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

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){