9.5. 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(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.Rationaland returns a newFractioninstance with valuenumerator/denominator. If denominator is0, it raises aZeroDivisionError. The second version requires that other_fraction is an instance ofnumbers.Rationaland returns aFractioninstance with the same value. The next two versions accept either afloator adecimal.Decimalinstance, and return aFractioninstance 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 toFraction(1.1)is not exactly equal to 11/10, and soFraction(1.1)does not returnFraction(11, 10)as one might expect. (But see the documentation for thelimit_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
signmay be either ‘+’ or ‘-’ andnumeratoranddenominator(if present) are strings of decimal digits. In addition, any string that represents a finite value and is accepted by thefloatconstructor is also accepted by theFractionconstructor. 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
Fractionclass inherits from the abstract base classnumbers.Rational, and implements all of the methods and operations from that class.Fractioninstances are hashable, and should be treated as immutable. In addition,Fractionhas the following properties and methods:Changed in version 3.2: The
Fractionconstructor now acceptsfloatanddecimal.Decimalinstances.-
numerator¶ Numerator of the Fraction in lowest term.
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denominator¶ Denominator of the Fraction in lowest term.
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from_float(flt)¶ This class method constructs a
Fractionrepresenting the exact value of flt, which must be afloat. Beware thatFraction.from_float(0.3)is not the same value asFraction(3, 10).
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from_decimal(dec)¶ This class method constructs a
Fractionrepresenting the exact value of dec, which must be adecimal.Decimalinstance.Note
From Python 3.2 onwards, you can also construct a
Fractioninstance directly from adecimal.Decimalinstance.
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limit_denominator(max_denominator=1000000)¶ Finds and returns the closest
Fractiontoselfthat 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)
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__floor__()¶ Returns the greatest
int<= self. This method can also be accessed through themath.floor()function:>>> from math import floor >>> floor(Fraction(355, 113)) 3
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__ceil__()¶ Returns the least
int>= self. This method can also be accessed through themath.ceil()function.
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__round__()¶ -
__round__(ndigits) The first version returns the nearest
inttoself, rounding half to even. The second version roundsselfto the nearest multiple ofFraction(1, 10**ndigits)(logically, ifndigitsis negative), again rounding half toward even. This method can also be accessed through theround()function.
-
-
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)returns0.Deprecated since version 3.5: Use
math.gcd()instead.
See also
- Module
numbers The abstract base classes making up the numeric tower.
