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

Author: markshannon

Lib/test/test_dis.py

@@ -204,7 +204,6 @@ def bug1333982(x=[]):
               LOAD_CONST               1 (<code object <genexpr> at 0x..., file "%s", line %d>)
               MAKE_FUNCTION
               LOAD_FAST_BORROW         0 (x)
-              GET_ITER
               CALL                     0
 
 %3d           LOAD_SMALL_INT           1
@@ -821,7 +820,6 @@ def foo(x):
                MAKE_FUNCTION
                SET_FUNCTION_ATTRIBUTE   8 (closure)
                LOAD_DEREF               1 (y)
-               GET_ITER
                CALL                     0
                CALL                     1
                RETURN_VALUE

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 = [

Lib/test/test_genexps.py

@@ -123,15 +123,6 @@
     >>> list(g)
     [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
 
-Verify that the outermost for-expression makes an immediate check
-for iterability
-
-    >>> (i for i in 6)
-    Traceback (most recent call last):
-      File "<pyshell#4>", line 1, in -toplevel-
-        (i for i in 6)
-    TypeError: 'int' object is not iterable
-
 Verify late binding for the outermost if-expression
 
     >>> include = (2,4,6,8)

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 from a non-interable will raise only when the
+generator expression is executed.
+This brings the behavior of generator expressions in line with other generators.

Python/codegen.c

@@ -4775,10 +4775,7 @@ codegen_comprehension(compiler *c, expr_ty e, int type,
     }
     Py_CLEAR(co);
 
-    if (codegen_comprehension_iter(c, outermost)) {
-        goto error;
-    }
-
+    VISIT(c, expr, outermost->iter);
     ADDOP_I(c, loc, CALL, 0);
 
     if (is_async_comprehension && type != COMP_GENEXP) {