pythongh-92203: Add closure support to exec(). (python#92204)

Add a closure keyword-only parameter to exec(). It can only be specified when exec-ing a code object that uses free variables. When specified, it must be a tuple, with exactly the number of cell variables referenced by the code object. closure has a default value of None, and it must be None if the code object doesn't refer to any free variables.

Doc/library/functions.rst

@@ -552,7 +552,7 @@ are always available.  They are listed here in alphabetical order.
 
 .. index:: builtin: exec
 
-.. function:: exec(object[, globals[, locals]])
+.. function:: exec(object[, globals[, locals]], *, closure=None)
 
    This function supports dynamic execution of Python code. *object* must be
    either a string or a code object.  If it is a string, the string is parsed as
@@ -581,6 +581,11 @@ are always available.  They are listed here in alphabetical order.
    builtins are available to the executed code by inserting your own
    ``__builtins__`` dictionary into *globals* before passing it to :func:`exec`.
 
+   The *closure* argument specifies a closure--a tuple of cellvars.
+   It's only valid when the *object* is a code object containing free variables.
+   The length of the tuple must exactly match the number of free variables
+   referenced by the code object.
+
    .. audit-event:: exec code_object exec
 
       Raises an :ref:`auditing event <auditing>` ``exec`` with the code object
@@ -599,6 +604,9 @@ are always available.  They are listed here in alphabetical order.
       Pass an explicit *locals* dictionary if you need to see effects of the
       code on *locals* after function :func:`exec` returns.
 
+   .. versionchanged:: 3.11
+      Added the *closure* parameter.
+
 
 .. function:: filter(function, iterable)
 

Lib/test/test_builtin.py

@@ -24,7 +24,7 @@
 from inspect import CO_COROUTINE
 from itertools import product
 from textwrap import dedent
-from types import AsyncGeneratorType, FunctionType
+from types import AsyncGeneratorType, FunctionType, CellType
 from operator import neg
 from test import support
 from test.support import (swap_attr, maybe_get_event_loop_policy)
@@ -772,6 +772,84 @@ def test_exec_redirected(self):
         finally:
             sys.stdout = savestdout
 
+    def test_exec_closure(self):
+        def function_without_closures():
+            return 3 * 5
+
+        result = 0
+        def make_closure_functions():
+            a = 2
+            b = 3
+            c = 5
+            def three_freevars():
+                nonlocal result
+                nonlocal a
+                nonlocal b
+                result = a*b
+            def four_freevars():
+                nonlocal result
+                nonlocal a
+                nonlocal b
+                nonlocal c
+                result = a*b*c
+            return three_freevars, four_freevars
+        three_freevars, four_freevars = make_closure_functions()
+
+        # "smoke" test
+        result = 0
+        exec(three_freevars.__code__,
+            three_freevars.__globals__,
+            closure=three_freevars.__closure__)
+        self.assertEqual(result, 6)
+
+        # should also work with a manually created closure
+        result = 0
+        my_closure = (CellType(35), CellType(72), three_freevars.__closure__[2])
+        exec(three_freevars.__code__,
+            three_freevars.__globals__,
+            closure=my_closure)
+        self.assertEqual(result, 2520)
+
+        # should fail: closure isn't allowed
+        # for functions without free vars
+        self.assertRaises(TypeError,
+            exec,
+            function_without_closures.__code__,
+            function_without_closures.__globals__,
+            closure=my_closure)
+
+        # should fail: closure required but wasn't specified
+        self.assertRaises(TypeError,
+            exec,
+            three_freevars.__code__,
+            three_freevars.__globals__,
+            closure=None)
+
+        # should fail: closure of wrong length
+        self.assertRaises(TypeError,
+            exec,
+            three_freevars.__code__,
+            three_freevars.__globals__,
+            closure=four_freevars.__closure__)
+
+        # should fail: closure using a list instead of a tuple
+        my_closure = list(my_closure)
+        self.assertRaises(TypeError,
+            exec,
+            three_freevars.__code__,
+            three_freevars.__globals__,
+            closure=my_closure)
+
+        # should fail: closure tuple with one non-cell-var
+        my_closure[0] = int
+        my_closure = tuple(my_closure)
+        self.assertRaises(TypeError,
+            exec,
+            three_freevars.__code__,
+            three_freevars.__globals__,
+            closure=my_closure)
+
+
     def test_filter(self):
         self.assertEqual(list(filter(lambda c: 'a' <= c <= 'z', 'Hello World')), list('elloorld'))
         self.assertEqual(list(filter(None, [1, 'hello', [], [3], '', None, 9, 0])), [1, 'hello', [3], 9])

