Issue 4796: Add from_float methods to the decimal module.
diff --git a/Lib/decimal.py b/Lib/decimal.py
index 7f1827a..6ad81b5 100644
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -135,6 +135,7 @@
]
import copy as _copy
+import math as _math
try:
from collections import namedtuple as _namedtuple
@@ -242,7 +243,7 @@
"""
def handle(self, context, sign, *args):
- return _SignedInfinity[sign]
+ return _Infsign[sign]
class DivisionImpossible(InvalidOperation):
"""Cannot perform the division adequately.
@@ -340,15 +341,15 @@
def handle(self, context, sign, *args):
if context.rounding in (ROUND_HALF_UP, ROUND_HALF_EVEN,
ROUND_HALF_DOWN, ROUND_UP):
- return _SignedInfinity[sign]
+ return _Infsign[sign]
if sign == 0:
if context.rounding == ROUND_CEILING:
- return _SignedInfinity[sign]
+ return _Infsign[sign]
return _dec_from_triple(sign, '9'*context.prec,
context.Emax-context.prec+1)
if sign == 1:
if context.rounding == ROUND_FLOOR:
- return _SignedInfinity[sign]
+ return _Infsign[sign]
return _dec_from_triple(sign, '9'*context.prec,
context.Emax-context.prec+1)
@@ -653,6 +654,38 @@
raise TypeError("Cannot convert %r to Decimal" % value)
+ @classmethod
+ def from_float(cls, f):
+ """Converts a float to a decimal number, exactly.
+
+ Note that Decimal.from_float(0.1) is not the same as Decimal('0.1').
+ Since 0.1 is not exactly representable in binary floating point, the
+ value is stored as the nearest representable value which is
+ 0x1.999999999999ap-4. The exact equivalent of the value in decimal
+ is 0.1000000000000000055511151231257827021181583404541015625.
+
+ >>> Decimal.from_float(0.1)
+ Decimal('0.1000000000000000055511151231257827021181583404541015625')
+ >>> Decimal.from_float(float('nan'))
+ Decimal('NaN')
+ >>> Decimal.from_float(float('inf'))
+ Decimal('Infinity')
+ >>> Decimal.from_float(-float('inf'))
+ Decimal('-Infinity')
+ >>> Decimal.from_float(-0.0)
+ Decimal('-0')
+
+ """
+ if isinstance(f, (int, long)): # handle integer inputs
+ return cls(f)
+ if _math.isinf(f) or _math.isnan(f): # raises TypeError if not a float
+ return cls(repr(f))
+ sign = 0 if _math.copysign(1.0, f) == 1.0 else 1
+ n, d = abs(f).as_integer_ratio()
+ k = d.bit_length() - 1
+ result = _dec_from_triple(sign, str(n*5**k), -k)
+ return result if cls is Decimal else cls(result)
+
def _isnan(self):
"""Returns whether the number is not actually one.
@@ -1171,12 +1204,12 @@
if self._isinfinity():
if not other:
return context._raise_error(InvalidOperation, '(+-)INF * 0')
- return _SignedInfinity[resultsign]
+ return _Infsign[resultsign]
if other._isinfinity():
if not self:
return context._raise_error(InvalidOperation, '0 * (+-)INF')
- return _SignedInfinity[resultsign]
+ return _Infsign[resultsign]
resultexp = self._exp + other._exp
@@ -1226,7 +1259,7 @@
return context._raise_error(InvalidOperation, '(+-)INF/(+-)INF')
if self._isinfinity():
- return _SignedInfinity[sign]
+ return _Infsign[sign]
if other._isinfinity():
context._raise_error(Clamped, 'Division by infinity')
@@ -1329,7 +1362,7 @@
ans = context._raise_error(InvalidOperation, 'divmod(INF, INF)')
return ans, ans
else:
- return (_SignedInfinity[sign],
+ return (_Infsign[sign],
context._raise_error(InvalidOperation, 'INF % x'))
if not other:
@@ -1477,7 +1510,7 @@
if other._isinfinity():
return context._raise_error(InvalidOperation, 'INF // INF')
else:
- return _SignedInfinity[self._sign ^ other._sign]
+ return _Infsign[self._sign ^ other._sign]
if not other:
if self:
@@ -1732,12 +1765,12 @@
if not other:
return context._raise_error(InvalidOperation,
'INF * 0 in fma')
- product = _SignedInfinity[self._sign ^ other._sign]
+ product = _Infsign[self._sign ^ other._sign]
elif other._exp == 'F':
if not self:
return context._raise_error(InvalidOperation,
'0 * INF in fma')
- product = _SignedInfinity[self._sign ^ other._sign]
+ product = _Infsign[self._sign ^ other._sign]
else:
product = _dec_from_triple(self._sign ^ other._sign,
str(int(self._int) * int(other._int)),
@@ -2087,7 +2120,7 @@
if not self:
return context._raise_error(InvalidOperation, '0 ** 0')
else:
- return _One
+ return _Dec_p1
# result has sign 1 iff self._sign is 1 and other is an odd integer
result_sign = 0
@@ -2109,19 +2142,19 @@
if other._sign == 0:
return _dec_from_triple(result_sign, '0', 0)
else:
- return _SignedInfinity[result_sign]
+ return _Infsign[result_sign]
# Inf**(+ve or Inf) = Inf; Inf**(-ve or -Inf) = 0
if self._isinfinity():
if other._sign == 0:
- return _SignedInfinity[result_sign]
+ return _Infsign[result_sign]
else:
return _dec_from_triple(result_sign, '0', 0)
# 1**other = 1, but the choice of exponent and the flags
# depend on the exponent of self, and on whether other is a
# positive integer, a negative integer, or neither
- if self == _One:
+ if self == _Dec_p1:
if other._isinteger():
# exp = max(self._exp*max(int(other), 0),
# 1-context.prec) but evaluating int(other) directly
@@ -2154,7 +2187,7 @@
if (other._sign == 0) == (self_adj < 0):
return _dec_from_triple(result_sign, '0', 0)
else:
- return _SignedInfinity[result_sign]
+ return _Infsign[result_sign]
