[mypyc] Make instance attribute read-only at runtime if Final (#21666)

Previously this was only enforced statically. I plan to add more
optimizations that take advantage of Final in the future.

Author: JukkaL

mypyc/codegen/emitclass.py

@@ -1065,12 +1065,14 @@ def generate_getseter_declarations(cl: ClassIR, emitter: Emitter) -> None:
                     getter_name(cl, attr, emitter.names), cl.struct_name(emitter.names)
                 )
             )
-            emitter.emit_line("static int")
-            emitter.emit_line(
-                "{}({} *self, PyObject *value, void *closure);".format(
-                    setter_name(cl, attr, emitter.names), cl.struct_name(emitter.names)
+            # Final attributes are read-only, so they have no setter.
+            if attr not in cl.final_attributes:
+                emitter.emit_line("static int")
+                emitter.emit_line(
+                    "{}({} *self, PyObject *value, void *closure);".format(
+                        setter_name(cl, attr, emitter.names), cl.struct_name(emitter.names)
+                    )
                 )
-            )
 
     for prop, (getter, setter) in cl.properties.items():
         if getter.decl.implicit:
@@ -1099,11 +1101,15 @@ def generate_getseters_table(cl: ClassIR, name: str, emitter: Emitter) -> None:
     if not cl.is_trait:
         for attr in cl.attributes:
             emitter.emit_line(f'{{"{attr}",')
-            emitter.emit_line(
-                " (getter){}, (setter){},".format(
-                    getter_name(cl, attr, emitter.names), setter_name(cl, attr, emitter.names)
+            if attr in cl.final_attributes:
+                # Final attributes are read-only, so emit a NULL setter.
+                emitter.emit_line(f" (getter){getter_name(cl, attr, emitter.names)}, NULL,")
+            else:
+                emitter.emit_line(
+                    " (getter){}, (setter){},".format(
+                        getter_name(cl, attr, emitter.names), setter_name(cl, attr, emitter.names)
+                    )
                 )
-            )
             emitter.emit_line(" NULL, NULL},")
     for prop, (getter, setter) in cl.properties.items():
         if getter.decl.implicit:
@@ -1129,8 +1135,10 @@ def generate_getseters(cl: ClassIR, emitter: Emitter) -> None:
     if not cl.is_trait:
         for i, (attr, rtype) in enumerate(cl.attributes.items()):
             generate_getter(cl, attr, rtype, emitter)
-            emitter.emit_line("")
-            generate_setter(cl, attr, rtype, emitter)
+            # Final attributes are read-only, so they have no setter.
+            if attr not in cl.final_attributes:
+                emitter.emit_line("")
+                generate_setter(cl, attr, rtype, emitter)
             if i < len(cl.attributes) - 1:
                 emitter.emit_line("")
     for prop, (getter, setter) in cl.properties.items():

mypyc/ir/class_ir.py

@@ -145,6 +145,8 @@ def __init__(
         )
         # Attributes defined in the class (not inherited)
         self.attributes: dict[str, RType] = {}
+        # Final attributes defined in the class (not inherited)
+        self.final_attributes: set[str] = set()
         # Deletable attributes
         self.deletable: list[str] = []
         # We populate method_types with the signatures of every method before
@@ -396,6 +398,7 @@ def serialize(self) -> JsonDict:
             "ctor": self.ctor.serialize(),
             # We serialize dicts as lists to ensure order is preserved
             "attributes": [(k, t.serialize()) for k, t in self.attributes.items()],
+            "final_attributes": sorted(self.final_attributes),
             # We try to serialize a name reference, but if the decl isn't in methods
             # then we can't be sure that will work so we serialize the whole decl.
             "method_decls": [
@@ -456,6 +459,7 @@ def deserialize(cls, data: JsonDict, ctx: DeserMaps) -> ClassIR:
         ir.builtin_base = data["builtin_base"]
         ir.ctor = FuncDecl.deserialize(data["ctor"], ctx)
         ir.attributes = {k: deserialize_type(t, ctx) for k, t in data["attributes"]}
+        ir.final_attributes = set(data["final_attributes"])
         ir.method_decls = {
             k: ctx.functions[v].decl if isinstance(v, str) else FuncDecl.deserialize(v, ctx)
             for k, v in data["method_decls"]

mypyc/irbuild/prepare.py

@@ -645,6 +645,8 @@ def prepare_methods_and_attributes(
                     add_getter_declaration(ir, name, attr_rtype, module_name)
                     add_setter_declaration(ir, name, attr_rtype, module_name)
                 ir.attributes[name] = attr_rtype
+                if node.node.is_final:
+                    ir.final_attributes.add(name)
         elif isinstance(node.node, (FuncDef, Decorator)):
             prepare_method_def(ir, module_name, cdef, mapper, node.node, options)
         elif isinstance(node.node, OverloadedFuncDef):

mypyc/test-data/run-classes.test

@@ -2825,6 +2825,46 @@ def test_final_attribute() -> None:
     assert C.b['x'] == 'y'
     assert C.a is C.b
 
+[case testFinalInstanceAttributeCannotBeRebound]
+from typing import Any, Final
+
+from testutil import assertRaises
+
+class C:
+    def __init__(self, x: int) -> None:
+        self.x: Final[int] = x
+
+class D(C):
+    def __init__(self, x: int, y: int) -> None:
+        super().__init__(x)
+        self.y: Final[int] = y
+
+def rebind_via_any(o: Any, value: int) -> None:
+    o.x = value
+
+def test_rebind_via_any() -> None:
+    c = C(1)
+    with assertRaises(AttributeError):
+        rebind_via_any(c, 2)
+    assert c.x == 1
+
+def test_rebind_via_setattr() -> None:
+    c = C(1)
+    with assertRaises(AttributeError):
+        setattr(c, "x", 3)
+    assert c.x == 1
+
+def test_rebind_inherited_via_setattr() -> None:
+    d = D(1, 2)
+    # Inherited Final attribute can't be modified.
+    with assertRaises(AttributeError):
+        setattr(d, "x", 3)
+    # The subclass's own Final attribute can't be modified either.
+    with assertRaises(AttributeError):
+        setattr(d, "y", 4)
+    assert d.x == 1
+    assert d.y == 2
+
 [case testClassDerivedFromIntEnum]
 from enum import IntEnum, auto