Add a subtract() method to collections.Counter()

Lib/collections.py

@@ -436,6 +436,34 @@ class Counter(dict):
         if kwds:
             self.update(kwds)
 
+    def subtract(self, iterable=None, **kwds):
+        '''Like dict.update() but subtracts counts instead of replacing them.
+        Counts can be reduced below zero.  Both the inputs and outputs are
+        allowed to contain zero and negative counts.
+
+        Source can be an iterable, a dictionary, or another Counter instance.
+
+        >>> c = Counter('which')
+        >>> c.subtract('witch')             # subtract elements from another iterable
+        >>> c.subtract(Counter('watch'))    # subtract elements from another counter
+        >>> c['h']                          # 2 in which, minus 1 in witch, minus 1 in watch
+        0
+        >>> c['w']                          # 1 in which, minus 1 in witch, minus 1 in watch
+        -1
+
+        '''
+        if iterable is not None:
+            if isinstance(iterable, Mapping):
+                self_get = self.get
+                for elem, count in iterable.items():
+                    self[elem] = self_get(elem, 0) - count
+            else:
+                self_get = self.get
+                for elem in iterable:
+                    self[elem] = self_get(elem, 0) - 1
+        if kwds:
+            self.subtract(kwds)
+
     def copy(self):
         'Like dict.copy() but returns a Counter instance instead of a dict.'
         return Counter(self)