Source code: Lib/fractions.py
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(float)
class fractions.Fraction(decimal)
class fractions.Fraction(string)
The first version requires that numerator and denominator are instances of numbers.Rational
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 a Fraction
instance with the same value. The next two versions accept either a float
or a decimal.Decimal
instance, and return a Fraction
instance with exactly the same value. Note that due to the usual issues with binary floating-point (see Floating Point Arithmetic: Issues and Limitations), 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 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 float
constructor is also accepted by the 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 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 properties and methods:
Changed in version 3.2: The Fraction
constructor now accepts float
and decimal.Decimal
instances.
Changed in version 3.9: The math.gcd()
function is now used to normalize the numerator and denominator. math.gcd()
always return a int
type. Previously, the GCD type depended on numerator and denominator.
numerator
Numerator of the Fraction in lowest term.
denominator
Denominator of the Fraction in lowest term.
as_integer_ratio()
Return a tuple of two integers, whose ratio is equal to the Fraction and with a positive denominator.
New in version 3.8.
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
instance.
Note
From Python 3.2 onwards, you can also construct a Fraction
instance directly from a decimal.Decimal
instance.
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(cos(pi/3)) Fraction(4503599627370497, 9007199254740992) >>> Fraction(cos(pi/3)).limit_denominator() Fraction(1, 2) >>> Fraction(1.1).limit_denominator() Fraction(11, 10)
__floor__()
Returns the greatest int
<= self
. This method can also be accessed through the math.floor()
function:
>>> from math import floor >>> floor(Fraction(355, 113)) 3
__ceil__()
Returns the least int
>= self
. This method can also be accessed through the math.ceil()
function.
__round__()
__round__(ndigits)
The first version returns the nearest int
to self
, rounding half to even. The second version rounds self
to the nearest multiple of Fraction(1, 10**ndigits)
(logically, if ndigits
is negative), again rounding half toward even. This method can also be accessed through the round()
function.
See also
Module
numbers
The abstract base classes making up the numeric tower.
© 2001–2020 Python Software Foundation
Licensed under the PSF License.
https://docs.python.org/3.9/library/fractions.html