Issue 4796: Add from_float methods to the decimal module.

Doc/library/decimal.rst

@@ -484,6 +484,29 @@ Decimal objects
 
       .. versionadded:: 2.6
 
+   .. 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
@@ -1007,6 +1030,26 @@ 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):
+              ...
+          Inexact: None
+
+      .. versionadded:: 2.7
+
    .. method:: Etiny()
 
       Returns a value equal to ``Emin - prec + 1`` which is the minimum exponent

Lib/test/test_decimal.py

@@ -1361,6 +1361,55 @@ 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):

Misc/NEWS

@@ -12,8 +12,6 @@ What's New in Python 2.7 alpha 1
 Core and Builtins
 -----------------
 
-- Issue #4817: Remove unused function PyOS_GetLastModificationTime.
-
 - Issue #4075: Use OutputDebugStringW in Py_FatalError.
 
 - Issue #4797: IOError.filename was not set when _fileio.FileIO failed to open
@@ -110,6 +108,9 @@ Core and Builtins
 Library
 -------
 
+- 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.)