bpo-42990: Add __builtins__ attribute to functions (GH-24559)

Expose the new PyFunctionObject.func_builtins member in Python as a
new __builtins__ attribute on functions.

Document also the behavior change in What's New in Python 3.10.

Doc/library/inspect.rst

@@ -95,6 +95,8 @@ attributes:
 |           | __globals__       | global namespace in which |
 |           |                   | this function was defined |
 +-----------+-------------------+---------------------------+
+|           | __builtins__      | builtins namespace        |
++-----------+-------------------+---------------------------+
 |           | __annotations__   | mapping of parameters     |
 |           |                   | names to annotations;     |
 |           |                   | ``"return"`` key is       |
@@ -251,6 +253,10 @@ attributes:
 
    Add ``cr_origin`` attribute to coroutines.
 
+.. versionchanged:: 3.10
+
+   Add ``__builtins__`` attribute to functions.
+
 .. function:: getmembers(object[, predicate])
 
    Return all the members of an object in a list of ``(name, value)``

Doc/whatsnew/3.10.rst

@@ -280,6 +280,11 @@ Other Language Changes
 * Assignment expressions can now be used unparenthesized within set literals
   and set comprehensions, as well as in sequence indexes (but not slices).
 
+* Functions have a new ``__builtins__`` attribute which is used to look for
+  builtin symbols when a function is executed, instead of looking into
+  ``__globals__['__builtins__']``.
+  (Contributed by Mark Shannon in :issue:`42990`.)
+
 
 New Modules
 ===========

Lib/test/test_collections.py

@@ -682,9 +682,10 @@ class TestNamedTuple(unittest.TestCase):
         self.assertEqual(np.y, 2)
 
     def test_new_builtins_issue_43102(self):
-        self.assertEqual(
+        obj = namedtuple('C', ())
-            namedtuple('C', ()).__new__.__globals__['__builtins__'],
+        new_func = obj.__new__
-            {})
+        self.assertEqual(new_func.__globals__['__builtins__'], {})
+        self.assertEqual(new_func.__builtins__, {})
 
 
 ################################################################################

Lib/test/test_funcattrs.py

@@ -73,6 +73,11 @@ class FunctionPropertiesTest(FuncAttrsTest):
         self.cannot_set_attr(self.b, '__globals__', 2,
                              (AttributeError, TypeError))
 
+    def test___builtins__(self):
+        self.assertIs(self.b.__builtins__, __builtins__)
+        self.cannot_set_attr(self.b, '__builtins__', 2,
+                             (AttributeError, TypeError))
+
     def test___closure__(self):
         a = 12
         def f(): print(a)

Misc/NEWS.d/next/Core and Builtins/2021-02-17-19-02-21.bpo-42990.SKXHiI.rst

@@ -0,0 +1,3 @@
+Functions have a new ``__builtins__`` attribute which is used to look for
+builtin symbols when a function is executed, instead of looking into
+``__globals__['__builtins__']``. Patch by Mark Shannon and Victor Stinner.

Objects/funcobject.c

@@ -250,6 +250,7 @@ static PyMemberDef func_memberlist[] = {
     {"__doc__",       T_OBJECT,     OFF(func_doc), 0},
     {"__globals__",   T_OBJECT,     OFF(func_globals), READONLY},
     {"__module__",    T_OBJECT,     OFF(func_module), 0},
+    {"__builtins__",  T_OBJECT,     OFF(func_builtins), READONLY},
     {NULL}  /* Sentinel */
 };