Issue #8294: Allow float and Decimal arguments in Fraction constructor.
diff --git a/Lib/fractions.py b/Lib/fractions.py
index 7a3e555..a0d86a4 100644
--- a/Lib/fractions.py
+++ b/Lib/fractions.py
@@ -4,6 +4,7 @@
"""Rational, infinite-precision, real numbers."""
from __future__ import division
+from decimal import Decimal
import math
import numbers
import operator
@@ -43,13 +44,21 @@
class Fraction(Rational):
"""This class implements rational numbers.
- Fraction(8, 6) will produce a rational number equivalent to
- 4/3. Both arguments must be Integral. The numerator defaults to 0
- and the denominator defaults to 1 so that Fraction(3) == 3 and
- Fraction() == 0.
+ In the two-argument form of the constructor, Fraction(8, 6) will
+ produce a rational number equivalent to 4/3. Both arguments must
+ be Rational. The numerator defaults to 0 and the denominator
+ defaults to 1 so that Fraction(3) == 3 and Fraction() == 0.
- Fractions can also be constructed from strings of the form
- '[-+]?[0-9]+((/|.)[0-9]+)?', optionally surrounded by spaces.
+ Fractions can also be constructed from:
+
+ - numeric strings similar to those accepted by the
+ float constructor (for example, '-2.3' or '1e10')
+
+ - strings of the form '123/456'
+
+ - float and Decimal instances
+
+ - other Rational instances (including integers)
"""
@@ -59,8 +68,32 @@
def __new__(cls, numerator=0, denominator=None):
"""Constructs a Fraction.
- Takes a string like '3/2' or '1.5', another Fraction, or a
- numerator/denominator pair.
+ Takes a string like '3/2' or '1.5', another Rational instance, a
+ numerator/denominator pair, or a float.
+
+ Examples
+ --------
+
+ >>> Fraction(10, -8)
+ Fraction(-5, 4)
+ >>> Fraction(Fraction(1, 7), 5)
+ Fraction(1, 35)
+ >>> Fraction(Fraction(1, 7), Fraction(2, 3))
+ Fraction(3, 14)
+ >>> Fraction('314')
+ Fraction(314, 1)
+ >>> Fraction('-35/4')
+ Fraction(-35, 4)
+ >>> Fraction('3.1415') # conversion from numeric string
+ Fraction(6283, 2000)
+ >>> Fraction('-47e-2') # string may include a decimal exponent
+ Fraction(-47, 100)
+ >>> Fraction(1.47) # direct construction from float (exact conversion)
+ Fraction(6620291452234629, 4503599627370496)
+ >>> Fraction(2.25)
+ Fraction(9, 4)
+ >>> Fraction(Decimal('1.47'))
+ Fraction(147, 100)
"""
self = super(Fraction, cls).__new__(cls)
@@ -71,6 +104,19 @@
self._denominator = numerator.denominator
return self
+ elif isinstance(numerator, float):
+ # Exact conversion from float
+ value = Fraction.from_float(numerator)
+ self._numerator = value._numerator
+ self._denominator = value._denominator
+ return self
+
+ elif isinstance(numerator, Decimal):
+ value = Fraction.from_decimal(numerator)
+ self._numerator = value._numerator
+ self._denominator = value._denominator
+ return self
+
elif isinstance(numerator, basestring):
# Handle construction from strings.
m = _RATIONAL_FORMAT.match(numerator)
diff --git a/Lib/test/test_fractions.py b/Lib/test/test_fractions.py
index a24fcd3..7f0c428 100644
--- a/Lib/test/test_fractions.py
+++ b/Lib/test/test_fractions.py
@@ -12,6 +12,11 @@
F = fractions.Fraction
gcd = fractions.gcd
+# decorator for skipping tests on non-IEEE 754 platforms
+requires_IEEE_754 = unittest.skipUnless(
+ float.__getformat__("double").startswith("IEEE"),
+ "test requires IEEE 754 doubles")
+
class DummyFloat(object):
"""Dummy float class for testing comparisons with Fractions"""
@@ -137,13 +142,33 @@
self.assertRaisesMessage(ZeroDivisionError, "Fraction(12, 0)",
F, 12, 0)
- self.assertRaises(TypeError, F, 1.5)
self.assertRaises(TypeError, F, 1.5 + 3j)
self.assertRaises(TypeError, F, "3/2", 3)
self.assertRaises(TypeError, F, 3, 0j)
self.assertRaises(TypeError, F, 3, 1j)
+ @requires_IEEE_754
+ def testInitFromFloat(self):
+ self.assertEquals((5, 2), _components(F(2.5)))
+ self.assertEquals((0, 1), _components(F(-0.0)))
+ self.assertEquals((3602879701896397, 36028797018963968),
+ _components(F(0.1)))
+ self.assertRaises(TypeError, F, float('nan'))
+ self.assertRaises(TypeError, F, float('inf'))
+ self.assertRaises(TypeError, F, float('-inf'))
+
+ def testInitFromDecimal(self):
+ self.assertEquals((11, 10),
+ _components(F(Decimal('1.1'))))
+ self.assertEquals((7, 200),
+ _components(F(Decimal('3.5e-2'))))
+ self.assertEquals((0, 1),
+ _components(F(Decimal('.000e20'))))
+ self.assertRaises(TypeError, F, Decimal('nan'))
+ self.assertRaises(TypeError, F, Decimal('snan'))
+ self.assertRaises(TypeError, F, Decimal('inf'))
+ self.assertRaises(TypeError, F, Decimal('-inf'))
def testFromString(self):
self.assertEquals((5, 1), _components(F("5")))