### Navigation

- index
- modules |
- next |
- previous |
- Python »
- Python v2.6.9 documentation »
- The Python Standard Library »
- 9. Numeric and Mathematical Modules »

# 9.5. `fractions` — Rational numbers

New in version 2.6.

The `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.

class `fractions.``Fraction`(numerator=0, denominator=1)
class `fractions.``Fraction`(other_fraction)
class `fractions.``Fraction`(string)

  The first version requires that numerator and denominator are instances of `numbers.Integral` and returns a new `Fraction` instance with value `numerator/denominator`. If denominator is `0`, it raises a `ZeroDivisionError`. The second version requires that other_fraction is an instance of `numbers.Rational` and returns an `Fraction` instance with the same value. The last version of the constructor expects a string or unicode instance in one of two possible forms. The first form is:

      [sign] numerator ['/' denominator]

  where the optional `sign` may be either ‘+’ or ‘-‘ and `numerator` and `denominator` (if present) are strings of decimal digits. The second permitted form is that of a number containing a decimal point:

      [sign] integer '.' [fraction] | [sign] '.' fraction

  where `integer` and `fraction` are strings of digits. 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)
      [40794 refs]
      >>> Fraction(' -3/7 ')
      Fraction(-3, 7)
      >>> Fraction('1.414213 \t\n')
      Fraction(1414213, 1000000)
      >>> Fraction('-.125')
      Fraction(-1, 8)

  The `Fraction` class inherits from the abstract base class `numbers.Rational`, and implements all of the methods and operations from that class. `Fraction` instances are hashable, and should be treated as immutable. In addition, `Fraction` has the following methods:

  `from_float`(flt)
    This class method constructs a `Fraction` representing the exact value of flt, which must be a `float`. Beware that `Fraction.from_float(0.3)` is not the same value as `Fraction(3, 10)`
  `from_decimal`(dec)
    This class method constructs a `Fraction` representing the exact value of dec, which must be a `decimal.Decimal`.
  `limit_denominator`(max_denominator=1000000)

    Finds and returns the closest `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.from_float(cos(pi/3))
        Fraction(4503599627370497, 9007199254740992)
        >>> Fraction.from_float(cos(pi/3)).limit_denominator()
        Fraction(1, 2)

`fractions.``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`.

See also

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

### Navigation

- index
- modules |
- next |
- previous |
- Python »
- Python v2.6.9 documentation »
- The Python Standard Library »
- 9. Numeric and Mathematical Modules »