#19304: Fix hash function of rationals
-------------------------------------+-------------------------------------
Reporter: vdelecroix | Owner:
Type: enhancement | Status: needs_info
Priority: critical | Milestone: sage-6.9
Component: misc | Resolution:
Keywords: | Merged in:
Authors: Jeroen Demeyer | Reviewers: Vincent Delecroix
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/jdemeyer/fix_hash_function_of_rationals|
c3d64c2bde1bf8a5c64c9d433e1d94004b8250ef
Dependencies: | Stopgaps:
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Changes (by vdelecroix):
* status: needs_review => needs_info
* reviewer: => Vincent Delecroix
Comment:
There are fractions in Python3, shouldn't we try to be complient with it?
{{{
>>> import fractions
>>> fractions.Fraction(2,3)
Fraction(2, 3)
>>> hash(fractions.Fraction(2,3))
768614336404564651
}}}
The code is actually pure python and takes care of exactly representable
float... and Python seems to have a different politics than Sage to that
respect.
{{{
def __hash__(self):
"""hash(self)
Tricky because values that are exactly representable as a
float must have the same hash as that float.
"""
# XXX since this method is expensive, consider caching the result
if self._denominator == 1:
# Get integers right.
return hash(self._numerator)
# Expensive check, but definitely correct.
if self == float(self):
return hash(float(self))
else:
# Use tuple's hash to avoid a high collision rate on
# simple fractions.
return hash((self._numerator, self._denominator))
}}}
--
Ticket URL: <http://trac.sagemath.org/ticket/19304#comment:9>
Sage <http://www.sagemath.org>
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