Issue #8644: Improve accuracy of timedelta.total_seconds, by doing intermediate

computations with integer arithmetic instead of floating point.
td.total_seconds() now agrees with td / timedelta(seconds = 1).

Thanks Alexander Belopolsky for the patch.

Doc/library/datetime.rst

@@ -287,7 +287,10 @@ Instance methods:
 .. method:: timedelta.total_seconds()
 
    Return the total number of seconds contained in the duration. Equivalent to
-   ``td.microseconds / 1000000 + td.seconds + td.days * 24 * 3600``.
+   ``td / timedelta(seconds=1)``.
+
+   Note that for very large time intervals (greater than 270 years on
+   most platforms) this method will lose microsecond accuracy.
 
    .. versionadded:: 3.2
 

Lib/test/test_datetime.py

@@ -264,6 +264,11 @@ class TestTimeDelta(HarmlessMixedComparison, unittest.TestCase):
         for total_seconds in [123456.789012, -123456.789012, 0.123456, 0, 1e6]:
             td = timedelta(seconds=total_seconds)
             self.assertEqual(td.total_seconds(), total_seconds)
+        # Issue8644: Test that td.total_seconds() has the same
+        # accuracy as td / timedelta(seconds=1).
+        for ms in [-1, -2, -123]:
+            td = timedelta(microseconds=ms)
+            self.assertEqual(td.total_seconds(), td / timedelta(seconds=1))
 
     def test_carries(self):
         t1 = timedelta(days=100,

Misc/NEWS

@@ -1105,6 +1105,11 @@ Library
 Extension Modules
 -----------------
 
+- Issue #8644: The accuracy of td.total_seconds() has been improved (by
+  calculating with integer arithmetic instead of float arithmetic internally):
+  the result is now always correctly rounded, and is equivalent to td /
+  timedelta(seconds=1).
+
 - Issue #2706: Allow division of a timedelta by another timedelta:
   timedelta / timedelta, timedelta % timedelta, timedelta // timedelta
   and divmod(timedelta, timedelta) are all supported.

Modules/datetimemodule.c

@@ -2211,9 +2211,25 @@ delta_getstate(PyDateTime_Delta *self)
 static PyObject *
 delta_total_seconds(PyObject *self)
 {
-	return PyFloat_FromDouble(GET_TD_MICROSECONDS(self) / 1000000.0 +
-				  GET_TD_SECONDS(self) +
-				  GET_TD_DAYS(self) * 24.0 * 3600.0);
+	PyObject *total_seconds;
+	PyObject *total_microseconds;
+	PyObject *one_million;
+
+	total_microseconds = delta_to_microseconds((PyDateTime_Delta *)self);
+	if (total_microseconds == NULL)
+		return NULL;
+
+	one_million = PyLong_FromLong(1000000L);
+	if (one_million == NULL) {
+		Py_DECREF(total_microseconds);
+		return NULL;
+	}
+
+	total_seconds = PyNumber_TrueDivide(total_microseconds, one_million);
+
+	Py_DECREF(total_microseconds);
+	Py_DECREF(one_million);
+	return total_seconds;
 }
 
 static PyObject *