gh-97933: inline list/dict/set comprehensions

Doc/library/dis.rst

@@ -1196,6 +1196,14 @@ iterations of the loop.
 
    .. versionadded:: 3.12
 
+.. opcode:: LOAD_FAST_AND_CLEAR (var_num)
+
+   Pushes a reference to the local ``co_varnames[var_num]`` onto the stack (or
+   pushes ``NULL`` onto the stack if the local variable has not been
+   initialized) and sets ``co_varnames[var_num]`` to ``NULL``.
+
+   .. versionadded:: 3.12
+
 .. opcode:: STORE_FAST (var_num)
 
    Stores ``STACK.pop()`` into the local ``co_varnames[var_num]``.

Doc/whatsnew/3.12.rst

@@ -153,6 +153,30 @@ New Features
   In Python 3.14, the default will switch to ``'data'``.
   (Contributed by Petr Viktorin in :pep:`706`.)
 
+.. _whatsnew312-pep709:
+
+PEP 709: Comprehension inlining
+-------------------------------
+
+Dictionary, list, and set comprehensions are now inlined, rather than creating a
+new single-use function object for each execution of the comprehension. This
+speeds up execution of a comprehension by up to 2x.
+
+Comprehension iteration variables remain isolated; they don't overwrite a
+variable of the same name in the outer scope, nor are they visible after the
+comprehension. This isolation is now maintained via stack/locals manipulation,
+not via separate function scope.
+
+Inlining does result in a few visible behavior changes:
+
+* There is no longer a separate frame for the comprehension in tracebacks,
+  and tracing/profiling no longer shows the comprehension as a function call.
+* Calling :func:`locals` inside a comprehension now includes variables
+  from outside the comprehension, and no longer includes the synthetic `.0`
+  variable for the comprehension "argument".
+
+Contributed by Carl Meyer and Vladimir Matveev in :pep:`709`.
+
 PEP 688: Making the buffer protocol accessible in Python
 --------------------------------------------------------
 

Include/internal/pycore_code.h

@@ -131,6 +131,7 @@ struct callable_cache {
 // Note that these all fit within a byte, as do combinations.
 // Later, we will use the smaller numbers to differentiate the different
 // kinds of locals (e.g. pos-only arg, varkwargs, local-only).
+#define CO_FAST_HIDDEN  0x10
 #define CO_FAST_LOCAL   0x20
 #define CO_FAST_CELL    0x40
 #define CO_FAST_FREE    0x80

Include/internal/pycore_compile.h

@@ -70,6 +70,9 @@ typedef struct {
     PyObject *u_varnames;  /* local variables */
     PyObject *u_cellvars;  /* cell variables */
     PyObject *u_freevars;  /* free variables */
+    PyObject *u_fasthidden; /* dict; keys are names that are fast-locals only
+                               temporarily within an inlined comprehension. When
+                               value is True, treat as fast-local. */
 
     Py_ssize_t u_argcount;        /* number of arguments for block */
     Py_ssize_t u_posonlyargcount;        /* number of positional only arguments for block */

Include/internal/pycore_flowgraph.h

@@ -94,7 +94,7 @@ _PyCfgInstruction* _PyCfg_BasicblockLastInstr(const _PyCfgBasicblock *b);
 int _PyCfg_OptimizeCodeUnit(_PyCfgBuilder *g, PyObject *consts, PyObject *const_cache,
                             int code_flags, int nlocals, int nparams, int firstlineno);
 int _PyCfg_Stackdepth(_PyCfgBasicblock *entryblock, int code_flags);
-void _PyCfg_ConvertExceptionHandlersToNops(_PyCfgBasicblock *entryblock);
+void _PyCfg_ConvertPseudoOps(_PyCfgBasicblock *entryblock);
 int _PyCfg_ResolveJumps(_PyCfgBuilder *g);
 
 

