Index: Doc/library/decimal.rst =================================================================== --- Doc/library/decimal.rst (revision 79479) +++ Doc/library/decimal.rst (working copy) @@ -308,7 +308,7 @@ Construct a new :class:`Decimal` object based from *value*. - *value* can be an integer, string, tuple, or another :class:`Decimal` + *value* can be an integer, string, tuple, :class:`float`, or another :class:`Decimal` object. If no *value* is given, returns ``Decimal('0')``. If *value* is a string, it should conform to the decimal numeric string syntax after leading and trailing whitespace characters are removed:: @@ -334,6 +334,11 @@ digits, and an integer exponent. For example, ``Decimal((0, (1, 4, 1, 4), -3))`` returns ``Decimal('1.414')``. + If *value* is a :class:`float`, the binary floating point value is losslessly + converted to its exact decimal equivalent. This conversion can often require + upto 53 digits of precision. For example, ``Decimal(float('1.1'))`` converts + to ``Decimal('1.100000000000000088817841970012523233890533447265625')``. + The *context* precision does not affect how many digits are stored. That is determined exclusively by the number of digits in *value*. For example, ``Decimal('3.00000')`` records all five zeros even if the context precision is @@ -1824,36 +1829,14 @@ Q. Is there a way to convert a regular float to a :class:`Decimal`? A. Yes, all binary floating point numbers can be exactly expressed as a -Decimal. An exact conversion may take more precision than intuition would -suggest, so we trap :const:`Inexact` to signal a need for more precision: +Decimal though an exact conversion may take more precision than intuition would +suggest: -.. testcode:: - - def float_to_decimal(f): - "Convert a floating point number to a Decimal with no loss of information" - n, d = f.as_integer_ratio() - with localcontext() as ctx: - ctx.traps[Inexact] = True - while True: - try: - return Decimal(n) / Decimal(d) - except Inexact: - ctx.prec += 1 - .. doctest:: - >>> float_to_decimal(math.pi) + >>> Decimal(math.pi) Decimal('3.141592653589793115997963468544185161590576171875') -Q. Why isn't the :func:`float_to_decimal` routine included in the module? - -A. There is some question about whether it is advisable to mix binary and -decimal floating point. Also, its use requires some care to avoid the -representation issues associated with binary floating point: - - >>> float_to_decimal(1.1) - Decimal('1.100000000000000088817841970012523233890533447265625') - Q. Within a complex calculation, how can I make sure that I haven't gotten a spurious result because of insufficient precision or rounding anomalies. Index: Lib/decimal.py =================================================================== --- Lib/decimal.py (revision 79479) +++ Lib/decimal.py (working copy) @@ -648,8 +648,12 @@ return self if isinstance(value, float): - raise TypeError("Cannot convert float in Decimal constructor. " - "Use from_float class method.") + value = Decimal.from_float(value) + self._exp = value._exp + self._sign = value._sign + self._int = value._int + self._is_special = value._is_special + return self raise TypeError("Cannot convert %r to Decimal" % value) Index: Lib/test/test_decimal.py =================================================================== --- Lib/test/test_decimal.py (revision 79479) +++ Lib/test/test_decimal.py (working copy) @@ -498,6 +498,26 @@ self.assertEqual(str(e), '0') self.assertNotEqual(id(d), id(e)) + def test_explicit_from_float(self): + r = Decimal(0.1) + self.assertEqual(type(r), Decimal) + self.assertEqual(str(r), + '0.1000000000000000055511151231257827021181583404541015625') + self.assertTrue(Decimal(float('nan')).is_qnan()) + self.assertTrue(Decimal(float('inf')).is_infinite()) + self.assertTrue(Decimal(float('-inf')).is_infinite()) + self.assertEqual(str(Decimal(float('nan'))), + str(Decimal('NaN'))) + self.assertEqual(str(Decimal(float('inf'))), + str(Decimal('Infinity'))) + self.assertEqual(str(Decimal(float('-inf'))), + str(Decimal('-Infinity'))) + self.assertEqual(str(Decimal(float('-0.0'))), + str(Decimal('-0'))) + for i in range(200): + x = random.expovariate(0.01) * (random.random() * 2.0 - 1.0) + self.assertEqual(x, float(Decimal(x))) # roundtrip + def test_explicit_context_create_decimal(self): nc = copy.copy(getcontext())