# HG changeset patch
# User Lars Buitinck <L.J.Buitinck@uva.nl>
# Date 1317977368 -7200
# Node ID f023dcafaf3b96ca4c2d1ab0c55ea1ae71dc0e34
# Parent  9c1b76936b7941c745ef157777e833106cb5b7ac
Added __iadd__ to collections.Counter for faster +=

Changes behavior: after

    from collections import Counter
    a = Counter([1,2,3])
    b = a
    a += Counter([3,4,5])

it used to be that (a is not b), but now (a is b).

diff -r 9c1b76936b79 -r f023dcafaf3b Lib/collections/__init__.py
--- a/Lib/collections/__init__.py	Fri Oct 07 10:01:28 2011 +0200
+++ b/Lib/collections/__init__.py	Fri Oct 07 10:49:28 2011 +0200
@@ -604,17 +604,27 @@
         Counter({'b': 4, 'c': 2, 'a': 1})
 
         '''
+        result = Counter()
+        result += self
+        result += other
+        return result
+
+    def __iadd__(self, other):
+        '''Add counts from other counter.
+
+        Like update, but only adds Counters and takes no keyword arguments.
+
+        >>> c = Counter('abbb')
+        >>> c += Counter('bcc')
+        Counter({'b': 4, 'c': 2, 'a': 1})
+
+        '''
         if not isinstance(other, Counter):
             return NotImplemented
-        result = Counter()
-        for elem, count in self.items():
-            newcount = count + other[elem]
-            if newcount > 0:
-                result[elem] = newcount
         for elem, count in other.items():
-            if elem not in self and count > 0:
-                result[elem] = count
-        return result
+            if count > 0:
+                self[elem] += count
+        return self
 
     def __sub__(self, other):
         ''' Subtract count, but keep only results with positive counts.