gh-127274: Defer nested methods (#128012)

Methods (functions defined in class scope) are likely to be cleaned
up by the GC anyway.

Add a new code flag, `CO_METHOD`, that is set for functions defined
in a class scope. Use that when deciding to defer functions.

Author: mpage

Doc/library/inspect.rst

@@ -1708,6 +1708,13 @@ which is a bitmap of the following flags:
 
    .. versionadded:: 3.14
 
+.. data:: CO_METHOD
+
+   The flag is set when the code object is a function defined in class
+   scope.
+
+   .. versionadded:: 3.14
+
 .. note::
    The flags are specific to CPython, and may not be defined in other
    Python implementations.  Furthermore, the flags are an implementation

Include/cpython/code.h

@@ -199,6 +199,9 @@ struct PyCodeObject _PyCode_DEF(1);
 */
 #define CO_HAS_DOCSTRING 0x4000000
 
+/* A function defined in class scope */
+#define CO_METHOD  0x8000000
+
 /* This should be defined if a future statement modifies the syntax.
    For example, when a keyword is added.
 */

Include/internal/pycore_symtable.h

@@ -124,6 +124,7 @@ typedef struct _symtable_entry {
     unsigned ste_can_see_class_scope : 1; /* true if this block can see names bound in an
                                              enclosing class scope */
     unsigned ste_has_docstring : 1; /* true if docstring present */
+    unsigned ste_method : 1; /* true if block is a function block defined in class scope */
     int ste_comp_iter_expr; /* non-zero if visiting a comprehension range expression */
     _Py_SourceLocation ste_loc; /* source location of block */
     struct _symtable_entry *ste_annotation_block; /* symbol table entry for this entry's annotations */

Lib/dis.py

@@ -162,6 +162,7 @@ def distb(tb=None, *, file=None, show_caches=False, adaptive=False, show_offsets
           256: "ITERABLE_COROUTINE",
           512: "ASYNC_GENERATOR",
     0x4000000: "HAS_DOCSTRING",
+    0x8000000: "METHOD",
 }
 
 def pretty_flags(flags):

Lib/inspect.py

@@ -57,6 +57,7 @@
     "CO_VARARGS",
     "CO_VARKEYWORDS",
     "CO_HAS_DOCSTRING",
+    "CO_METHOD",
     "ClassFoundException",
     "ClosureVars",
     "EndOfBlock",

Lib/test/test_monitoring.py

@@ -850,12 +850,6 @@ def __init__(self, events):
     def __call__(self, code, offset, val):
         self.events.append(("return", code.co_name, val))
 
-# gh-127274: CALL_ALLOC_AND_ENTER_INIT will only cache __init__ methods that
-# are deferred. We only defer functions defined at the top-level.
-class ValueErrorRaiser:
-    def __init__(self):
-        raise ValueError()
-
 
 class ExceptionMonitoringTest(CheckEvents):
 
@@ -1054,6 +1048,9 @@ def func():
 
     @requires_specialization_ft
     def test_no_unwind_for_shim_frame(self):
+        class ValueErrorRaiser:
+            def __init__(self):
+                raise ValueError()
 
         def f():
             try:

Lib/test/test_opcache.py

@@ -493,13 +493,6 @@ def f():
             self.assertFalse(f())
 
 
-# gh-127274: CALL_ALLOC_AND_ENTER_INIT will only cache __init__ methods that
-# are deferred. We only defer functions defined at the top-level.
-class MyClass:
-    def __init__(self):
-        pass
-
-
 class InitTakesArg:
     def __init__(self, arg):
         self.arg = arg
@@ -536,6 +529,10 @@ def f(x, y):
     @disabling_optimizer
     @requires_specialization_ft
     def test_assign_init_code(self):
+        class MyClass:
+            def __init__(self):
+                pass
+
         def instantiate():
             return MyClass()
 

Misc/NEWS.d/next/Core_and_Builtins/2024-12-17-13-45-33.gh-issue-127274.deNxNC.rst

@@ -0,0 +1,3 @@
+Add a new flag, ``CO_METHOD``, to :attr:`~codeobject.co_flags` that
+indicates whether the code object belongs to a function defined in class
+scope.

Objects/funcobject.c

@@ -210,10 +210,14 @@ PyFunction_NewWithQualName(PyObject *code, PyObject *globals, PyObject *qualname
     op->func_typeparams = NULL;
     op->vectorcall = _PyFunction_Vectorcall;
     op->func_version = FUNC_VERSION_UNSET;
-    if ((code_obj->co_flags & CO_NESTED) == 0) {
+    if (((code_obj->co_flags & CO_NESTED) == 0) ||
+        (code_obj->co_flags & CO_METHOD)) {
         // Use deferred reference counting for top-level functions, but not
         // nested functions because they are more likely to capture variables,
         // which makes prompt deallocation more important.
+        //
+        // Nested methods (functions defined in class scope) are also deferred,
+        // since they will likely be cleaned up by GC anyway.
         _PyObject_SetDeferredRefcount((PyObject *)op);
     }
     _PyObject_GC_TRACK(op);

Python/compile.c

@@ -1289,6 +1289,8 @@ compute_code_flags(compiler *c)
             flags |= CO_VARKEYWORDS;
         if (ste->ste_has_docstring)
             flags |= CO_HAS_DOCSTRING;
+        if (ste->ste_method)
+            flags |= CO_METHOD;
     }
 
     if (ste->ste_coroutine && !ste->ste_generator) {

Python/symtable.c

@@ -138,6 +138,13 @@ ste_new(struct symtable *st, identifier name, _Py_block_ty block,
 
     ste->ste_has_docstring = 0;
 
+    ste->ste_method = 0;
+    if (st->st_cur != NULL &&
+        st->st_cur->ste_type == ClassBlock &&
+        block == FunctionBlock) {
+        ste->ste_method = 1;
+    }
+
     ste->ste_symbols = PyDict_New();
     ste->ste_varnames = PyList_New(0);
     ste->ste_children = PyList_New(0);