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  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.
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Mark Dickinson 2010-04-03 11:18:52 +00:00
parent ac256ab284
commit 98127c3716
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49
Doc/library/fractions.rst
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@ -15,17 +15,24 @@ 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 +62,13 @@ 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 +77,10 @@ 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 +88,19 @@ 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 +115,12 @@ 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__()