The constructor from tuple was way too permissive: it allowed bad

coefficient numbers, floats in the sign, and other details that
generated directly the wrong number in the best case, or triggered
misfunctionality in the alorithms.

Test cases added for these issues. Thanks Mark Dickinson.

Lib/decimal.py

@@ -562,20 +562,46 @@ class Decimal(object):
         # tuple/list conversion (possibly from as_tuple())
         if isinstance(value, (list,tuple)):
             if len(value) != 3:
-                raise ValueError('Invalid arguments')
-            if value[0] not in (0,1):
-                raise ValueError('Invalid sign')
-            for digit in value[1]:
-                if not isinstance(digit, (int,long)) or digit < 0:
-                    raise ValueError("The second value in the tuple must be "
-                                "composed of non negative integer elements.")
+                raise ValueError('Invalid tuple size in creation of Decimal '
+                                 'from list or tuple.  The list or tuple '
+                                 'should have exactly three elements.')
+            # process sign.  The isinstance test rejects floats
+            if not (isinstance(value[0], (int, long)) and value[0] in (0,1)):
+                raise ValueError("Invalid sign.  The first value in the tuple "
+                                 "should be an integer; either 0 for a "
+                                 "positive number or 1 for a negative number.")
             self._sign = value[0]
-            self._int  = tuple(value[1])
-            if value[2] in ('F','n','N'):
+            if value[2] == 'F':
+                # infinity: value[1] is ignored
+                self._int = (0,)
                 self._exp = value[2]
                 self._is_special = True
             else:
-                self._exp  = int(value[2])
+                # process and validate the digits in value[1]
+                digits = []
+                for digit in value[1]:
+                    if isinstance(digit, (int, long)) and 0 <= digit <= 9:
+                        # skip leading zeros
+                        if digits or digit != 0:
+                            digits.append(digit)
+                    else:
+                        raise ValueError("The second value in the tuple must "
+                                         "be composed of integers in the range "
+                                         "0 through 9.")
+                if value[2] in ('n', 'N'):
+                    # NaN: digits form the diagnostic
+                    self._int = tuple(digits)
+                    self._exp = value[2]
+                    self._is_special = True
+                elif isinstance(value[2], (int, long)):
+                    # finite number: digits give the coefficient
+                    self._int = tuple(digits or [0])
+                    self._exp = value[2]
+                    self._is_special = False
+                else:
+                    raise ValueError("The third value in the tuple must "
+                                     "be an integer, or one of the "
+                                     "strings 'F', 'n', 'N'.")
             return self
 
         if isinstance(value, float):

Lib/test/test_decimal.py

@@ -452,13 +452,18 @@ class DecimalExplicitConstructionTest(unittest.TestCase):
 
         #bad sign
         self.assertRaises(ValueError, Decimal, (8, (4, 3, 4, 9, 1), 2) )
+        self.assertRaises(ValueError, Decimal, (0., (4, 3, 4, 9, 1), 2) )
+        self.assertRaises(ValueError, Decimal, (Decimal(1), (4, 3, 4, 9, 1), 2))
 
         #bad exp
         self.assertRaises(ValueError, Decimal, (1, (4, 3, 4, 9, 1), 'wrong!') )
+        self.assertRaises(ValueError, Decimal, (1, (4, 3, 4, 9, 1), 0.) )
+        self.assertRaises(ValueError, Decimal, (1, (4, 3, 4, 9, 1), '1') )
 
         #bad coefficients
         self.assertRaises(ValueError, Decimal, (1, (4, 3, 4, None, 1), 2) )
         self.assertRaises(ValueError, Decimal, (1, (4, -3, 4, 9, 1), 2) )
+        self.assertRaises(ValueError, Decimal, (1, (4, 10, 4, 9, 1), 2) )
 
     def test_explicit_from_Decimal(self):
 
@@ -1060,6 +1065,28 @@ class DecimalUsabilityTest(unittest.TestCase):
         d = Decimal("Infinity")
         self.assertEqual(d.as_tuple(), (0, (0,), 'F') )
 
+        #leading zeros in coefficient should be stripped
+        d = Decimal( (0, (0, 0, 4, 0, 5, 3, 4), -2) )
+        self.assertEqual(d.as_tuple(), (0, (4, 0, 5, 3, 4), -2) )
+        d = Decimal( (1, (0, 0, 0), 37) )
+        self.assertEqual(d.as_tuple(), (1, (0,), 37))
+        d = Decimal( (1, (), 37) )
+        self.assertEqual(d.as_tuple(), (1, (0,), 37))
+
+        #leading zeros in NaN diagnostic info should be stripped
+        d = Decimal( (0, (0, 0, 4, 0, 5, 3, 4), 'n') )
+        self.assertEqual(d.as_tuple(), (0, (4, 0, 5, 3, 4), 'n') )
+        d = Decimal( (1, (0, 0, 0), 'N') )
+        self.assertEqual(d.as_tuple(), (1, (), 'N') )
+        d = Decimal( (1, (), 'n') )
+        self.assertEqual(d.as_tuple(), (1, (), 'n') )
+
+        #coefficient in infinity should be ignored
+        d = Decimal( (0, (4, 5, 3, 4), 'F') )
+        self.assertEqual(d.as_tuple(), (0, (0,), 'F'))
+        d = Decimal( (1, (0, 2, 7, 1), 'F') )
+        self.assertEqual(d.as_tuple(), (1, (0,), 'F'))
+
     def test_immutability_operations(self):
         # Do operations and check that it didn't change change internal objects.