diff --git a/Doc/library/fractions.rst b/Doc/library/fractions.rst index 2ee15495254..fc7f9a6301a 100644 --- a/Doc/library/fractions.rst +++ b/Doc/library/fractions.rst @@ -166,6 +166,16 @@ another rational number, or from a string. instance. + .. classmethod:: from_number(number) + + Alternative constructor which only accepts instances of + :class:`numbers.Integral`, :class:`numbers.Rational`, + :class:`float` or :class:`decimal.Decimal`, and objects with + the :meth:`!as_integer_ratio` method, but not strings. + + .. versionadded:: 3.14 + + .. method:: limit_denominator(max_denominator=1000000) Finds and returns the closest :class:`Fraction` to ``self`` that has diff --git a/Doc/whatsnew/3.14.rst b/Doc/whatsnew/3.14.rst index c62a3ca5872..b22d1bd1e99 100644 --- a/Doc/whatsnew/3.14.rst +++ b/Doc/whatsnew/3.14.rst @@ -263,6 +263,10 @@ fractions :meth:`!as_integer_ratio` method to a :class:`~fractions.Fraction`. (Contributed by Serhiy Storchaka in :gh:`82017`.) +* Add alternative :class:`~fractions.Fraction` constructor + :meth:`Fraction.from_number() `. + (Contributed by Serhiy Storchaka in :gh:`121797`.) + functools --------- diff --git a/Lib/fractions.py b/Lib/fractions.py index 34fd0803d1b..f0cbc8c2e6c 100644 --- a/Lib/fractions.py +++ b/Lib/fractions.py @@ -279,7 +279,8 @@ class Fraction(numbers.Rational): numerator = -numerator else: - raise TypeError("argument should be a string or a number") + raise TypeError("argument should be a string or a Rational " + "instance or have the as_integer_ratio() method") elif type(numerator) is int is type(denominator): pass # *very* normal case @@ -305,6 +306,28 @@ class Fraction(numbers.Rational): self._denominator = denominator return self + @classmethod + def from_number(cls, number): + """Converts a finite real number to a rational number, exactly. + + Beware that Fraction.from_number(0.3) != Fraction(3, 10). + + """ + if type(number) is int: + return cls._from_coprime_ints(number, 1) + + elif isinstance(number, numbers.Rational): + return cls._from_coprime_ints(number.numerator, number.denominator) + + elif (isinstance(number, float) or + (not isinstance(number, type) and + hasattr(number, 'as_integer_ratio'))): + return cls._from_coprime_ints(*number.as_integer_ratio()) + + else: + raise TypeError("argument should be a Rational instance or " + "have the as_integer_ratio() method") + @classmethod def from_float(cls, f): """Converts a finite float to a rational number, exactly. diff --git a/Lib/test/test_fractions.py b/Lib/test/test_fractions.py index 4907f4093f5..98dccbec956 100644 --- a/Lib/test/test_fractions.py +++ b/Lib/test/test_fractions.py @@ -283,6 +283,13 @@ numbers.Complex.register(Rect) class RectComplex(Rect, complex): pass +class Ratio: + def __init__(self, ratio): + self._ratio = ratio + def as_integer_ratio(self): + return self._ratio + + class FractionTest(unittest.TestCase): def assertTypedEquals(self, expected, actual): @@ -355,14 +362,9 @@ class FractionTest(unittest.TestCase): self.assertRaises(OverflowError, F, Decimal('-inf')) def testInitFromIntegerRatio(self): - class Ratio: - def __init__(self, ratio): - self._ratio = ratio - def as_integer_ratio(self): - return self._ratio - self.assertEqual((7, 3), _components(F(Ratio((7, 3))))) - errmsg = "argument should be a string or a number" + errmsg = (r"argument should be a string or a Rational instance or " + r"have the as_integer_ratio\(\) method") # the type also has an "as_integer_ratio" attribute. self.assertRaisesRegex(TypeError, errmsg, F, Ratio) # bad ratio @@ -388,6 +390,8 @@ class FractionTest(unittest.TestCase): pass self.assertRaisesRegex(TypeError, errmsg, F, B) self.assertRaisesRegex(TypeError, errmsg, F, B()) + self.assertRaises(TypeError, F.from_number, B) + self.assertRaises(TypeError, F.from_number, B()) def testFromString(self): self.assertEqual((5, 1), _components(F("5"))) @@ -594,6 +598,37 @@ class FractionTest(unittest.TestCase): ValueError, "cannot convert NaN to integer ratio", F.from_decimal, Decimal("snan")) + def testFromNumber(self, cls=F): + def check(arg, numerator, denominator): + f = cls.from_number(arg) + self.assertIs(type(f), cls) + self.assertEqual(f.numerator, numerator) + self.assertEqual(f.denominator, denominator) + + check(10, 10, 1) + check(2.5, 5, 2) + check(Decimal('2.5'), 5, 2) + check(F(22, 7), 22, 7) + check(DummyFraction(22, 7), 22, 7) + check(Rat(22, 7), 22, 7) + check(Ratio((22, 7)), 22, 7) + self.assertRaises(TypeError, cls.from_number, 3+4j) + self.assertRaises(TypeError, cls.from_number, '5/2') + self.assertRaises(TypeError, cls.from_number, []) + self.assertRaises(OverflowError, cls.from_number, float('inf')) + self.assertRaises(OverflowError, cls.from_number, Decimal('inf')) + + # as_integer_ratio not defined in a class + class A: + pass + a = A() + a.as_integer_ratio = lambda: (9, 5) + check(a, 9, 5) + + def testFromNumber_subclass(self): + self.testFromNumber(DummyFraction) + + def test_is_integer(self): self.assertTrue(F(1, 1).is_integer()) self.assertTrue(F(-1, 1).is_integer()) diff --git a/Misc/NEWS.d/next/Library/2024-07-15-19-34-56.gh-issue-121797.qDqj59.rst b/Misc/NEWS.d/next/Library/2024-07-15-19-34-56.gh-issue-121797.qDqj59.rst new file mode 100644 index 00000000000..9525379587f --- /dev/null +++ b/Misc/NEWS.d/next/Library/2024-07-15-19-34-56.gh-issue-121797.qDqj59.rst @@ -0,0 +1,2 @@ +Add alternative :class:`~fractions.Fraction` constructor +:meth:`Fraction.from_number() `.