gh-156133: Add PyUnstable_InterpreterFrame_GetLocal

Doc/c-api/code.rst

@@ -44,6 +44,15 @@ bound into a function.
       The old name is deprecated, but will remain available until the
       signature changes again.
 
+.. c:function:: PyObject* PyUnstable_Code_GetLocalPlusNames(PyCodeObject *co)
+
+   Return a new :term:`strong reference` to the tuple of names of the local,
+   cell and free variables of a code object.  The tuple is indexed like the
+   *localsplus* array of a frame, so it can be used together with
+   :c:func:`PyUnstable_InterpreterFrame_GetLocal`.
+
+   .. versionadded:: next
+
 .. c:function:: PyCodeObject* PyUnstable_Code_New(int argcount, int kwonlyargcount, int nlocals, int stacksize, int flags, PyObject *code, PyObject *consts, PyObject *names, PyObject *varnames, PyObject *freevars, PyObject *cellvars, PyObject *filename, PyObject *name, PyObject *qualname, int firstlineno, PyObject *linetable, PyObject *exceptiontable)
 
    Return a new code object.  If you need a dummy code object to create a frame,

Doc/c-api/frame.rst

@@ -243,3 +243,17 @@ Unless using :pep:`523`, you will not need this.
    Return the currently executing line number, or -1 if there is no line number.
 
    .. versionadded:: 3.12
+
+
+.. c:function:: int PyUnstable_InterpreterFrame_GetLocal(struct _PyInterpreterFrame *frame, Py_ssize_t index, PyObject **result)
+
+   Retrieve the local variable at *index* in the frame's localsplus array, with
+   cell and free variables unboxed to their contents.  Free variables are
+   resolved from the function closure, so this also works on a frame that has
+   not started executing.
+
+   * On success, store a new :term:`strong reference` in *result* and return ``1``.
+   * If the slot is unset or hidden, store ``NULL`` in *result* and return ``0``.
+   * On error, raise an exception, store ``NULL`` in *result* and return ``-1``.
+
+   .. versionadded:: next

Doc/whatsnew/3.16.rst

@@ -1177,6 +1177,12 @@ New features
 * Add :c:func:`Py_HashBuffer` to the limited C API.
   (Contributed by Victor Stinner in :gh:`148233`.)
 
+* Add :c:func:`PyUnstable_InterpreterFrame_GetLocal` to read a local variable
+  of an internal interpreter frame by its localsplus index, and
+  :c:func:`PyUnstable_Code_GetLocalPlusNames` to get the matching variable
+  names.
+  (Contributed by Guilherme Leobas in :gh:`156133`.)
+
 Porting to Python 3.16
 ----------------------
 

Include/cpython/code.h

@@ -177,6 +177,11 @@ static inline int PyUnstable_Code_GetFirstFree(PyCodeObject *op) {
     return op->co_nlocalsplus - op->co_nfreevars;
 }
 
+static inline PyObject *PyUnstable_Code_GetLocalPlusNames(PyCodeObject *op) {
+    assert(PyCode_Check(op));
+    return Py_NewRef(op->co_localsplusnames);
+}
+
 Py_DEPRECATED(3.13) static inline int PyCode_GetFirstFree(PyCodeObject *op) {
     return PyUnstable_Code_GetFirstFree(op);
 }

Include/cpython/pyframe.h

@@ -35,3 +35,8 @@ PyAPI_FUNC(int) PyUnstable_InterpreterFrame_GetLasti(struct _PyInterpreterFrame
 /* Returns the currently executing line number, or -1 if there is no line number.
  * Does not raise an exception. */
 PyAPI_FUNC(int) PyUnstable_InterpreterFrame_GetLine(struct _PyInterpreterFrame *frame);
+
+/* Returns a new (strong) reference to the local variable at `index` in the
+ * frame's localsplus array. */
+PyAPI_FUNC(int) PyUnstable_InterpreterFrame_GetLocal(
+    struct _PyInterpreterFrame *frame, Py_ssize_t index, PyObject **result);

Lib/test/test_capi/test_misc.py

@@ -2800,6 +2800,92 @@ def test_line(self):
         firstline = self.func.__code__.co_firstlineno
         self.assertEqual(line, firstline + 2)
 
