Add itertools.accumulate().

Doc/library/itertools.rst

@@ -46,6 +46,7 @@ Iterator            Arguments               Results
 ====================    ============================    =================================================   =============================================================
 Iterator                Arguments                       Results                                             Example
 ====================    ============================    =================================================   =============================================================
+:func:`accumulate`      p[, start=0]                    p0, p0+p1, p0+p1+p2 ...                       `     ``accumulate([1,2,3,4,5]) --> 1 3 6 10 15``
 :func:`chain`           p, q, ...                       p0, p1, ... plast, q0, q1, ...                      ``chain('ABC', 'DEF') --> A B C D E F``
 :func:`compress`        data, selectors                 (d[0] if s[0]), (d[1] if s[1]), ...                 ``compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F``
 :func:`dropwhile`       pred, seq                       seq[n], seq[n+1], starting when pred fails          ``dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1``
@@ -83,6 +84,21 @@ The following module functions all construct and return iterators. Some provide
 streams of infinite length, so they should only be accessed by functions or
 loops that truncate the stream.
 
+.. function:: accumulate(iterable, start=0)
+
+    Make an iterator that returns accumulated sums plus the value of the *start*
+    parameter (which defaults to :const:`0`). Elements may be any addable type
+    including :class:`Decimal` or :class:`Fraction`.  Equivalent to::
+
+        def accumulate(iterable, start=0):
+            'Return running totals'
+                # accumulate([1,2,3,4,5]) --> 1 3 6 10 15
+                total = start
+                for element in iterable:
+                    total += element
+                    yield total
+
+    .. versionadded:: 3.2
 
 .. function:: chain(*iterables)
 
@@ -653,14 +669,6 @@ which incur interpreter overhead.
                 pending -= 1
                 nexts = cycle(islice(nexts, pending))
 
-   def accumulate(iterable):
-       'Emit a running total'
-       # accumulate([1,2,3,4,5]) --> 1 3 6 10 15
-       total = 0
-       for element in iterable:
-           total += element
-           yield total
-
     def partition(pred, iterable):
         'Use a predicate to partition entries into false entries and true entries'
         # partition(is_odd, range(10)) --> 0 2 4 6 8   and  1 3 5 7 9