Issue 19933: Provide default argument for ndigits in round. Patch by Vajrasky Kok.

Doc/library/functions.rst

@@ -1225,8 +1225,8 @@ are always available.  They are listed here in alphabetical order.
 .. function:: round(number[, ndigits])
 
    Return the floating point value *number* rounded to *ndigits* digits after
-   the decimal point.  If *ndigits* is omitted, it defaults to zero. Delegates
+   the decimal point.  If *ndigits* is omitted, it returns the nearest integer
-   to ``number.__round__(ndigits)``.
+   to its input.  Delegates to ``number.__round__(ndigits)``.
 
    For the built-in types supporting :func:`round`, values are rounded to the
    closest multiple of 10 to the power minus *ndigits*; if two multiples are

Lib/test/test_float.py

@@ -773,6 +773,14 @@ class RoundTestCase(unittest.TestCase):
             test(sfmt, NAN, ' nan')
             test(sfmt, -NAN, ' nan')
 
+    def test_None_ndigits(self):
+        for x in round(1.23), round(1.23, None), round(1.23, ndigits=None):
+            self.assertEqual(x, 1)
+            self.assertIsInstance(x, int)
+        for x in round(1.78), round(1.78, None), round(1.78, ndigits=None):
+            self.assertEqual(x, 2)
+            self.assertIsInstance(x, int)
+
 
 # Beginning with Python 2.6 float has cross platform compatible
 # ways to create and represent inf and nan

Misc/NEWS

@@ -36,6 +36,9 @@ Core and Builtins
 Library
 -------
 
+- Issue 19933: Provide default argument for ndigits in round. Patch by
+  Vajrasky Kok.
+
 - Issue #23193: Add a numeric_owner parameter to
   tarfile.TarFile.extract and tarfile.TarFile.extractall. Patch by
   Michael Vogt and Eric Smith.

Objects/floatobject.c

@@ -986,8 +986,9 @@ float_round(PyObject *v, PyObject *args)
     x = PyFloat_AsDouble(v);
     if (!PyArg_ParseTuple(args, "|O", &o_ndigits))
         return NULL;
-    if (o_ndigits == NULL) {
+    if (o_ndigits == NULL || o_ndigits == Py_None) {
-        /* single-argument round: round to nearest integer */
+        /* single-argument round or with None ndigits:
+         * round to nearest integer */
         rounded = round(x);
         if (fabs(x-rounded) == 0.5)
             /* halfway case: round to even */