+    # get_frame_locals() returns the caller frame's locals as a name -> value
+    # dict via PyUnstable_InterpreterFrame_GetLocal (one strong reference per
+    # localsplus index).
+    def helper_plain(self, a, b):
+        c = a + b
+        return _testinternalcapi.get_frame_locals()
+
+    def test_get_local_plain(self):
+        d = self.helper_plain(3, 4)
+        self.assertEqual(d['a'], 3)
+        self.assertEqual(d['b'], 4)
+        self.assertEqual(d['c'], 7)
+        self.assertIs(d['self'], self)
+
+    def test_get_local_cell(self):
+        # y is a cell variable of this frame because inner closes over it.
+        y = 100
+
+        def inner():
+            return y
+
+        d = _testinternalcapi.get_frame_locals()
+        self.assertEqual(d['y'], 100)
+        self.assertIs(d['inner'], inner)
+
+    def test_get_local_free(self):
+        # z is a free variable of inner, read from the closure.
+        z = 7
+
+        def inner():
+            _ = z
+            return _testinternalcapi.get_frame_locals()
+
+        d = inner()
+        self.assertEqual(d['z'], 7)
+
+    def test_get_local_index_out_of_range(self):
+        def f(index):
+            return _testinternalcapi.get_frame_local(index)
+
+        # f has no cell or free variables, so co_nlocalsplus == co_nlocals.
+        code = f.__code__
+        self.assertFalse(code.co_cellvars or code.co_freevars)
+        nlocalsplus = code.co_nlocals
+        for index in (-1, nlocalsplus, nlocalsplus + 1):
+            with self.subTest(index=index):
+                with self.assertRaises(IndexError):
+                    f(index)
+
+    def test_get_local_unset(self):
+        def f():
+            if False:
+                unset = 1
+            names = f.__code__.co_varnames
+            return _testinternalcapi.get_frame_local(names.index('unset'))
+
+        self.assertEqual(f(), (0, None))
+
+    def test_get_local_set(self):
+        x = 5
+        index = self.test_get_local_set.__code__.co_varnames.index('x')
+        self.assertEqual(_testinternalcapi.get_frame_local(index), (1, 5))
+
+    def test_code_get_localsplus_names(self):
+        def outer(a, b):
+            c = a
+            def inner():
+                return a, d
+            d = b
+            return inner
+
+        names = _testinternalcapi.code_get_localsplus_names(outer.__code__)
+        self.assertIsInstance(names, tuple)
+        # Arguments and locals come first, followed by cells that are not
+        # arguments; here a and d are cells.
+        self.assertEqual(sorted(names), sorted(['a', 'b', 'c', 'd', 'inner']))
+        self.assertEqual(names[:2], ('a', 'b'))
+
+        inner = outer(1, 2)
+        names = _testinternalcapi.code_get_localsplus_names(inner.__code__)
+        self.assertEqual(names, ('a', 'd'))
+        self.assertEqual(names[inner.__code__.co_nlocals:], ('a', 'd'))
+
+        with self.assertRaises(TypeError):
+            _testinternalcapi.code_get_localsplus_names(None)
+
 
 SUFFICIENT_TO_DEOPT_AND_SPECIALIZE = 100
 

Misc/NEWS.d/next/C_API/2026-08-20-12-00-00.gh-issue-156133.GetLoc.rst

@@ -0,0 +1,3 @@
+Add :c:func:`PyUnstable_InterpreterFrame_GetLocal` to read a local variable of
+an internal interpreter frame by its localsplus index, and
+:c:func:`PyUnstable_Code_GetLocalPlusNames` to get the matching variable names.

Modules/_testinternalcapi.c

@@ -1535,6 +1535,87 @@ iframe_getlasti(PyObject *self, PyObject *frame)
     return PyLong_FromLong(PyUnstable_InterpreterFrame_GetLasti(f));
 }
 
