Issue 27936: Fix inconsistent round() behavior between float and int

Lib/test/test_builtin.py

@@ -3,6 +3,8 @@
 import ast
 import builtins
 import collections
+import decimal
+import fractions
 import io
 import locale
 import os
@@ -1244,6 +1246,15 @@ class BuiltinTest(unittest.TestCase):
         self.assertEqual(round(5e15+2), 5e15+2)
         self.assertEqual(round(5e15+3), 5e15+3)
 
+    def test_bug_27936(self):
+        # Verify that ndigits=None means the same as passing in no argument
+        for x in [1234,
+                  1234.56,
+                  decimal.Decimal('1234.56'),
+                  fractions.Fraction(123456, 100)]:
+            self.assertEqual(round(x, None), round(x))
+            self.assertEqual(type(round(x, None)), type(round(x)))
+
     def test_setattr(self):
         setattr(sys, 'spam', 1)
         self.assertEqual(sys.spam, 1)

Lib/test/test_long.py

@@ -967,7 +967,7 @@ class LongTest(unittest.TestCase):
             self.assertIs(type(got), int)
 
         # bad second argument
-        bad_exponents = ('brian', 2.0, 0j, None)
+        bad_exponents = ('brian', 2.0, 0j)
         for e in bad_exponents:
             self.assertRaises(TypeError, round, 3, e)
 

Misc/NEWS

@@ -18,6 +18,10 @@ Core and Builtins
   ``m_methods`` field to be used to add module level functions to instances
   of non-module types returned from ``Py_create_mod``. Patch by Xiang Zhang.
 
+- Issue #27936: The round() function accepted a second None argument
+  for some types but not for others.  Fixed the inconsistency by
+  accepting None for all numeric types.
+
 - Issue #27487: Warn if a submodule argument to "python -m" or
   runpy.run_module() is found in sys.modules after parent packages are
   imported, but before the submodule is executed.

Python/bltinmodule.c

@@ -2039,7 +2039,7 @@ builtin_round(PyObject *self, PyObject *args, PyObject *kwds)
         return NULL;
     }
 
-    if (ndigits == NULL)
+    if (ndigits == NULL || ndigits == Py_None)
         result = PyObject_CallFunctionObjArgs(round, NULL);
     else
         result = PyObject_CallFunctionObjArgs(round, ndigits, NULL);