Issue #25928: Add Decimal.as_integer_ratio(). Python parts and docs by
Mark Dickinson.
diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py
index 05ba4ee..eb7bba8 100644
--- a/Lib/_pydecimal.py
+++ b/Lib/_pydecimal.py
@@ -1010,6 +1010,58 @@
"""
return DecimalTuple(self._sign, tuple(map(int, self._int)), self._exp)
+ def as_integer_ratio(self):
+ """Express a finite Decimal instance in the form n / d.
+
+ Returns a pair (n, d) of integers. When called on an infinity
+ or NaN, raises OverflowError or ValueError respectively.
+
+ >>> Decimal('3.14').as_integer_ratio()
+ (157, 50)
+ >>> Decimal('-123e5').as_integer_ratio()
+ (-12300000, 1)
+ >>> Decimal('0.00').as_integer_ratio()
+ (0, 1)
+
+ """
+ if self._is_special:
+ if self.is_nan():
+ raise ValueError("Cannot pass NaN "
+ "to decimal.as_integer_ratio.")
+ else:
+ raise OverflowError("Cannot pass infinity "
+ "to decimal.as_integer_ratio.")
+
+ if not self:
+ return 0, 1
+
+ # Find n, d in lowest terms such that abs(self) == n / d;
+ # we'll deal with the sign later.
+ n = int(self._int)
+ if self._exp >= 0:
+ # self is an integer.
+ n, d = n * 10**self._exp, 1
+ else:
+ # Find d2, d5 such that abs(self) = n / (2**d2 * 5**d5).
+ d5 = -self._exp
+ while d5 > 0 and n % 5 == 0:
+ n //= 5
+ d5 -= 1
+
+ # (n & -n).bit_length() - 1 counts trailing zeros in binary
+ # representation of n (provided n is nonzero).
+ d2 = -self._exp
+ shift2 = min((n & -n).bit_length() - 1, d2)
+ if shift2:
+ n >>= shift2
+ d2 -= shift2
+
+ d = 5**d5 << d2
+
+ if self._sign:
+ n = -n
+ return n, d
+
def __repr__(self):
"""Represents the number as an instance of Decimal."""
# Invariant: eval(repr(d)) == d
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 137aaa5..f6d58ff 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -2047,6 +2047,39 @@
d = Decimal( (1, (0, 2, 7, 1), 'F') )
self.assertEqual(d.as_tuple(), (1, (0,), 'F'))
+ def test_as_integer_ratio(self):
+ Decimal = self.decimal.Decimal
+
+ # exceptional cases
+ self.assertRaises(OverflowError,
+ Decimal.as_integer_ratio, Decimal('inf'))
+ self.assertRaises(OverflowError,
+ Decimal.as_integer_ratio, Decimal('-inf'))
+ self.assertRaises(ValueError,
+ Decimal.as_integer_ratio, Decimal('-nan'))
+ self.assertRaises(ValueError,
+ Decimal.as_integer_ratio, Decimal('snan123'))
+
+ for exp in range(-4, 2):
+ for coeff in range(1000):
+ for sign in '+', '-':
+ d = Decimal('%s%dE%d' % (sign, coeff, exp))
+ pq = d.as_integer_ratio()
+ p, q = pq
+
+ # check return type
+ self.assertIsInstance(pq, tuple)
+ self.assertIsInstance(p, int)
+ self.assertIsInstance(q, int)
+
+ # check normalization: q should be positive;
+ # p should be relatively prime to q.
+ self.assertGreater(q, 0)
+ self.assertEqual(math.gcd(p, q), 1)
+
+ # check that p/q actually gives the correct value
+ self.assertEqual(Decimal(p) / Decimal(q), d)
+
def test_subclassing(self):
# Different behaviours when subclassing Decimal
Decimal = self.decimal.Decimal