Merge pull request #253 from llllllllll/scanl

ENH: Adds `start` argument to accumulate

toolz/itertoolz.py

@@ -3,8 +3,8 @@
 import collections
 import operator
 from functools import partial
-from toolz.compatibility import (map, filter, filterfalse, zip, zip_longest,
-                                 iteritems)
+from toolz.compatibility import (map, filterfalse, zip, zip_longest, iteritems)
+from toolz.utils import no_default
 
 
 __all__ = ('remove', 'accumulate', 'groupby', 'merge_sorted', 'interleave',
@@ -26,7 +26,7 @@ def remove(predicate, seq):
     return filterfalse(predicate, seq)
 
 
-def accumulate(binop, seq):
+def accumulate(binop, seq, initial=no_default):
     """ Repeatedly apply binary function to a sequence, accumulating results
 
     >>> from operator import add, mul
@@ -42,11 +42,19 @@ def accumulate(binop, seq):
     >>> sum    = partial(reduce, add)
     >>> cumsum = partial(accumulate, add)
 
+    Accumulate also takes an optional argument that will be used as the first
+    value. This is similar to reduce.
+
+    >>> list(accumulate(add, [1, 2, 3], -1))
+    [-1, 0, 2, 5]
+    >>> list(accumulate(add, [], 1))
+    [1]
+
     See Also:
         itertools.accumulate :  In standard itertools for Python 3.2+
     """
     seq = iter(seq)
-    result = next(seq)
+    result = next(seq) if initial is no_default else initial
     yield result
     for elem in seq:
         result = binop(result, elem)
@@ -343,9 +351,6 @@ def last(seq):
 rest = partial(drop, 1)
 
 
-no_default = '__no__default__'
-
-
 def _get(ind, seq, default):
     try:
         return seq[ind]

toolz/tests/test_itertoolz.py

@@ -266,6 +266,13 @@ def test_iterate():
 def test_accumulate():
     assert list(accumulate(add, [1, 2, 3, 4, 5])) == [1, 3, 6, 10, 15]
     assert list(accumulate(mul, [1, 2, 3, 4, 5])) == [1, 2, 6, 24, 120]
+    assert list(accumulate(add, [1, 2, 3, 4, 5], -1)) == [-1, 0, 2, 5, 9, 14]
+
+    def binop(a, b):
+        raise AssertionError('binop should not be called')
+
+    start = object()
+    assert list(accumulate(binop, [], start)) == [start]
 
 
 def test_accumulate_works_on_consumable_iterables():