Include/internal/pycore_symtable.h

@@ -64,6 +64,7 @@ typedef struct _symtable_entry {
     unsigned ste_needs_class_closure : 1; /* for class scopes, true if a
                                              closure over __class__
                                              should be created */
+    unsigned ste_comp_inlined : 1; /* true if this comprehension is inlined */
     unsigned ste_comp_iter_target : 1; /* true if visiting comprehension target */
     int ste_comp_iter_expr; /* non-zero if visiting a comprehension range expression */
     int ste_lineno;          /* first line of block */

Lib/importlib/_bootstrap_external.py

@@ -442,6 +442,7 @@ def _write_atomic(path, data, mode=0o666):
 #     Python 3.12b1 3526 (Add instrumentation support)
 #     Python 3.12b1 3527 (Add LOAD_SUPER_ATTR)
 #     Python 3.12b1 3528 (Add LOAD_SUPER_ATTR_METHOD specialization)
+#     Python 3.12b1 3529 (Inline list/dict/set comprehensions)
 
 #     Python 3.13 will start with 3550
 
@@ -458,7 +459,7 @@ def _write_atomic(path, data, mode=0o666):
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
 
-MAGIC_NUMBER = (3528).to_bytes(2, 'little') + b'\r\n'
+MAGIC_NUMBER = (3529).to_bytes(2, 'little') + b'\r\n'
 
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c
 

Lib/opcode.py

@@ -198,6 +198,8 @@ def pseudo_op(name, op, real_ops):
 jrel_op('JUMP_BACKWARD', 140)    # Number of words to skip (backwards)
 name_op('LOAD_SUPER_ATTR', 141)
 def_op('CALL_FUNCTION_EX', 142)  # Flags
+def_op('LOAD_FAST_AND_CLEAR', 143)  # Local variable number
+haslocal.append(143)
 
 def_op('EXTENDED_ARG', 144)
 EXTENDED_ARG = 144
@@ -268,6 +270,8 @@ def pseudo_op(name, op, real_ops):
 pseudo_op('LOAD_ZERO_SUPER_METHOD', 264, ['LOAD_SUPER_ATTR'])
 pseudo_op('LOAD_ZERO_SUPER_ATTR', 265, ['LOAD_SUPER_ATTR'])
 
+pseudo_op('STORE_FAST_MAYBE_NULL', 266, ['STORE_FAST'])
+
 MAX_PSEUDO_OPCODE = MIN_PSEUDO_OPCODE + len(_pseudo_ops) - 1
 
 del def_op, name_op, jrel_op, jabs_op, pseudo_op

Lib/test/test_compile.py

@@ -1352,14 +1352,11 @@ def test_multiline_list_comprehension(self):
                                and x != 50)]
             """)
         compiled_code, _ = self.check_positions_against_ast(snippet)
-        compiled_code = compiled_code.co_consts[0]
         self.assertIsInstance(compiled_code, types.CodeType)
         self.assertOpcodeSourcePositionIs(compiled_code, 'LIST_APPEND',
             line=1, end_line=2, column=1, end_column=8, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
             line=1, end_line=2, column=1, end_column=8, occurrence=1)
-        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_VALUE',
-            line=1, end_line=6, column=0, end_column=32, occurrence=1)
 
     def test_multiline_async_list_comprehension(self):
         snippet = textwrap.dedent("""\
@@ -1374,13 +1371,13 @@ async def f():
         compiled_code, _ = self.check_positions_against_ast(snippet)
         g = {}
         eval(compiled_code, g)
-        compiled_code = g['f'].__code__.co_consts[1]
+        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)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
             line=2, end_line=3, column=5, end_column=12, occurrence=1)
-        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_VALUE',
+        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_CONST',
             line=2, end_line=7, column=4, end_column=36, occurrence=1)
 
     def test_multiline_set_comprehension(self):
@@ -1393,14 +1390,11 @@ def test_multiline_set_comprehension(self):
                                and x != 50)}
             """)
         compiled_code, _ = self.check_positions_against_ast(snippet)
-        compiled_code = compiled_code.co_consts[0]
         self.assertIsInstance(compiled_code, types.CodeType)
         self.assertOpcodeSourcePositionIs(compiled_code, 'SET_ADD',
             line=1, end_line=2, column=1, end_column=8, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
             line=1, end_line=2, column=1, end_column=8, occurrence=1)
-        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_VALUE',
-            line=1, end_line=6, column=0, end_column=32, occurrence=1)
 
