Index: Doc/library/decimal.rst
===================================================================
--- Doc/library/decimal.rst	(revision 68185)
+++ Doc/library/decimal.rst	(working copy)
@@ -484,6 +484,29 @@
 
       .. 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 @@
       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
Index: Lib/decimal.py
===================================================================
--- Lib/decimal.py	(revision 68185)
+++ Lib/decimal.py	(working copy)
@@ -135,6 +135,7 @@
 ]
 
 import copy as _copy
+import math as _math
 
 try:
     from collections import namedtuple as _namedtuple
@@ -653,6 +654,37 @@
 
         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 equilvalent of the value in decimal
+        is Decimal('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')
+        
+        """
+        if _math.isinf(f) or _math.isnan(f):  # raises TypeError if f is not a float
+            return cls(repr(f))
+        n, d = f.as_integer_ratio()
+        numerator, denominator = Decimal(n), Decimal(d)
+        with localcontext(Context(prec=60, traps=[Inexact, Overflow])) as ctx:
+            while True:
+                try:
+                   return cls(numerator / denominator)
+                except Inexact:
+                    ctx.prec += 15
+
     def _isnan(self):
         """Returns whether the number is not actually one.
 
@@ -3744,6 +3776,23 @@
                                      "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 self.plus(d)             # Apply the context rounding
+
     # Methods
     def abs(self, a):
         """Returns the absolute value of the operand.
Index: Lib/test/test_decimal.py
===================================================================
--- Lib/test/test_decimal.py	(revision 68185)
+++ Lib/test/test_decimal.py	(working copy)
@@ -1361,6 +1361,44 @@
             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')
+        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')
+
+    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
+        )
+
 class ContextAPItests(unittest.TestCase):
 
     def test_pickle(self):