# from here on, the result always goes through the call
# to _fix at the end of this function.
@@ -2674,9 +2707,9 @@
"""
# if one is negative and the other is positive, it's easy
if self._sign and not other._sign:
- return _NegativeOne
+ return _Dec_n1
if not self._sign and other._sign:
- return _One
+ return _Dec_p1
sign = self._sign
# let's handle both NaN types
@@ -2686,51 +2719,51 @@
if self_nan == other_nan:
if self._int < other._int:
if sign:
- return _One
+ return _Dec_p1
else:
- return _NegativeOne
+ return _Dec_n1
if self._int > other._int:
if sign:
- return _NegativeOne
+ return _Dec_n1
else:
- return _One
- return _Zero
+ return _Dec_p1
+ return _Dec_0
if sign:
if self_nan == 1:
- return _NegativeOne
+ return _Dec_n1
if other_nan == 1:
- return _One
+ return _Dec_p1
if self_nan == 2:
- return _NegativeOne
+ return _Dec_n1
if other_nan == 2:
- return _One
+ return _Dec_p1
else:
if self_nan == 1:
- return _One
+ return _Dec_p1
if other_nan == 1:
- return _NegativeOne
+ return _Dec_n1
if self_nan == 2:
- return _One
+ return _Dec_p1
if other_nan == 2:
- return _NegativeOne
+ return _Dec_n1
if self < other:
- return _NegativeOne
+ return _Dec_n1
if self > other:
- return _One
+ return _Dec_p1
if self._exp < other._exp:
if sign:
- return _One
+ return _Dec_p1
else:
- return _NegativeOne
+ return _Dec_n1
if self._exp > other._exp:
if sign:
- return _NegativeOne
+ return _Dec_n1
else:
- return _One
- return _Zero
+ return _Dec_p1
+ return _Dec_0
def compare_total_mag(self, other):
@@ -2771,11 +2804,11 @@
# exp(-Infinity) = 0
if self._isinfinity() == -1:
- return _Zero
+ return _Dec_0
# exp(0) = 1
if not self:
- return _One
+ return _Dec_p1
# exp(Infinity) = Infinity
if self._isinfinity() == 1:
@@ -2927,15 +2960,15 @@
# ln(0.0) == -Infinity
if not self:
- return _NegativeInfinity
+ return _negInf
# ln(Infinity) = Infinity
if self._isinfinity() == 1:
- return _Infinity
+ return _Inf
# ln(1.0) == 0.0
- if self == _One:
- return _Zero
+ if self == _Dec_p1:
+ return _Dec_0
# ln(negative) raises InvalidOperation
if self._sign == 1:
@@ -3007,11 +3040,11 @@
# log10(0.0) == -Infinity
if not self:
- return _NegativeInfinity
+ return _negInf
# log10(Infinity) = Infinity
if self._isinfinity() == 1:
- return _Infinity
+ return _Inf
# log10(negative or -Infinity) raises InvalidOperation
if self._sign == 1:
@@ -3063,7 +3096,7 @@
# logb(+/-Inf) = +Inf
if self._isinfinity():
- return _Infinity
+ return _Inf
# logb(0) = -Inf, DivisionByZero
if not self:
@@ -3220,7 +3253,7 @@
return ans
if self._isinfinity() == -1:
- return _NegativeInfinity
+ return _negInf
if self._isinfinity() == 1:
return _dec_from_triple(0, '9'*context.prec, context.Etop())
@@ -3243,7 +3276,7 @@
return ans
if self._isinfinity() == 1:
- return _Infinity
+ return _Inf
if self._isinfinity() == -1:
return _dec_from_triple(1, '9'*context.prec, context.Etop())
@@ -3744,6 +3777,23 @@
"diagnostic info too long in NaN")
return d._fix(self)
+ def create_decimal_from_float(self, f):
+ """Creates a new Decimal instance from a float but rounding using self
+ as the context.
+
+ >>> context = Context(prec=5, rounding=ROUND_DOWN)
+ >>> context.create_decimal_from_float(3.1415926535897932)
+ Decimal('3.1415')
+ >>> context = Context(prec=5, traps=[Inexact])
+ >>> context.create_decimal_from_float(3.1415926535897932)
+ Traceback (most recent call last):
+ ...
+ Inexact: None
+
+ """
+ d = Decimal.from_float(f) # An exact conversion
+ return d._fix(self) # Apply the context rounding
+
# Methods
def abs(self, a):
"""Returns the absolute value of the operand.
@@ -5490,15 +5540,15 @@
##### Useful Constants (internal use only) ################################
# Reusable defaults
-_Infinity = Decimal('Inf')
-_NegativeInfinity = Decimal('-Inf')
+_Inf = Decimal('Inf')
+_negInf = Decimal('-Inf')
_NaN = Decimal('NaN')
-_Zero = Decimal(0)
-_One = Decimal(1)
-_NegativeOne = Decimal(-1)
+_Dec_0 = Decimal(0)
+_Dec_p1 = Decimal(1)
+_Dec_n1 = Decimal(-1)
-# _SignedInfinity[sign] is infinity w/ that sign
-_SignedInfinity = (_Infinity, _NegativeInfinity)
+# _Infsign[sign] is infinity w/ that sign
+_Infsign = (_Inf, _negInf)