Several tweaks: add construction from strings and .from_decimal(), change
__init__ to __new__ to enforce immutability, and remove "rational." from repr
and the parens from str.
diff --git a/Lib/rational.py b/Lib/rational.py
index d455dc6..19e7f14 100755
--- a/Lib/rational.py
+++ b/Lib/rational.py
@@ -7,6 +7,7 @@
import math
import numbers
import operator
+import re
__all__ = ["Rational"]
@@ -76,6 +77,10 @@
return (top, 2 ** -e)
+_RATIONAL_FORMAT = re.compile(
+ r'^\s*(?P<sign>[-+]?)(?P<num>\d+)(?:/(?P<denom>\d+))?\s*$')
+
+
class Rational(RationalAbc):
"""This class implements rational numbers.
@@ -84,18 +89,41 @@
and the denominator defaults to 1 so that Rational(3) == 3 and
Rational() == 0.
+ Rationals can also be constructed from strings of the form
+ '[-+]?[0-9]+(/[0-9]+)?', optionally surrounded by spaces.
+
"""
__slots__ = ('_numerator', '_denominator')
- def __init__(self, numerator=0, denominator=1):
- if (not isinstance(numerator, numbers.Integral) and
- isinstance(numerator, RationalAbc) and
- denominator == 1):
- # Handle copies from other rationals.
- other_rational = numerator
- numerator = other_rational.numerator
- denominator = other_rational.denominator
+ # We're immutable, so use __new__ not __init__
+ def __new__(cls, numerator=0, denominator=1):
+ """Constructs a Rational.
+
+ Takes a string, another Rational, or a numerator/denominator pair.
+
+ """
+ self = super(Rational, cls).__new__(cls)
+
+ if denominator == 1:
+ if isinstance(numerator, basestring):
+ # Handle construction from strings.
+ input = numerator
+ m = _RATIONAL_FORMAT.match(input)
+ if m is None:
+ raise ValueError('Invalid literal for Rational: ' + input)
+ numerator = int(m.group('num'))
+ # Default denominator to 1. That's the only optional group.
+ denominator = int(m.group('denom') or 1)
+ if m.group('sign') == '-':
+ numerator = -numerator
+
+ elif (not isinstance(numerator, numbers.Integral) and
+ isinstance(numerator, RationalAbc)):
+ # Handle copies from other rationals.
+ other_rational = numerator
+ numerator = other_rational.numerator
+ denominator = other_rational.denominator
if (not isinstance(numerator, numbers.Integral) or
not isinstance(denominator, numbers.Integral)):
@@ -108,10 +136,15 @@
g = _gcd(numerator, denominator)
self._numerator = int(numerator // g)
self._denominator = int(denominator // g)
+ return self
@classmethod
def from_float(cls, f):
- """Converts a float to a rational number, exactly."""
+ """Converts a finite float to a rational number, exactly.
+
+ Beware that Rational.from_float(0.3) != Rational(3, 10).
+
+ """
if not isinstance(f, float):
raise TypeError("%s.from_float() only takes floats, not %r (%s)" %
(cls.__name__, f, type(f).__name__))
@@ -119,6 +152,26 @@
raise TypeError("Cannot convert %r to %s." % (f, cls.__name__))
return cls(*_binary_float_to_ratio(f))
+ @classmethod
+ def from_decimal(cls, dec):
+ """Converts a finite Decimal instance to a rational number, exactly."""
+ from decimal import Decimal
+ if not isinstance(dec, Decimal):
+ raise TypeError(
+ "%s.from_decimal() only takes Decimals, not %r (%s)" %
+ (cls.__name__, dec, type(dec).__name__))
+ if not dec.is_finite():
+ # Catches infinities and nans.
+ raise TypeError("Cannot convert %s to %s." % (dec, cls.__name__))
+ sign, digits, exp = dec.as_tuple()
+ digits = int(''.join(map(str, digits)))
+ if sign:
+ digits = -digits
+ if exp >= 0:
+ return cls(digits * 10 ** exp)
+ else:
+ return cls(digits, 10 ** -exp)
+
@property
def numerator(a):
return a._numerator
@@ -129,15 +182,14 @@
def __repr__(self):
"""repr(self)"""
- return ('rational.Rational(%r,%r)' %
- (self.numerator, self.denominator))
+ return ('Rational(%r,%r)' % (self.numerator, self.denominator))
def __str__(self):
"""str(self)"""
if self.denominator == 1:
return str(self.numerator)
else:
- return '(%s/%s)' % (self.numerator, self.denominator)
+ return '%s/%s' % (self.numerator, self.denominator)
def _operator_fallbacks(monomorphic_operator, fallback_operator):
"""Generates forward and reverse operators given a purely-rational