Add test and news entry

Author: markshannon

Lib/test/test_generators.py

@@ -268,6 +268,28 @@ def loop():
         #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 = [

Misc/NEWS.d/next/Core_and_Builtins/2025-04-10-10-29-45.gh-issue-127682.X0HoGz.rst

@@ -0,0 +1,4 @@
+No longer call ``__iter__`` twice when creating and executing a generator
+expression. Creating a generator expression with non-interable will only
+raise when the generator expression is exeuted. This brings the behavior of
+generator expressions in line with other generators