Add support for unary plus and unary minus to collections.Counter()

2 files changed, 16 insertions, 0 deletions

diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index 03bd2b3..2007e68 100644
--- a/Lib/collections/__init__.py
+++ b/Lib/collections/__init__.py
@@ -672,6 +672,17 @@ class Counter(dict):
                 result[elem] = newcount
         return result
 
+    def __pos__(self):
+        'Adds an empty counter, effectively stripping negative and zero counts'
+        return self + Counter()
+
+    def __neg__(self):
+        '''Subtracts from an empty counter.  Strips positive and zero counts,
+        and flips the sign on negative counts.
+
+        '''
+        return Counter() - self
+
 
 ########################################################################
 ###  ChainMap (helper for configparser and string.Template)
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index eb6d4d7..04c4d97 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -943,6 +943,11 @@ class TestCounter(unittest.TestCase):
         c.subtract('aaaabbcce')
         self.assertEqual(c, Counter(a=-1, b=0, c=-1, d=1, e=-1))
 
+    def test_unary(self):
+        c = Counter(a=-5, b=0, c=5, d=10, e=15,g=40)
+        self.assertEqual(dict(+c), dict(c=5, d=10, e=15, g=40))
+        self.assertEqual(dict(-c), dict(a=5))
+
     def test_helper_function(self):
         # two paths, one for real dicts and one for other mappings
         elems = list('abracadabra')