Misc/NEWS.d/next/Core and Builtins/2022-05-02-17-12-49.gh-issue-92203.-igcjS.rst

@@ -0,0 +1,5 @@
+Add a closure keyword-only parameter to exec(). It can only be specified
+when exec-ing a code object that uses free variables. When specified, it
+must be a tuple, with exactly the number of cell variables referenced by the
+code object. closure has a default value of None, and it must be None if the
+code object doesn't refer to any free variables.

Python/bltinmodule.c

@@ -977,6 +977,8 @@ exec as builtin_exec
     globals: object = None
     locals: object = None
     /
+    *
+    closure: object(c_default="NULL") = None
 
 Execute the given source in the context of globals and locals.
 
@@ -985,12 +987,14 @@ or a code object as returned by compile().
 The globals must be a dictionary and locals can be any mapping,
 defaulting to the current globals and locals.
 If only globals is given, locals defaults to it.
+The closure must be a tuple of cellvars, and can only be used
+when source is a code object requiring exactly that many cellvars.
 [clinic start generated code]*/
 
 static PyObject *
 builtin_exec_impl(PyObject *module, PyObject *source, PyObject *globals,
-                  PyObject *locals)
-/*[clinic end generated code: output=3c90efc6ab68ef5d input=01ca3e1c01692829]*/
+                  PyObject *locals, PyObject *closure)
+/*[clinic end generated code: output=7579eb4e7646743d input=f13a7e2b503d1d9a]*/
 {
     PyObject *v;
 
@@ -1029,20 +1033,60 @@ builtin_exec_impl(PyObject *module, PyObject *source, PyObject *globals,
         return NULL;
     }
 
+    if (closure == Py_None) {
+        closure = NULL;
+    }
+
     if (PyCode_Check(source)) {
+        Py_ssize_t num_free = PyCode_GetNumFree((PyCodeObject *)source);
+        if (num_free == 0) {
+            if (closure) {
+                PyErr_SetString(PyExc_TypeError,
+                    "cannot use a closure with this code object");
+                return NULL;
+            }
+        } else {
+            int closure_is_ok =
+                closure
+                && PyTuple_CheckExact(closure)
+                && (PyTuple_GET_SIZE(closure) == num_free);
+            if (closure_is_ok) {
+                for (Py_ssize_t i = 0; i < num_free; i++) {
+                    PyObject *cell = PyTuple_GET_ITEM(closure, i);
+                    if (!PyCell_Check(cell)) {
+                        closure_is_ok = 0;
+                        break;
+                    }
+                }
+            }
+            if (!closure_is_ok) {
+                PyErr_Format(PyExc_TypeError,
+                    "code object requires a closure of exactly length %zd",
+                    num_free);
+                return NULL;
+            }
+        }
+
         if (PySys_Audit("exec", "O", source) < 0) {
             return NULL;
         }
 
-        if (PyCode_GetNumFree((PyCodeObject *)source) > 0) {
-            PyErr_SetString(PyExc_TypeError,
-                "code object passed to exec() may not "
-                "contain free variables");
-            return NULL;
+        if (!closure) {
+            v = PyEval_EvalCode(source, globals, locals);
+        } else {
+            v = PyEval_EvalCodeEx(source, globals, locals,
+                NULL, 0,
+                NULL, 0,
+                NULL, 0,
+                NULL,
+                closure);
         }
-        v = PyEval_EvalCode(source, globals, locals);
     }
     else {
+        if (closure != NULL) {
+            PyErr_SetString(PyExc_TypeError,
+                "closure can only be used when source is a code object");
+        }
         PyObject *source_copy;
         const char *str;
         PyCompilerFlags cf = _PyCompilerFlags_INIT;