Merged revisions 79629 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
........
r79629 | mark.dickinson | 2010-04-02 23:27:36 +0100 (Fri, 02 Apr 2010) | 2 lines
Issue #8294: Allow float and Decimal arguments in Fraction constructor.
........
diff --git a/Doc/library/fractions.rst b/Doc/library/fractions.rst
index 7960026..f08cfaf 100644
--- a/Doc/library/fractions.rst
+++ b/Doc/library/fractions.rst
@@ -15,17 +15,24 @@
.. 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 +62,13 @@
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 +77,10 @@
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 +88,19 @@
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 +115,12 @@
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__()