[3.13] gh-128161: Remove redundant GET_ITER from list comprehension code (backport of GH-134778) (GH-134788)

Author: markshannon

Lib/test/test_coroutines.py

@@ -2252,6 +2252,31 @@ def c():
         # before fixing, visible stack from throw would be shorter than from send.
         self.assertEqual(len_send, len_throw)
 
+    def test_call_aiter_once_in_comprehension(self):
+
+        class Iterator:
+
+            def __init__(self):
+                self.val = 0
+
+            async def __anext__(self):
+                if self.val == 2:
+                    raise StopAsyncIteration
+                self.val += 1
+                return self.val
+
+            # No __aiter__ method
+
+        class C:
+
+            def __aiter__(self):
+                return Iterator()
+
+        async def run():
+            return [i async for i in C()]
+
+        self.assertEqual(run_async(run()), ([], [1,2]))
+
 
 @unittest.skipIf(
     support.is_emscripten or support.is_wasi,

Lib/test/test_listcomps.py

@@ -750,6 +750,28 @@ def iter_raises():
                 self.assertEqual(f.line[f.colno - indent : f.end_colno - indent],
                                  expected)
 
+    def test_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], [i for i in C()])
+
 __test__ = {'doctests' : doctests}
 
 def load_tests(loader, tests, pattern):

Misc/NEWS.d/next/Core_and_Builtins/2025-05-27-09-19-21.gh-issue-127682.9WwFrM.rst

@@ -0,0 +1,2 @@
+No longer call ``__iter__`` twice in list comprehensions. This brings the
+behavior of list comprehensions in line with other forms of iteration

Python/compile.c

@@ -5501,9 +5501,9 @@ compiler_async_comprehension_generator(struct compiler *c, location loc,
         else {
             /* Sub-iter - calculate on the fly */
             VISIT(c, expr, gen->iter);
-            ADDOP(c, LOC(gen->iter), GET_AITER);
         }
     }
+    ADDOP(c, LOC(gen->iter), GET_AITER);
 
     USE_LABEL(c, start);
     /* Runtime will push a block here, so we need to account for that */
@@ -5790,19 +5790,6 @@ pop_inlined_comprehension_state(struct compiler *c, location loc,
     return SUCCESS;
 }
 
-static inline int
-compiler_comprehension_iter(struct compiler *c, comprehension_ty comp)
-{
-    VISIT(c, expr, comp->iter);
-    if (comp->is_async) {
-        ADDOP(c, LOC(comp->iter), GET_AITER);
-    }
-    else {
-        ADDOP(c, LOC(comp->iter), GET_ITER);
-    }
-    return SUCCESS;
-}
-
 static int
 compiler_comprehension(struct compiler *c, expr_ty e, int type,
                        identifier name, asdl_comprehension_seq *generators, expr_ty elt,
@@ -5824,9 +5811,7 @@ compiler_comprehension(struct compiler *c, expr_ty e, int type,
 
     outermost = (comprehension_ty) asdl_seq_GET(generators, 0);
     if (is_inlined) {
-        if (compiler_comprehension_iter(c, outermost)) {
-            goto error;
-        }
+        VISIT(c, expr, outermost->iter);
         if (push_inlined_comprehension_state(c, loc, entry, &inline_state)) {
             goto error;
         }