diff options
Diffstat (limited to 'Lib/decimal.py')
| -rw-r--r-- | Lib/decimal.py | 162 |
1 files changed, 106 insertions, 56 deletions
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 @@ __all__ = [ ] import copy as _copy +import math as _math try: from collections import namedtuple as _namedtuple @@ -242,7 +243,7 @@ class DivisionByZero(DecimalException, ZeroDivisionError): """ def handle(self, context, sign, *args): - return _SignedInfinity[sign] + return _Infsign[sign] class DivisionImpossible(InvalidOperation): """Cannot perform the division adequately. @@ -340,15 +341,15 @@ class Overflow(Inexact, Rounded): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Decimal(object): """ # 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 @@ class Decimal(object): 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 @@ class Decimal(object): # 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 @@ class Decimal(object): # 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 @@ class Decimal(object): # 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 @@ class Decimal(object): # logb(+/-Inf) = +Inf if self._isinfinity(): - return _Infinity + return _Inf # logb(0) = -Inf, DivisionByZero if not self: @@ -3220,7 +3253,7 @@ class Decimal(object): 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 @@ class Decimal(object): 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 @@ class Context(object): "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 @@ def _format_align(body, spec_dict): ##### 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) |