Issue #13121: Support in-place math operators for collections.Counter().

1 files changed, 21 insertions, 0 deletions

diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index 04c4d97c55..ec20938917 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -932,6 +932,27 @@ class TestCounter(unittest.TestCase):
                 set_result = setop(set(p.elements()), set(q.elements()))
                 self.assertEqual(counter_result, dict.fromkeys(set_result, 1))
 
+    def test_inplace_operations(self):
+        elements = 'abcd'
+        for i in range(1000):
+            # test random pairs of multisets
+            p = Counter(dict((elem, randrange(-2,4)) for elem in elements))
+            p.update(e=1, f=-1, g=0)
+            q = Counter(dict((elem, randrange(-2,4)) for elem in elements))
+            q.update(h=1, i=-1, j=0)
+            for inplace_op, regular_op in [
+                (Counter.__iadd__, Counter.__add__),
+                (Counter.__isub__, Counter.__sub__),
+                (Counter.__ior__, Counter.__or__),
+                (Counter.__iand__, Counter.__and__),
+            ]:
+                c = p.copy()
+                c_id = id(c)
+                regular_result = regular_op(c, q)
+                inplace_result = inplace_op(c, q)
+                self.assertEqual(inplace_result, regular_result)
+                self.assertEqual(id(inplace_result), c_id)
+
     def test_subtract(self):
         c = Counter(a=-5, b=0, c=5, d=10, e=15,g=40)
         c.subtract(a=1, b=2, c=-3, d=10, e=20, f=30, h=-50)