:mod:`fractions` --- Rational numbers

.. module:: fractions
   :synopsis: Rational numbers.

.. moduleauthor:: Jeffrey Yasskin <jyasskin at gmail.com>

.. sectionauthor:: Jeffrey Yasskin <jyasskin at gmail.com>

.. versionadded:: 2.6

Source code: :source:`Lib/fractions.py`

The :mod:`fractions` module provides support for rational number arithmetic.

A Fraction instance can be constructed from a pair of integers, from another rational number, or from a 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 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]

where the optional sign may be either '+' or '-' and numerator and denominator (if present) are strings of decimal digits. In addition, any string that represents a finite value and is accepted by the :class:`float` constructor is also accepted by the :class:`Fraction` constructor. In either form the input string may also have leading and/or trailing whitespace. Here are some examples:

>>> from fractions import Fraction
>>> Fraction(16, -10)
Fraction(-8, 5)
>>> Fraction(123)
Fraction(123, 1)
>>> Fraction()
Fraction(0, 1)
>>> Fraction('3/7')
Fraction(3, 7)
>>> Fraction(' -3/7 ')
Fraction(-3, 7)
>>> Fraction('1.414213 \t\n')
Fraction(1414213, 1000000)
>>> Fraction('-.125')
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 :class:`numbers.Rational`, and implements all of the methods and operations from that class. :class:`Fraction` instances are hashable, and should be treated as immutable. In addition, :class:`Fraction` has the following methods:

.. versionchanged:: 2.7
   The :class:`Fraction` constructor now accepts :class:`float` and
   :class:`decimal.Decimal` instances.

.. method:: from_float(flt)

   This class method constructs a :class:`Fraction` representing the exact
   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 2.7 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`.

   .. note:: From Python 2.7 onwards, you can also construct a
      :class:`Fraction` instance directly from a :class:`decimal.Decimal`
      instance.

.. method:: limit_denominator(max_denominator=1000000)

   Finds and returns the closest :class:`Fraction` to ``self`` that has
   denominator at most max_denominator.  This method is useful for finding
   rational approximations to a given floating-point number:

      >>> from fractions import Fraction
      >>> Fraction('3.1415926535897932').limit_denominator(1000)
      Fraction(355, 113)

   or for recovering a rational number that's represented as a float:

      >>> from math import pi, cos
      >>> Fraction(cos(pi/3))
      Fraction(4503599627370497, 9007199254740992)
      >>> Fraction(cos(pi/3)).limit_denominator()
      Fraction(1, 2)
      >>> Fraction(1.1).limit_denominator()
      Fraction(11, 10)

.. function:: gcd(a, b)

   Return the greatest common divisor of the integers *a* and *b*.  If either
   *a* or *b* is nonzero, then the absolute value of ``gcd(a, b)`` is the
   largest integer that divides both *a* and *b*.  ``gcd(a,b)`` has the same
   sign as *b* if *b* is nonzero; otherwise it takes the sign of *a*.  ``gcd(0,
   0)`` returns ``0``.

.. seealso::

   Module :mod:`numbers`
      The abstract base classes making up the numeric tower.