     def test_multiline_async_set_comprehension(self):
         snippet = textwrap.dedent("""\
@@ -1415,13 +1409,13 @@ async def f():
         compiled_code, _ = self.check_positions_against_ast(snippet)
         g = {}
         eval(compiled_code, g)
-        compiled_code = g['f'].__code__.co_consts[1]
+        compiled_code = g['f'].__code__
         self.assertIsInstance(compiled_code, types.CodeType)
         self.assertOpcodeSourcePositionIs(compiled_code, 'SET_ADD',
             line=2, end_line=3, column=5, end_column=12, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
             line=2, end_line=3, column=5, end_column=12, occurrence=1)
-        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_VALUE',
+        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_CONST',
             line=2, end_line=7, column=4, end_column=36, occurrence=1)
 
     def test_multiline_dict_comprehension(self):
@@ -1434,14 +1428,11 @@ def test_multiline_dict_comprehension(self):
                                and x != 50)}
             """)
         compiled_code, _ = self.check_positions_against_ast(snippet)
-        compiled_code = compiled_code.co_consts[0]
         self.assertIsInstance(compiled_code, types.CodeType)
         self.assertOpcodeSourcePositionIs(compiled_code, 'MAP_ADD',
             line=1, end_line=2, column=1, end_column=7, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
             line=1, end_line=2, column=1, end_column=7, occurrence=1)
-        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_VALUE',
-            line=1, end_line=6, column=0, end_column=32, occurrence=1)
 
     def test_multiline_async_dict_comprehension(self):
         snippet = textwrap.dedent("""\
@@ -1456,13 +1447,13 @@ async def f():
         compiled_code, _ = self.check_positions_against_ast(snippet)
         g = {}
         eval(compiled_code, g)
-        compiled_code = g['f'].__code__.co_consts[1]
+        compiled_code = g['f'].__code__
         self.assertIsInstance(compiled_code, types.CodeType)
         self.assertOpcodeSourcePositionIs(compiled_code, 'MAP_ADD',
             line=2, end_line=3, column=5, end_column=11, occurrence=1)
         self.assertOpcodeSourcePositionIs(compiled_code, 'JUMP_BACKWARD',
             line=2, end_line=3, column=5, end_column=11, occurrence=1)
-        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_VALUE',
+        self.assertOpcodeSourcePositionIs(compiled_code, 'RETURN_CONST',
             line=2, end_line=7, column=4, end_column=36, occurrence=1)
 
     def test_matchcase_sequence(self):
@@ -1711,9 +1702,6 @@ def test_column_offset_deduplication(self):
         for source in [
             "lambda: a",
             "(a for b in c)",
-            "[a for b in c]",
-            "{a for b in c}",
-            "{a: b for c in d}",
         ]:
             with self.subTest(source):
                 code = compile(f"{source}, {source}", "<test>", "eval")

Lib/test/test_compiler_assemble.py

@@ -16,7 +16,7 @@ def complete_metadata(self, metadata, filename="myfile.py"):
             metadata.setdefault(key, key)
         for key in ['consts']:
             metadata.setdefault(key, [])
-        for key in ['names', 'varnames', 'cellvars', 'freevars']:
+        for key in ['names', 'varnames', 'cellvars', 'freevars', 'fasthidden']:
             metadata.setdefault(key, {})
         for key in ['argcount', 'posonlyargcount', 'kwonlyargcount']:
             metadata.setdefault(key, 0)
@@ -33,6 +33,9 @@ def assemble_test(self, insts, metadata, expected):
 
         expected_metadata = {}
         for key, value in metadata.items():
+            if key == "fasthidden":
+                # not exposed on code object
+                continue
             if isinstance(value, list):
                 expected_metadata[key] = tuple(value)
             elif isinstance(value, dict):