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Diffstat (limited to 'Doc/library/fractions.rst')
| -rw-r--r-- | Doc/library/fractions.rst | 52 |
1 files changed, 41 insertions, 11 deletions
diff --git a/Doc/library/fractions.rst b/Doc/library/fractions.rst index 7960026..a3ad44a 100644 --- a/Doc/library/fractions.rst +++ b/Doc/library/fractions.rst @@ -6,6 +6,9 @@ .. moduleauthor:: Jeffrey Yasskin <jyasskin at gmail.com> .. sectionauthor:: Jeffrey Yasskin <jyasskin at gmail.com> +**Source code:** :source:`Lib/fractions.py` + +-------------- The :mod:`fractions` module provides support for rational number arithmetic. @@ -15,17 +18,24 @@ another rational number, or from a string. .. class:: Fraction(numerator=0, denominator=1) Fraction(other_fraction) + Fraction(float) + Fraction(decimal) Fraction(string) - The first version requires that *numerator* and *denominator* are - instances of :class:`numbers.Rational` and returns a new - :class:`Fraction` instance with value ``numerator/denominator``. If - *denominator* is :const:`0`, it raises a - :exc:`ZeroDivisionError`. The second version requires that - *other_fraction* is an instance of :class:`numbers.Rational` and - returns an :class:`Fraction` instance with the same value. The - last version of the constructor expects a string instance. The - usual form for this string is:: + The first version requires that *numerator* and *denominator* are instances + of :class:`numbers.Rational` and returns a new :class:`Fraction` instance + with value ``numerator/denominator``. If *denominator* is :const:`0`, it + raises a :exc:`ZeroDivisionError`. The second version requires that + *other_fraction* is an instance of :class:`numbers.Rational` and returns a + :class:`Fraction` instance with the same value. The next two versions accept + either a :class:`float` or a :class:`decimal.Decimal` instance, and return a + :class:`Fraction` instance with exactly the same value. Note that due to the + usual issues with binary floating-point (see :ref:`tut-fp-issues`), the + argument to ``Fraction(1.1)`` is not exactly equal to 11/10, and so + ``Fraction(1.1)`` does *not* return ``Fraction(11, 10)`` as one might expect. + (But see the documentation for the :meth:`limit_denominator` method below.) + The last version of the constructor expects a string or unicode instance. + The usual form for this instance is:: [sign] numerator ['/' denominator] @@ -55,6 +65,13 @@ another rational number, or from a string. Fraction(-1, 8) >>> Fraction('7e-6') Fraction(7, 1000000) + >>> Fraction(2.25) + Fraction(9, 4) + >>> Fraction(1.1) + Fraction(2476979795053773, 2251799813685248) + >>> from decimal import Decimal + >>> Fraction(Decimal('1.1')) + Fraction(11, 10) The :class:`Fraction` class inherits from the abstract base class @@ -63,6 +80,10 @@ another rational number, or from a string. and should be treated as immutable. In addition, :class:`Fraction` has the following methods: + .. versionchanged:: 3.2 + The :class:`Fraction` constructor now accepts :class:`float` and + :class:`decimal.Decimal` instances. + .. method:: from_float(flt) @@ -70,12 +91,19 @@ another rational number, or from a string. value of *flt*, which must be a :class:`float`. Beware that ``Fraction.from_float(0.3)`` is not the same value as ``Fraction(3, 10)`` + .. note:: From Python 3.2 onwards, you can also construct a + :class:`Fraction` instance directly from a :class:`float`. + .. method:: from_decimal(dec) This class method constructs a :class:`Fraction` representing the exact value of *dec*, which must be a :class:`decimal.Decimal` instance. + .. note:: From Python 3.2 onwards, you can also construct a + :class:`Fraction` instance directly from a :class:`decimal.Decimal` + instance. + .. method:: limit_denominator(max_denominator=1000000) @@ -90,10 +118,12 @@ another rational number, or from a string. or for recovering a rational number that's represented as a float: >>> from math import pi, cos - >>> Fraction.from_float(cos(pi/3)) + >>> Fraction(cos(pi/3)) Fraction(4503599627370497, 9007199254740992) - >>> Fraction.from_float(cos(pi/3)).limit_denominator() + >>> Fraction(cos(pi/3)).limit_denominator() Fraction(1, 2) + >>> Fraction(1.1).limit_denominator() + Fraction(11, 10) .. method:: __floor__() |