Issue #1757: The hash of a Decimal instance is no longer affected

by the current context.  Thanks Mark Dickinson.

Lib/decimal.py

@@ -788,8 +788,10 @@ class Decimal(object):
     def __hash__(self):
         """x.__hash__() <==> hash(x)"""
         # Decimal integers must hash the same as the ints
-        # Non-integer decimals are normalized and hashed as strings
-        # Normalization assures that hash(100E-1) == hash(10)
+        #
+        # The hash of a nonspecial noninteger Decimal must depend only
+        # on the value of that Decimal, and not on its representation.
+        # For example: hash(Decimal("100E-1")) == hash(Decimal("10")).
         if self._is_special:
             if self._isnan():
                 raise TypeError('Cannot hash a NaN value.')
@@ -805,7 +807,13 @@ class Decimal(object):
             # 2**64-1.  So we can replace hash((-1)**s*c*10**e) with
             # hash((-1)**s*c*pow(10, e, 2**64-1).
             return hash((-1)**op.sign*op.int*pow(10, op.exp, 2**64-1))
-        return hash(str(self.normalize()))
+        # The value of a nonzero nonspecial Decimal instance is
+        # faithfully represented by the triple consisting of its sign,
+        # its adjusted exponent, and its coefficient with trailing
+        # zeros removed.
+        return hash((self._sign,
+                     self._exp+len(self._int),
+                     self._int.rstrip('0')))
 
     def as_tuple(self):
         """Represents the number as a triple tuple.

Lib/test/test_decimal.py

@@ -980,6 +980,23 @@ class DecimalUsabilityTest(unittest.TestCase):
         self.assert_(hash(Decimal('Inf')))
         self.assert_(hash(Decimal('-Inf')))
 
+        # check that the value of the hash doesn't depend on the
+        # current context (issue #1757)
+        c = getcontext()
+        old_precision = c.prec
+        x = Decimal("123456789.1")
+
+        c.prec = 6
+        h1 = hash(x)
+        c.prec = 10
+        h2 = hash(x)
+        c.prec = 16
+        h3 = hash(x)
+
+        self.assertEqual(h1, h2)
+        self.assertEqual(h1, h3)
+        c.prec = old_precision
+
     def test_min_and_max_methods(self):
 
         d1 = Decimal('15.32')

Misc/NEWS

@@ -348,6 +348,9 @@ Core and builtins
 Library
 -------
 
+- Issue #1757: The hash of a Decimal instance is no longer affected by
+  the current context.
+
 - Patch #467924: add ZipFile.extract() and ZipFile.extractall() in the
   zipfile module.