gh-151907: Avoid creating temporary objects in some list comprehensions (GH-151908)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Author: ZeroIntensity

Lib/test/test_compile.py

@@ -2063,7 +2063,7 @@ def test_multiline_async_generator_expression(self):
 
     def test_multiline_list_comprehension(self):
         snippet = textwrap.dedent("""\
-            [(x,
+            _ = [(x,
                 2*x)
                 for x
                 in [1,2,3] if (x > 0
@@ -2073,14 +2073,14 @@ def test_multiline_list_comprehension(self):
         compiled_code, _ = self.check_positions_against_ast(snippet)
         self.assertIsInstance(compiled_code, types.CodeType)
         self.assertOpcodeSourcePositionIs(compiled_code, 'LIST_APPEND',
-            line=1, end_line=2, column=1, end_column=8, occurrence=1)
+            line=1, end_line=2, column=5, end_column=8, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
-            line=1, end_line=2, column=1, end_column=8, occurrence=1)
+            line=1, end_line=2, column=5, end_column=8, occurrence=1)
 
     def test_multiline_async_list_comprehension(self):
         snippet = textwrap.dedent("""\
             async def f():
-                [(x,
+                _ = [(x,
                     2*x)
                     async for x
                     in [1,2,3] if (x > 0
@@ -2093,11 +2093,11 @@ async def f():
         compiled_code = g['f'].__code__
         self.assertIsInstance(compiled_code, types.CodeType)
         self.assertOpcodeSourcePositionIs(compiled_code, 'LIST_APPEND',
-            line=2, end_line=3, column=5, end_column=12, occurrence=1)
+            line=2, end_line=3, column=9, end_column=12, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
-            line=2, end_line=3, column=5, end_column=12, occurrence=1)
+            line=2, end_line=3, column=9, end_column=12, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_VALUE',
-            line=2, end_line=7, column=4, end_column=36, occurrence=1)
+            line=2, end_line=2, column=4, end_column=5, occurrence=1)
 
     def test_multiline_set_comprehension(self):
         snippet = textwrap.dedent("""\

Lib/test/test_compiler_codegen.py

@@ -216,3 +216,39 @@ def test_frozenset_optimization(self):
             ('RETURN_VALUE', None)
         ]
         self.codegen_test(snippet, expected)
+
+    def test_comp_without_target_optimization(self):
+        snippet = "[i for i in range(10)]"
+        expected = [
+            ('RESUME', 0),
+            ('ANNOTATIONS_PLACEHOLDER', None),
+            ('LOAD_NAME', 0),
+            ('PUSH_NULL', None),
+            ('LOAD_CONST', 0),
+            ('CALL', 1),
+            ('LOAD_FAST_AND_CLEAR', 0),
+            ('SWAP', 2),
+            ('SETUP_FINALLY', 20),
+            ('COPY', 1),
+            ('GET_ITER', 0),
+            ('FOR_ITER', 16),
+            ('STORE_FAST', 0),
+            ('LOAD_FAST', 0),
+            ('POP_TOP', None),
+            ('JUMP', 11),
+            ('END_FOR', None),
+            ('POP_ITER', None),
+            ('POP_BLOCK', None),
+            ('JUMP_NO_INTERRUPT', 25),
+            ('SWAP', 2),
+            ('POP_TOP', None),
+            ('SWAP', 2),
+            ('STORE_FAST_MAYBE_NULL', 0),
+            ('RERAISE', 0),
+            ('SWAP', 2),
+            ('STORE_FAST_MAYBE_NULL', 0),
+            ('POP_TOP', None),
+            ('LOAD_CONST', 1),
+            ('RETURN_VALUE', None),
+        ]
+        self.codegen_test(snippet, expected)

Lib/test/test_dictcomps.py

@@ -165,6 +165,14 @@ def iter_raises():
                 self.assertEqual(f.line[f.colno - indent : f.end_colno - indent],
                                  expected)
 
+    def test_hash_error(self):
+        class Unhashable:
+            def __hash__(self):
+                0/0
+
+        with self.assertRaises(ZeroDivisionError):
+            {unhashable: 1 for unhashable in [Unhashable()]}
+
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_listcomps.py

@@ -793,6 +793,75 @@ def iter_raises():
                 self.assertEqual(f.line[f.colno - indent : f.end_colno - indent],
                                  expected)
 
+    def test_optimization_with_side_effects(self):
+        # List comprehensions that aren't used as a value are optimized
+        # to avoid creating a list. Ensure that side effects are still
+        # retained when this happens.
+        with self.assertRaises(ZeroDivisionError):
+            [0/0 for _ in [1]]
+
+        count = 0
+        def increment():
+            nonlocal count
+            count += 1
+
+        [increment() for _ in range(5)]
+        self.assertEqual(count, 5)
+
+    def test_async_optimization_with_side_effects(self):
+        async def gen1(aiterator):
+            with self.assertRaises(ZeroDivisionError):
+                [0/0 async for _ in aiterator]
+
+        async def gen2(aiterator):
+            [increment() async for _ in aiterator]
+
+        async def numbers():
+            for i in range(5):
+                yield i
+
+        count = 0
+        def increment():
+            nonlocal count
+            count += 1
+
+        def exhaust(coro):
+            try:
+                coro.send(None)
+            except StopIteration:
+                pass
+
+        exhaust(gen1(numbers()))
+        exhaust(gen2(numbers()))
+        self.assertEqual(count, 5)
+
+    def test_optimization_with_starred_unpack(self):
+        with self.assertRaises(TypeError):
+            [*i for i in [1, 2, 3]]
+
+        async def coro():
+            async def gen():
+                yield 1
+
+            with self.assertRaises(TypeError):
+                [*i async for i in gen()]
+
+        c = coro()
+        while True:
+            try:
+                c.send(None)
+            except StopIteration:
+                break
+
+        count = 0
+        def weird():
+            nonlocal count
+            count += 1
+            yield 0
+
+        [*weird() for _ in range(5)]
+        self.assertEqual(count, 5)
+
 __test__ = {'doctests' : doctests}
 
 def load_tests(loader, tests, pattern):

Lib/test/test_setcomps.py

@@ -188,6 +188,18 @@ def iter_raises():
                 self.assertEqual(f.line[f.colno - indent : f.end_colno - indent],
                                  expected)
 
+    def test_hash_error(self):
+        class Unhashable:
+            def __hash__(self):
+                0/0
+
+        with self.assertRaises(ZeroDivisionError):
+            {unhashable for unhashable in [Unhashable()]}
+
+
+if __name__ == "__main__":
+    unittest.main()
+
 __test__ = {'doctests' : doctests}
 
 def load_tests(loader, tests, pattern):

Misc/NEWS.d/next/Core_and_Builtins/2026-06-22-02-42-38.gh-issue-151907.6Tol_J.rst

@@ -0,0 +1,2 @@
+Avoid creating :class:`list` objects in comprehensions when the comprehension
+is not used as a value.