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gh-72902: speedup Fraction.from_decimal/float in typical cases#133251

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Merged
serhiy-storchaka merged 6 commits into from
May 25, 2026
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gh-72902: speedup Fraction.from_decimal/float in typical cases #133251

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caa608f
gh-72902: speedup Fraction.from_Decimal/float in typical cases
skirpichev May 1, 2025
1ea8f6a
+1
skirpichev May 1, 2025
23d0b52
Merge branch 'main' into opt-Fraction.from_Decimal/72902
skirpichev May 23, 2026
bf0842d
Apply suggestion from @eendebakpt
skirpichev May 24, 2026
ba889d8
+ cleanup news
skirpichev May 24, 2026
cbb2d3f
address review: rewrite without nested if
skirpichev May 24, 2026
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20 changes: 10 additions & 10 deletions Lib/fractions.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -339,24 +339,24 @@ def from_float(cls, f):
Beware that Fraction.from_float(0.3) != Fraction(3, 10).

"""
if isinstance(f, float):
return cls._from_coprime_ints(*f.as_integer_ratio())
Comment on lines +342 to +343

@scoderscoderJun 26, 2026

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For a subclass of float, f.as_integer_ratio() doesn't necessarily return a normalised integer pair. I think we should restrict this optimisation to exact floats.

Suggested change
ifisinstance(f, float):
returncls._from_coprime_ints(*f.as_integer_ratio())
iftype(f) isfloat:
returncls._from_coprime_ints(*f.as_integer_ratio())

The intention is to optimise the common case, and that is certainly (and by a margin) exact float, not arbitrary subclasses.
We then need to keep the fallback code that handles subclasses, as it was before.

@skirpichevskirpichevJun 27, 2026

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This affects only float subclasses. The float.as_integer_ration was already documented to return coprime ints.

if isinstance(f, numbers.Integral):
return cls(f)
elif not isinstance(f, float):
raise TypeError("%s.from_float() only takes floats, not %r (%s)" %
(cls.__name__, f, type(f).__name__))
return cls._from_coprime_ints(*f.as_integer_ratio())
raise TypeError("%s.from_float() only takes floats, not %r (%s)" %
(cls.__name__, f, type(f).__name__))

@classmethod
def from_decimal(cls, dec):
"""Converts a finite Decimal instance to a rational number, exactly."""
from decimal import Decimal
if isinstance(dec, Decimal):
return cls._from_coprime_ints(*dec.as_integer_ratio())
Comment on lines +353 to +354

@scoderscoderJun 26, 2026

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Suggested change
ifisinstance(dec, Decimal):
returncls._from_coprime_ints(*dec.as_integer_ratio())
iftype(dec) isDecimal:
returncls._from_coprime_ints(*dec.as_integer_ratio())

@skirpichevskirpichevJun 27, 2026

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Same here.

if isinstance(dec, numbers.Integral):
dec = Decimal(int(dec))
elif not isinstance(dec, Decimal):
raise TypeError(
"%s.from_decimal() only takes Decimals, not %r (%s)" %
(cls.__name__, dec, type(dec).__name__))
return cls._from_coprime_ints(*dec.as_integer_ratio())
dec = int(dec)
return cls._from_coprime_ints(*dec.as_integer_ratio())
raise TypeError("%s.from_decimal() only takes Decimals, not %r (%s)" %
(cls.__name__, dec, type(dec).__name__))

@classmethod
def _from_coprime_ints(cls, numerator, denominator, /):
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2025-05-01-16-44-16.gh-issue-72902.19qwJW.rst
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,3 @@
Optimize (~x1.4 speedup) :meth:`fractions.Fraction.from_decimal` and
:meth:`fractions.Fraction.from_float` for :class:`~decimal.Decimal` and
:class:`float` inputs, respectively. Patch by Sergey B Kirpichev.
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