diff --git a/Lib/test/support.py b/Lib/test/support.py
index a05f420..28f7e27 100644
--- a/Lib/test/support.py
+++ b/Lib/test/support.py
@@ -30,7 +30,8 @@
            "run_with_locale",
            "set_memlimit", "bigmemtest", "bigaddrspacetest", "BasicTestRunner",
            "run_unittest", "run_doctest", "threading_setup", "threading_cleanup",
-           "reap_children", "cpython_only", "check_impl_detail", "get_attribute"]
+           "reap_children", "cpython_only", "check_impl_detail", "get_attribute",
+           "swap_item", "swap_attr"]
 
 class Error(Exception):
     """Base class for regression test exceptions."""
@@ -1074,3 +1075,57 @@
                     break
             except:
                 break
+
+@contextlib.contextmanager
+def swap_attr(obj, attr, new_val):
+    """Temporary swap out an attribute with a new object.
+
+    Usage:
+        with swap_attr(obj, "attr", 5):
+            ...
+
+        This will set obj.attr to 5 for the duration of the with: block,
+        restoring the old value at the end of the block. If `attr` doesn't
+        exist on `obj`, it will be created and then deleted at the end of the
+        block.
+    """
+    if hasattr(obj, attr):
+        real_val = getattr(obj, attr)
+        setattr(obj, attr, new_val)
+        try:
+            yield
+        finally:
+            setattr(obj, attr, real_val)
+    else:
+        setattr(obj, attr, new_val)
+        try:
+            yield
+        finally:
+            delattr(obj, attr)
+
+@contextlib.contextmanager
+def swap_item(obj, item, new_val):
+    """Temporary swap out an item with a new object.
+
+    Usage:
+        with swap_item(obj, "item", 5):
+            ...
+
+        This will set obj["item"] to 5 for the duration of the with: block,
+        restoring the old value at the end of the block. If `item` doesn't
+        exist on `obj`, it will be created and then deleted at the end of the
+        block.
+    """
+    if item in obj:
+        real_val = obj[item]
+        obj[item] = new_val
+        try:
+            yield
+        finally:
+            obj[item] = real_val
+    else:
+        obj[item] = new_val
+        try:
+            yield
+        finally:
+            del obj[item]
diff --git a/Lib/test/test_dynamic.py b/Lib/test/test_dynamic.py
new file mode 100644
index 0000000..beb7b1c
--- /dev/null
+++ b/Lib/test/test_dynamic.py
@@ -0,0 +1,143 @@
+# Test the most dynamic corner cases of Python's runtime semantics.
+
+import builtins
+import contextlib
+import unittest
+
+from test.support import run_unittest, swap_item, swap_attr
+
+
+class RebindBuiltinsTests(unittest.TestCase):
+
+    """Test all the ways that we can change/shadow globals/builtins."""
+
+    def configure_func(self, func, *args):
+        """Perform TestCase-specific configuration on a function before testing.
+
+        By default, this does nothing. Example usage: spinning a function so
+        that a JIT will optimize it. Subclasses should override this as needed.
+
+        Args:
+            func: function to configure.
+            *args: any arguments that should be passed to func, if calling it.
+
+        Returns:
+            Nothing. Work will be performed on func in-place.
+        """
+        pass
+
+    def test_globals_shadow_builtins(self):
+        # Modify globals() to shadow an entry in builtins.
+        def foo():
+            return len([1, 2, 3])
+        self.configure_func(foo)
+
+        self.assertEqual(foo(), 3)
+        with swap_item(globals(), "len", lambda x: 7):
+            self.assertEqual(foo(), 7)
+
+    def test_modify_builtins(self):
+        # Modify the builtins module directly.
+        def foo():
+            return len([1, 2, 3])
+        self.configure_func(foo)
+
+        self.assertEqual(foo(), 3)
+        with swap_attr(builtins, "len", lambda x: 7):
+            self.assertEqual(foo(), 7)
+
+    def test_modify_builtins_while_generator_active(self):
+        # Modify the builtins out from under a live generator.
+        def foo():
+            x = range(3)
+            yield len(x)
+            yield len(x)
+        self.configure_func(foo)
+
+        g = foo()
+        self.assertEqual(next(g), 3)
+        with swap_attr(builtins, "len", lambda x: 7):
+            self.assertEqual(next(g), 7)
+
+    def test_modify_builtins_from_leaf_function(self):
+        # Verify that modifications made by leaf functions percolate up the
+        # callstack.
+        with swap_attr(builtins, "len", len):
+            def bar():
+                builtins.len = lambda x: 4
+
+            def foo(modifier):
+                l = []
+                l.append(len(range(7)))
+                modifier()
+                l.append(len(range(7)))
+                return l
+            self.configure_func(foo, lambda: None)
+
+            self.assertEqual(foo(bar), [7, 4])
+
+    def test_cannot_change_globals_or_builtins_with_eval(self):
+        def foo():
+            return len([1, 2, 3])
+        self.configure_func(foo)
+
+        # Note that this *doesn't* change the definition of len() seen by foo().
+        builtins_dict = {"len": lambda x: 7}
+        globals_dict = {"foo": foo, "__builtins__": builtins_dict,
+                        "len": lambda x: 8}
+        self.assertEqual(eval("foo()", globals_dict), 3)
+
+        self.assertEqual(eval("foo()", {"foo": foo}), 3)
+
+    def test_cannot_change_globals_or_builtins_with_exec(self):
+        def foo():
+            return len([1, 2, 3])
+        self.configure_func(foo)
+
+        globals_dict = {"foo": foo}
+        exec("x = foo()", globals_dict)
+        self.assertEqual(globals_dict["x"], 3)
+
+        # Note that this *doesn't* change the definition of len() seen by foo().
+        builtins_dict = {"len": lambda x: 7}
+        globals_dict = {"foo": foo, "__builtins__": builtins_dict,
+                        "len": lambda x: 8}
+
+        exec("x = foo()", globals_dict)
+        self.assertEqual(globals_dict["x"], 3)
+
+    def test_cannot_replace_builtins_dict_while_active(self):
+        def foo():
+            x = range(3)
+            yield len(x)
+            yield len(x)
+        self.configure_func(foo)
+
+        g = foo()
+        self.assertEqual(next(g), 3)
+        with swap_item(globals(), "__builtins__", {"len": lambda x: 7}):
+            self.assertEqual(next(g), 3)
+
+    def test_cannot_replace_builtins_dict_between_calls(self):
+        def foo():
+            return len([1, 2, 3])
+        self.configure_func(foo)
+
+        self.assertEqual(foo(), 3)
+        with swap_item(globals(), "__builtins__", {"len": lambda x: 7}):
+            self.assertEqual(foo(), 3)
+
+    def test_eval_gives_lambda_custom_globals(self):
+        globals_dict = {"len": lambda x: 7}
+        foo = eval("lambda: len([])", globals_dict)
+        self.configure_func(foo)
+
+        self.assertEqual(foo(), 7)
+
+
+def test_main():
+    run_unittest(RebindBuiltinsTests)
+
+
+if __name__ == "__main__":
+    test_main()