21:59 CET 2009

Antoine Pitrou <solipsis at pitrou.net> wrote:

Stefan Krah <stefan-usenet bytereef.org> writes: > > >>> d = {0:Decimal("NaN")} > >>> Decimal("NaN") in d.values() > False > > So, since non-decimal use cases are limited at best, the equality/inequality > operators might as well have the behavior of the other comparison operators, > which is safer for the user.

The problem is when searching for /another/ object which hashes the same as Decimal("NaN"). Here is a made-up situation to show you the problem: >>> class H(object): ... def hash(self): return hash(1) ... def eq(self, other): raise ValueError ... >>> h = H() >>> d = {h: ""} >>> d[1] Traceback (most recent call last): File "", line 1, in File "", line 3, in eq ValueError >>> d[1] = 2 Traceback (most recent call last): File "", line 1, in File "", line 3, in eq ValueError

I see the point, but Decimal("NaN") does not hash:

hash(Decimal("NaN")) Traceback (most recent call last): File "", line 1, in File "/usr/lib/python2.7/decimal.py", line 937, in hash raise TypeError('Cannot hash a NaN value.') TypeError: Cannot hash a NaN value.

Also, NaNs cause problems in non-decimal comparisons virtually everywhere:

L = [1, 2, Decimal("NaN")] L.sort() Traceback (most recent call last): File "", line 1, in File "/usr/lib/python2.7/decimal.py", line 877, in lt ans = self._compare_check_nans(other, context) File "/usr/lib/python2.7/decimal.py", line 782, in _compare_check_nans self) File "/usr/lib/python2.7/decimal.py", line 3755, in _raise_error raise error(explanation) decimal.InvalidOperation: comparison involving NaN

I think problems like these would best be avoided by having a separate totalorder or lexorder method instead of using eq, lt, etc., but this is of course outside the scope of this discussion (and I have no idea how difficult it would be to implement that).

Stefan Krah

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