GH-127682: Only call __iter__ once in generator expressions. (GH-132351)

Lib/test/test_generators.py

@@ -268,6 +268,28 @@ class GeneratorTest(unittest.TestCase):
         #This should not raise
         loop()
 
+    def test_genexpr_only_calls_dunder_iter_once(self):
+
+        class Iterator:
+
+            def __init__(self):
+                self.val = 0
+
+            def __next__(self):
+                if self.val == 2:
+                    raise StopIteration
+                self.val += 1
+                return self.val
+
+            # No __iter__ method
+
+        class C:
+
+            def __iter__(self):
+                return Iterator()
+
+        self.assertEqual([1,2], list(i for i in C()))
+
 
 class ModifyUnderlyingIterableTest(unittest.TestCase):
     iterables = [