+// Reads the locals of the Python frame that called this C function using
+// PyUnstable_InterpreterFrame_GetLocal and returns them as a name -> value
+// dict, skipping NULL (unset or hidden) slots.
+static PyObject *
+get_frame_locals(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    _PyInterpreterFrame *frame = _PyThreadState_GetFrame(tstate);
+    if (frame == NULL) {
+        PyErr_SetString(PyExc_RuntimeError, "no caller frame");
+        return NULL;
+    }
+    PyCodeObject *co = _PyFrame_GetCode(frame);
+    Py_ssize_t n = co->co_nlocalsplus;
+    PyObject *dict = PyDict_New();
+    if (dict == NULL) {
+        return NULL;
+    }
+    PyObject *names = PyUnstable_Code_GetLocalPlusNames(co);
+    for (Py_ssize_t i = 0; i < n; i++) {
+        PyObject *value;
+        int rc = PyUnstable_InterpreterFrame_GetLocal(frame, i, &value);
+        if (rc < 0) {
+            Py_DECREF(names);
+            Py_DECREF(dict);
+            return NULL;
+        }
+        if (rc == 0) {
+            continue;  // unset or hidden slot
+        }
+        PyObject *name = PyTuple_GET_ITEM(names, i);
+        int err = PyDict_SetItem(dict, name, value);
+        Py_DECREF(value);
+        if (err < 0) {
+            Py_DECREF(names);
+            Py_DECREF(dict);
+            return NULL;
+        }
+    }
+    Py_DECREF(names);
+    return dict;
+}
+
+// Calls PyUnstable_InterpreterFrame_GetLocal on the caller's frame for a single
+// index and returns (rc, value), or raises if rc is -1.
+static PyObject *
+get_frame_local(PyObject *self, PyObject *arg)
+{
+    Py_ssize_t index = PyLong_AsSsize_t(arg);
+    if (index == -1 && PyErr_Occurred()) {
+        return NULL;
+    }
+    PyThreadState *tstate = _PyThreadState_GET();
+    _PyInterpreterFrame *frame = _PyThreadState_GetFrame(tstate);
+    if (frame == NULL) {
+        PyErr_SetString(PyExc_RuntimeError, "no caller frame");
+        return NULL;
+    }
+    PyObject *value;
+    int rc = PyUnstable_InterpreterFrame_GetLocal(frame, index, &value);
+    if (rc < 0) {
+        assert(value == NULL);
+        return NULL;
+    }
+    if (rc == 0) {
+        assert(value == NULL);
+        return Py_BuildValue("iO", rc, Py_None);
+    }
+    return Py_BuildValue("iN", rc, value);
+}
+
+static PyObject *
+code_get_localsplus_names(PyObject *self, PyObject *arg)
+{
+    if (!PyCode_Check(arg)) {
+        PyErr_SetString(PyExc_TypeError, "argument must be a code object");
+        return NULL;
+    }
+    return PyUnstable_Code_GetLocalPlusNames((PyCodeObject *)arg);
+}
+
 static PyObject *
 code_returns_only_none(PyObject *self, PyObject *arg)
 {
@@ -3349,6 +3430,9 @@ static PyMethodDef module_functions[] = {
     {"iframe_getcode", iframe_getcode, METH_O, NULL},
     {"iframe_getline", iframe_getline, METH_O, NULL},
     {"iframe_getlasti", iframe_getlasti, METH_O, NULL},
+    {"get_frame_locals", get_frame_locals, METH_NOARGS, NULL},
+    {"get_frame_local", get_frame_local, METH_O, NULL},
+    {"code_get_localsplus_names", code_get_localsplus_names, METH_O, NULL},
     {"code_returns_only_none", code_returns_only_none, METH_O, NULL},
     {"get_co_framesize", get_co_framesize, METH_O, NULL},
     {"get_co_localskinds", get_co_localskinds, METH_O, NULL},

Objects/frameobject.c

@@ -2326,6 +2326,53 @@ frame_get_var(_PyInterpreterFrame *frame, PyCodeObject *co, int i,
 }
 
 
+int
+PyUnstable_InterpreterFrame_GetLocal(_PyInterpreterFrame *frame,
+                                     Py_ssize_t index, PyObject **result)
+{
+    PyCodeObject *co = _PyFrame_GetCode(frame);
+    *result = NULL;
+    if (index < 0 || index >= co->co_nlocalsplus) {
+        PyErr_Format(
+            PyExc_IndexError,
+            "PyUnstable_InterpreterFrame_GetLocal: index %zd out of range [0, %d)",
+            index, co->co_nlocalsplus);
+        return -1;
+    }
+
+    int offset = PyUnstable_Code_GetFirstFree(co);  // co_nlocalsplus - co_nfreevars
+    if (index < offset) {
+        // Local or cell variable. frame_get_var unboxes cells and copes with
+        // not-yet-started frames and arguments not yet promoted by MAKE_CELL.
+        if (_PyLocals_GetKind(co->co_localspluskinds, (int)index) & CO_FAST_HIDDEN) {
+            return 0;
+        }
+        PyObject *value = NULL;
+        frame_get_var(frame, co, (int)index, &value);
+        if (value == NULL) {
+            return 0;
+        }
+        *result = value;  // strong reference
+        return 1;
+    }
+
+    // Free variable: read from the function closure rather than localsplus.
+    if ((co->co_flags & CO_OPTIMIZED)
+         && PyStackRef_FunctionCheck(frame->f_funcobj))
+    {
+        PyFunctionObject *func = _PyFrame_GetFunction(frame);
+        PyObject *cell = PyTuple_GET_ITEM(func->func_closure, index - offset);
+        PyObject *value = PyCell_GetRef((PyCellObject *)cell);
+        if (value == NULL) {
+            return 0;
+        }
+        *result = value;
+        return 1;
+    }
+    return 0;
+}
+
+
 bool
 _PyFrame_HasHiddenLocals(_PyInterpreterFrame *frame)
 {