Issue 2748: fix __ceil__, __floor__ and __round__ magic methods in

Decimal, and add tests.

Lib/decimal.py

@@ -1645,9 +1645,6 @@ class Decimal(_numbers.Real):
         else:
             return -1
 
-    def __round__(self):
-        return self._round_down(0)
-
     def _round_up(self, prec):
         """Rounds away from 0."""
         return -self._round_down(prec)
@@ -1683,9 +1680,6 @@ class Decimal(_numbers.Real):
         else:
             return -self._round_down(prec)
 
-    def __ceil__(self):
-        return self._round_ceiling(0)
-
     def _round_floor(self, prec):
         """Rounds down (not towards 0 if negative)"""
         if not self._sign:
@@ -1693,9 +1687,6 @@ class Decimal(_numbers.Real):
         else:
             return -self._round_down(prec)
 
-    def __floor__(self):
-        return self._round_floor(0)
-
     def _round_05up(self, prec):
         """Round down unless digit prec-1 is 0 or 5."""
         if prec and self._int[prec-1] not in '05':
@@ -1703,6 +1694,102 @@ class Decimal(_numbers.Real):
         else:
             return -self._round_down(prec)
 
+    def __round__(self, n=None):
+        """Round self to the nearest integer, or to a given precision.
+
+        If only one argument is supplied, round a finite Decimal
+        instance self to the nearest integer.  If self is infinite or
+        a NaN then a Python exception is raised.  If self is finite
+        and lies exactly halfway between two integers then it is
+        rounded to the integer with even last digit.
+
+        >>> round(Decimal('123.456'))
+        123
+        >>> round(Decimal('-456.789'))
+        -457
+        >>> round(Decimal('-3.0'))
+        -3
+        >>> round(Decimal('2.5'))
+        2
+        >>> round(Decimal('3.5'))
+        4
+        >>> round(Decimal('Inf'))
+        Traceback (most recent call last):
+          ...
+          ...
+          ...
+        OverflowError: cannot round an infinity
+        >>> round(Decimal('NaN'))
+        Traceback (most recent call last):
+          ...
+          ...
+          ...
+        ValueError: cannot round a NaN
+
+        If a second argument n is supplied, self is rounded to n
+        decimal places using the rounding mode for the current
+        context.
+
+        For an integer n, round(self, -n) is exactly equivalent to
+        self.quantize(Decimal('1En')).
+
+        >>> round(Decimal('123.456'), 0)
+        Decimal('123')
+        >>> round(Decimal('123.456'), 2)
+        Decimal('123.46')
+        >>> round(Decimal('123.456'), -2)
+        Decimal('1E+2')
+        >>> round(Decimal('-Infinity'), 37)
+        Decimal('NaN')
+        >>> round(Decimal('sNaN123'), 0)
+        Decimal('NaN123')
+
+        """
+        if n is not None:
+            # two-argument form: use the equivalent quantize call
+            if not isinstance(n, int):
+                raise TypeError('Second argument to round should be integral')
+            exp = _dec_from_triple(0, '1', -n)
+            return self.quantize(exp)
+
+        # one-argument form
+        if self._is_special:
+            if self.is_nan():
+                raise ValueError("cannot round a NaN")
+            else:
+                raise OverflowError("cannot round an infinity")
+        return int(self._rescale(0, ROUND_HALF_EVEN))
+
+    def __floor__(self):
+        """Return the floor of self, as an integer.
+
+        For a finite Decimal instance self, return the greatest
+        integer n such that n <= self.  If self is infinite or a NaN
+        then a Python exception is raised.
+
+        """
+        if self._is_special:
+            if self.is_nan():
+                raise ValueError("cannot round a NaN")
+            else:
+                raise OverflowError("cannot round an infinity")
+        return int(self._rescale(0, ROUND_FLOOR))
+
+    def __ceil__(self):
+        """Return the ceiling of self, as an integer.
+
+        For a finite Decimal instance self, return the least integer n
+        such that n >= self.  If self is infinite or a NaN then a
+        Python exception is raised.
+
+        """
+        if self._is_special:
+            if self.is_nan():
+                raise ValueError("cannot round a NaN")
+            else:
+                raise OverflowError("cannot round an infinity")
+        return int(self._rescale(0, ROUND_CEILING))
+
     def fma(self, other, third, context=None):
         """Fused multiply-add.
 

