Add link to math.fsum().

diff --git a/Doc/tutorial/floatingpoint.rst b/Doc/tutorial/floatingpoint.rst
index 98e299db98bf44c2b45e64e9f388ffc659932ae5..a8a42029fe49532a3a975a8ed1cee540aa520a2b 100644 (file)
--- a/Doc/tutorial/floatingpoint.rst
+++ b/Doc/tutorial/floatingpoint.rst
@@ -189,6 +189,16 @@ Since the representation is exact, it is useful for reliably porting values
 across different versions of Python (platform independence) and exchanging
 data with other languages that support the same format (such as Java and C99).
 
+Another helpful tool is the :func:`math.fsum` function which helps mitigate
+loss-of-precision during summation.  It tracks "lost digits" as values are
+added onto a running total.  That can make a difference in overall accuracy
+so that the errors do not accumulate to the point where they affect the
+final total:
+
+   >>> sum([0.1] * 10) == 1.0
+   False
+   >>> math.fsum([0.1] * 10) == 1.0
+   True
 
 .. _tut-fp-error: