diff options
| -rw-r--r-- | Doc/library/decimal.rst | 43 | ||||
| -rw-r--r-- | Lib/decimal.py | 50 | ||||
| -rw-r--r-- | Lib/test/test_decimal.py | 49 | ||||
| -rw-r--r-- | Misc/NEWS | 5 |
4 files changed, 145 insertions, 2 deletions
diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst index ad8b65c..76ba2ee 100644 --- a/Doc/library/decimal.rst +++ b/Doc/library/decimal.rst @@ -453,6 +453,29 @@ Decimal objects >>> Decimal(321).exp() Decimal('2.561702493119680037517373933E+139') + .. method:: from_float(f) + + Classmethod that converts a float to a decimal number, exactly. + + Note `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`. That equivalent value in decimal is + `0.1000000000000000055511151231257827021181583404541015625`. + + .. doctest:: + + >>> 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') + + .. versionadded:: 2.7 + .. method:: fma(other, third[, context]) Fused multiply-add. Return self*other+third with no rounding of the @@ -910,6 +933,26 @@ In addition to the three supplied contexts, new contexts can be created with the If the argument is a string, no leading or trailing whitespace is permitted. +.. method:: create_decimal_from_float(f) + + Creates a new Decimal instance from a float *f* but rounding using *self* + as the context. Unlike the :method:`Decimal.from_float` class method, + the context precision, rounding method, flags, and traps are applied to + the conversion. + + .. doctest:: + + >>> context = Context(prec=5, rounding=ROUND_DOWN) + >>> context.create_decimal_from_float(math.pi) + Decimal('3.1415') + >>> context = Context(prec=5, traps=[Inexact]) + >>> context.create_decimal_from_float(math.pi) + Traceback (most recent call last): + ... + decimal.Inexact: None + + .. versionadded:: 2.7 + .. method:: Etiny() Returns a value equal to ``Emin - prec + 1`` which is the minimum exponent diff --git a/Lib/decimal.py b/Lib/decimal.py index 1f6218c..197269c 100644 --- a/Lib/decimal.py +++ b/Lib/decimal.py @@ -136,6 +136,7 @@ __all__ = [ import numbers as _numbers import copy as _copy +import math as _math try: from collections import namedtuple as _namedtuple @@ -654,6 +655,38 @@ class Decimal(_numbers.Real): 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): # 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. @@ -3830,6 +3863,23 @@ 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): + ... + decimal.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. diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py index d0de64d..ade356c 100644 --- a/Lib/test/test_decimal.py +++ b/Lib/test/test_decimal.py @@ -1421,6 +1421,55 @@ class DecimalPythonAPItests(unittest.TestCase): r = d.to_integral(ROUND_DOWN) self.assertEqual(Decimal(math.trunc(d)), r) + def test_from_float(self): + + class MyDecimal(Decimal): + pass + + r = MyDecimal.from_float(0.1) + self.assertEqual(type(r), MyDecimal) + self.assertEqual(str(r), + '0.1000000000000000055511151231257827021181583404541015625') + bigint = 12345678901234567890123456789 + self.assertEqual(MyDecimal.from_float(bigint), MyDecimal(bigint)) + self.assert_(MyDecimal.from_float(float('nan')).is_qnan()) + self.assert_(MyDecimal.from_float(float('inf')).is_infinite()) + self.assert_(MyDecimal.from_float(float('-inf')).is_infinite()) + self.assertEqual(str(MyDecimal.from_float(float('nan'))), + str(Decimal('NaN'))) + self.assertEqual(str(MyDecimal.from_float(float('inf'))), + str(Decimal('Infinity'))) + self.assertEqual(str(MyDecimal.from_float(float('-inf'))), + str(Decimal('-Infinity'))) + self.assertRaises(TypeError, MyDecimal.from_float, 'abc') + for i in range(200): + x = random.expovariate(0.01) * (random.random() * 2.0 - 1.0) + self.assertEqual(x, float(MyDecimal.from_float(x))) # roundtrip + + def test_create_decimal_from_float(self): + context = Context(prec=5, rounding=ROUND_DOWN) + self.assertEqual( + context.create_decimal_from_float(math.pi), + Decimal('3.1415') + ) + context = Context(prec=5, rounding=ROUND_UP) + self.assertEqual( + context.create_decimal_from_float(math.pi), + Decimal('3.1416') + ) + context = Context(prec=5, traps=[Inexact]) + self.assertRaises( + Inexact, + context.create_decimal_from_float, + math.pi + ) + self.assertEqual(repr(context.create_decimal_from_float(-0.0)), + "Decimal('-0')") + self.assertEqual(repr(context.create_decimal_from_float(1.0)), + "Decimal('1')") + self.assertEqual(repr(context.create_decimal_from_float(10)), + "Decimal('10')") + class ContextAPItests(unittest.TestCase): def test_pickle(self): @@ -12,8 +12,6 @@ What's New in Python 3.1 alpha 0 Core and Builtins ----------------- -- Issue #4817: Remove unused function PyOS_GetLastModificationTime. - - Issue #4580: Fix slicing of memoryviews when the item size is greater than one byte. Also fixes the meaning of len() so that it returns the number of items, rather than the size in bytes. @@ -88,6 +86,9 @@ Library Python 3.x, in accordance with the `official amendments of the spec <http://www.wsgi.org/wsgi/Amendments_1.0>`_. +- Issue #4796: Added Decimal.from_float() and Context.create_decimal_from_float() + to the decimal module. + - Issue #4812: add missing underscore prefix to some internal-use-only constants in the decimal module. (Dec_0 becomes _Dec_0, etc.) |