Issue 8257: Decimal constructor to accept float argument.

2 files changed, 32 insertions, 2 deletions

diff --git a/Lib/decimal.py b/Lib/decimal.py
index 370eb82..ab38ed4 100644
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -648,8 +648,12 @@ class Decimal(object):
             return self
 
         if isinstance(value, float):
-            raise TypeError("Cannot convert float in Decimal constructor. "
-                            "Use from_float class method.")
+            value = Decimal.from_float(value)
+            self._exp  = value._exp
+            self._sign = value._sign
+            self._int  = value._int
+            self._is_special  = value._is_special
+            return self
 
         raise TypeError("Cannot convert %r to Decimal" % value)
 
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 360a5e8..51bdf9c 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -52,6 +52,11 @@ def init():
         )
     setcontext(DefaultTestContext)
 
+# decorator for skipping tests on non-IEEE 754 platforms
+requires_IEEE_754 = unittest.skipUnless(
+    float.__getformat__("double").startswith("IEEE"),
+    "test requires IEEE 754 doubles")
+
 TESTDATADIR = 'decimaltestdata'
 if __name__ == '__main__':
     file = sys.argv[0]
@@ -504,6 +509,27 @@ class DecimalExplicitConstructionTest(unittest.TestCase):
         self.assertEqual(str(e), '0')
         self.assertNotEqual(id(d), id(e))
 
+    @requires_IEEE_754
+    def test_explicit_from_float(self):
+        r = Decimal(0.1)
+        self.assertEqual(type(r), Decimal)
+        self.assertEqual(str(r),
+                '0.1000000000000000055511151231257827021181583404541015625')
+        self.assertTrue(Decimal(float('nan')).is_qnan())
+        self.assertTrue(Decimal(float('inf')).is_infinite())
+        self.assertTrue(Decimal(float('-inf')).is_infinite())
+        self.assertEqual(str(Decimal(float('nan'))),
+                         str(Decimal('NaN')))
+        self.assertEqual(str(Decimal(float('inf'))),
+                         str(Decimal('Infinity')))
+        self.assertEqual(str(Decimal(float('-inf'))),
+                         str(Decimal('-Infinity')))
+        self.assertEqual(str(Decimal(float('-0.0'))),
+                         str(Decimal('-0')))
+        for i in range(200):
+            x = random.expovariate(0.01) * (random.random() * 2.0 - 1.0)
+            self.assertEqual(x, float(Decimal(x))) # roundtrip
+
     def test_explicit_context_create_decimal(self):
 
         nc = copy.copy(getcontext())