Lib/test/test_decimal.py

@@ -1150,6 +1150,94 @@ class DecimalUsabilityTest(unittest.TestCase):
         self.assertEqual(float(d1), 66)
         self.assertEqual(float(d2), 15.32)
 
+        #floor
+        test_pairs = [
+            ('123.00', 123),
+            ('3.2', 3),
+            ('3.54', 3),
+            ('3.899', 3),
+            ('-2.3', -3),
+            ('-11.0', -11),
+            ('0.0', 0),
+            ('-0E3', 0),
+            ]
+        for d, i in test_pairs:
+            self.assertEqual(math.floor(Decimal(d)), i)
+        self.assertRaises(ValueError, math.floor, Decimal('-NaN'))
+        self.assertRaises(ValueError, math.floor, Decimal('sNaN'))
+        self.assertRaises(ValueError, math.floor, Decimal('NaN123'))
+        self.assertRaises(OverflowError, math.floor, Decimal('Inf'))
+        self.assertRaises(OverflowError, math.floor, Decimal('-Inf'))
+
+        #ceiling
+        test_pairs = [
+            ('123.00', 123),
+            ('3.2', 4),
+            ('3.54', 4),
+            ('3.899', 4),
+            ('-2.3', -2),
+            ('-11.0', -11),
+            ('0.0', 0),
+            ('-0E3', 0),
+            ]
+        for d, i in test_pairs:
+            self.assertEqual(math.ceil(Decimal(d)), i)
+        self.assertRaises(ValueError, math.ceil, Decimal('-NaN'))
+        self.assertRaises(ValueError, math.ceil, Decimal('sNaN'))
+        self.assertRaises(ValueError, math.ceil, Decimal('NaN123'))
+        self.assertRaises(OverflowError, math.ceil, Decimal('Inf'))
+        self.assertRaises(OverflowError, math.ceil, Decimal('-Inf'))
+
+        #round, single argument
+        test_pairs = [
+            ('123.00', 123),
+            ('3.2', 3),
+            ('3.54', 4),
+            ('3.899', 4),
+            ('-2.3', -2),
+            ('-11.0', -11),
+            ('0.0', 0),
+            ('-0E3', 0),
+            ('-3.5', -4),
+            ('-2.5', -2),
+            ('-1.5', -2),
+            ('-0.5', 0),
+            ('0.5', 0),
+            ('1.5', 2),
+            ('2.5', 2),
+            ('3.5', 4),
+            ]
+        for d, i in test_pairs:
+            self.assertEqual(round(Decimal(d)), i)
+        self.assertRaises(ValueError, round, Decimal('-NaN'))
+        self.assertRaises(ValueError, round, Decimal('sNaN'))
+        self.assertRaises(ValueError, round, Decimal('NaN123'))
+        self.assertRaises(OverflowError, round, Decimal('Inf'))
+        self.assertRaises(OverflowError, round, Decimal('-Inf'))
+
+        #round, two arguments;  this is essentially equivalent
+        #to quantize, which is already extensively tested
+        test_triples = [
+            ('123.456', -4, '0E+4'),
+            ('123.456', -3, '0E+3'),
+            ('123.456', -2, '1E+2'),
+            ('123.456', -1, '1.2E+2'),
+            ('123.456', 0, '123'),
+            ('123.456', 1, '123.5'),
+            ('123.456', 2, '123.46'),
+            ('123.456', 3, '123.456'),
+            ('123.456', 4, '123.4560'),
+            ('123.455', 2, '123.46'),
+            ('123.445', 2, '123.44'),
+            ('Inf', 4, 'NaN'),
+            ('-Inf', -23, 'NaN'),
+            ('sNaN314', 3, 'NaN314'),
+            ]
+        for d, n, r in test_triples:
+            self.assertEqual(str(round(Decimal(d), n)), r)
+
+
+
     def test_eval_round_trip(self):
 
         #with zero

Misc/NEWS

@@ -18,6 +18,10 @@ Extension Modules
 Library
 -------
 
+- The Decimal module gained the magic methods __round__, __ceil__,
+  __floor__ and __trunc__, to give support for round, math.ceil,
+  math.floor and math.trunc.
+
 - The user module has been removed.
 
 - The mutex module has been removed.