THRIFT-5715 Python Exceptions mutable with slots

In Python 3.11 exceptions generated by the compiler can't be used with a
context manager because they are immutable. As of Python 3.11
`contextlib.contextmanager` sets `exc.__traceback__` in the event that
the code in the context manager errors.

As of Thrift v0.18.1 exceptions are generated as immutable by default.
See [PR#1835](#1835) for more
information about why exceptions were made immutable by default.

This change makes all non-Thrift fields mutable when slots is used
without dynamic. This will allow exceptions to be re-raised properly by
the contextmanager in Python 3.11.

compiler/cpp/src/thrift/generate/t_py_generator.cc

@@ -893,12 +893,42 @@ void t_py_generator::generate_py_struct_definition(ostream& out,
 
   if (is_immutable(tstruct)) {
     out << endl;
-    out << indent() << "def __setattr__(self, *args):" << endl
-        << indent() << indent_str() << "raise TypeError(\"can't modify immutable instance\")" << endl
-        << endl;
-    out << indent() << "def __delattr__(self, *args):" << endl
-        << indent() << indent_str() << "raise TypeError(\"can't modify immutable instance\")" << endl
-        << endl;
+    out << indent() << "def __setattr__(self, *args):" << endl;
+    indent_up();
+
+    // Not user-provided fields should be editable so that the Python Standard Library can edit
+    // internal fields of std library base classes. For example, in Python 3.11 ContextManager
+    // edits the `__traceback__` field on Exceptions. Allowing this to work with `__slots__` is
+    // trivial because we know which fields are user-provided, without slots we need to build a
+    // way to know which fields are user-provided.
+    if (gen_slots_ && !gen_dynamic_) {
+        out << indent() << "if args[0] not in self.__slots__:" << endl;
+        indent_up();
+        out << indent() << "super().__setattr__(*args)" << endl
+            << indent() << "return" << endl;
+        indent_down();
+    }
+    out << indent() << "raise TypeError(\"can't modify immutable instance\")" << endl;
+    indent_down();
+    out << endl;
+    out << indent() << "def __delattr__(self, *args):" << endl;
+    indent_up();
+
+    // Not user-provided fields should be editable so that the Python Standard Library can edit
+    // internal fields of std library base classes. For example, in Python 3.11 ContextManager
+    // edits the `__traceback__` field on Exceptions. Allowing this to work with `__slots__` is
+    // trivial because we know which fields are user-provided, without slots we need to build a
+    // way to know which fields are user-provided.
+    if (gen_slots_ && !gen_dynamic_) {
+        out << indent() << "if args[0] not in self.__slots__:" << endl;
+        indent_up();
+        out << indent() << "super().__delattr__(*args)" << endl
+            << indent() << "return" << endl;
+        indent_down();
+    }
+    out << indent() << "raise TypeError(\"can't modify immutable instance\")" << endl;
+    indent_down();
+    out << endl;
 
     // Hash all of the members in order, and also hash in the class
     // to avoid collisions for stuff like single-field structures.

test/py/TestClient.py

@@ -256,6 +256,35 @@ def testMultiException(self):
         y = self.client.testMultiException('success', 'foobar')
         self.assertEqual(y.string_thing, 'foobar')
 
+    def testException__traceback__(self):
+        print('testException__traceback__')
+        self.client.testException('Safe')
+        expect_slots = uses_slots = False
+        expect_dynamic = uses_dynamic = False
+        try:
+            self.client.testException('Xception')
+            self.fail("should have gotten exception")
+        except Xception as x:
+            uses_slots = hasattr(x, '__slots__')
+            uses_dynamic = (not isinstance(x, TException))
+            # We set expected values here so that we get clean tracebackes when
+            # the assertions fail.
+            try:
+                x.__traceback__ = x.__traceback__
+                # If `__traceback__` was set without errors than we expect that
+                # the slots option was used and that dynamic classes were not.
+                expect_slots = True
+                expect_dynamic = False
+            except Exception as e:
+                self.assertTrue(isinstance(e, TypeError))
+                # There are no other meaningful tests we can preform because we
+                # are unable to determine the desired state of either `__slots__`
+                # or `dynamic`.
+                return
+
+        self.assertEqual(expect_slots, uses_slots)
+        self.assertEqual(expect_dynamic, uses_dynamic)
+
     def testOneway(self):
         print('testOneway')
         start = time.time()