diff options
Diffstat (limited to 'Lib/fractions.py')
| -rw-r--r-- | Lib/fractions.py | 31 |
1 files changed, 22 insertions, 9 deletions
diff --git a/Lib/fractions.py b/Lib/fractions.py index fc8a12c..51e67e2 100644 --- a/Lib/fractions.py +++ b/Lib/fractions.py @@ -8,6 +8,7 @@ import math import numbers import operator import re +import sys __all__ = ['Fraction', 'gcd'] @@ -23,6 +24,12 @@ def gcd(a, b): a, b = b, a%b return a +# Constants related to the hash implementation; hash(x) is based +# on the reduction of x modulo the prime _PyHASH_MODULUS. +_PyHASH_MODULUS = sys.hash_info.modulus +# Value to be used for rationals that reduce to infinity modulo +# _PyHASH_MODULUS. +_PyHASH_INF = sys.hash_info.inf _RATIONAL_FORMAT = re.compile(r""" \A\s* # optional whitespace at the start, then @@ -528,16 +535,22 @@ class Fraction(numbers.Rational): """ # 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)) + + # In order to make sure that the hash of a Fraction agrees + # with the hash of a numerically equal integer, float or + # Decimal instance, we follow the rules for numeric hashes + # outlined in the documentation. (See library docs, 'Built-in + # Types'). + + # dinv is the inverse of self._denominator modulo the prime + # _PyHASH_MODULUS, or 0 if self._denominator is divisible by + # _PyHASH_MODULUS. + dinv = pow(self._denominator, _PyHASH_MODULUS - 2, _PyHASH_MODULUS) + if not dinv: + hash_ = _PyHASH_INF else: - # Use tuple's hash to avoid a high collision rate on - # simple fractions. - return hash((self._numerator, self._denominator)) + hash_ = abs(self._numerator) * dinv % _PyHASH_MODULUS + return hash_ if self >= 0 else -hash_ def __eq__(a, b): """a == b""" |