fractions — Rational numbers

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(number)
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 number is an instance of numbers.Rational or has the as_integer_ratio() method (this includes float and decimal.Decimal). It returns a Fraction instance with exactly the same value. Assumed, that the as_integer_ratio() method returns a pair of coprime integers and last one is positive. Note that due to the usual issues with binary 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. 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 (underscores may be used to delimit digits as with integral literals in code). 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 returns an int type. Previously, the GCD type depended on numerator and denominator.

Changed in version 3.11: Underscores are now permitted when creating a Fraction instance from a string, following PEP 515 rules.

Changed in version 3.11: Fraction implements __int__ now to satisfy typing.SupportsInt instance checks.

Changed in version 3.12: Space is allowed around the slash for string inputs: Fraction('2 / 3').

Changed in version 3.12: Fraction instances now support float-style formatting, with presentation types "e", "E", "f", "F", "g", "G" and "%"".

Changed in version 3.13: Formatting of Fraction instances without a presentation type now supports fill, alignment, sign handling, minimum width and grouping.

Changed in version 3.14: The Fraction constructor now accepts any objects with the as_integer_ratio() method.

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 original Fraction. The ratio is in lowest terms and has a positive denominator.

Added in version 3.8.

is_integer()

Return True if the Fraction is an integer.

Added in version 3.12.

classmethod from_float(flt)

Alternative constructor which only accepts instances of float or numbers.Integral. 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 Fraction instance directly from a float.

classmethod from_decimal(dec)

Alternative constructor which only accepts instances of decimal.Decimal or numbers.Integral.

Note

From Python 3.2 onwards, you can also construct a Fraction instance directly from a decimal.Decimal instance.

classmethod from_number(number)

Alternative constructor which only accepts instances of numbers.Integral, numbers.Rational, float or decimal.Decimal, and objects with the as_integer_ratio() method, but not strings.

Added in version 3.14.

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.

__format__(format_spec, /)

Provides support for formatting of Fraction instances via the str.format() method, the format() built-in function, or Formatted string literals.

If the format_spec format specification string does not end with one of the presentation types 'e', 'E', 'f', 'F', 'g', 'G' or '%' then formatting follows the general rules for fill, alignment, sign handling, minimum width, and grouping as described in the format specification mini-language. The “alternate form” flag '#' is supported: if present, it forces the output string to always include an explicit denominator, even when the value being formatted is an exact integer. The zero-fill flag '0' is not supported.

If the format_spec format specification string ends with one of the presentation types 'e', 'E', 'f', 'F', 'g', 'G' or '%' then formatting follows the rules outlined for the float type in the Format Specification Mini-Language section.

Here are some examples:

>>> from fractions import Fraction
>>> format(Fraction(103993, 33102), '_')
'103_993/33_102'
>>> format(Fraction(1, 7), '.^+10')
'...+1/7...'
>>> format(Fraction(3, 1), '')
'3'
>>> format(Fraction(3, 1), '#')
'3/1'
>>> format(Fraction(1, 7), '.40g')
'0.1428571428571428571428571428571428571429'
>>> format(Fraction('1234567.855'), '_.2f')
'1_234_567.86'
>>> f"{Fraction(355, 113):*>20.6e}"
'********3.141593e+00'
>>> old_price, new_price = 499, 672
>>> "{:.2%} price increase".format(Fraction(new_price, old_price) - 1)
'34.67% price increase'

See also

Module numbers

The abstract base classes making up the numeric tower.