[mypyc] Support deleting attributes in __setattr__ wrapper (#19997)

The `__setattr__` wrapper that mypyc generates needs to handle deleting
attributes as well because `del` statements go through the same
`tp_setattro` pointer but with the value argument set to `NULL`.

The wrapper calls `__delattr__` in this case if it's overridden in the
native class (or its parent). Handling of dynamic attributes is
different without `__dict__` which makes a custom `__delattr__` required
if the dynamic attributes are stored in a custom dictionary.

If `__delattr__` is not overridden it calls the implementation of
`object.__delattr__` which results in `AttributeError` because there's
no `__dict__`.

If it's defined without `__setattr__`, mypyc reports an error. It's
possible to support just `__delattr__` but since it shares a slot with
`__setattr__`, the wrapper generation would be more complicated. It
seems like an unlikely use case to only need `__delattr__` so I think it
makes sense to leave it for later.

Author: p-sawicki

mypyc/irbuild/function.py

@@ -83,7 +83,7 @@
     dict_new_op,
     exact_dict_set_item_op,
 )
-from mypyc.primitives.generic_ops import generic_getattr, py_setattr_op
+from mypyc.primitives.generic_ops import generic_getattr, generic_setattr, py_setattr_op
 from mypyc.primitives.misc_ops import register_function
 from mypyc.primitives.registry import builtin_names
 from mypyc.sametype import is_same_method_signature, is_same_type
@@ -423,8 +423,10 @@ def generate_setattr_wrapper(builder: IRBuilder, cdef: ClassDef, setattr: FuncDe
     Returns 0 on success and -1 on failure. Restrictions are similar to the __getattr__
     wrapper above.
 
-    This one is simpler because to match interpreted python semantics it's enough to always
-    call the user-provided function, including for names matching regular attributes.
+    The wrapper calls the user-defined __setattr__ when the value to set is not NULL.
+    When it's NULL, this means that the call to tp_setattro comes from a del statement,
+    so it calls __delattr__ instead. If __delattr__ is not overridden in the native class,
+    this will call the base implementation in object which doesn't work without __dict__.
     """
     name = setattr.name + "__wrapper"
     ir = builder.mapper.type_to_ir[cdef.info]
@@ -440,6 +442,27 @@ def generate_setattr_wrapper(builder: IRBuilder, cdef: ClassDef, setattr: FuncDe
         attr_arg = builder.add_argument("attr", object_rprimitive)
         value_arg = builder.add_argument("value", object_rprimitive)
 
+        call_delattr, call_setattr = BasicBlock(), BasicBlock()
+        null = Integer(0, object_rprimitive, line)
+        is_delattr = builder.add(ComparisonOp(value_arg, null, ComparisonOp.EQ, line))
+        builder.add_bool_branch(is_delattr, call_delattr, call_setattr)
+
+        builder.activate_block(call_delattr)
+        delattr_symbol = cdef.info.get("__delattr__")
+        delattr = delattr_symbol.node if delattr_symbol else None
+        delattr_override = delattr is not None and not delattr.fullname.startswith("builtins.")
+        if delattr_override:
+            builder.gen_method_call(builder.self(), "__delattr__", [attr_arg], None, line)
+        else:
+            # Call internal function that cpython normally calls when deleting an attribute.
+            # Cannot call object.__delattr__ here because it calls PyObject_SetAttr internally
+            # which in turn calls our wrapper and recurses infinitely.
+            # Note that since native classes don't have __dict__, this will raise AttributeError
+            # for dynamic attributes.
+            builder.call_c(generic_setattr, [builder.self(), attr_arg, null], line)
+        builder.add(Return(Integer(0, c_int_rprimitive), line))
+
+        builder.activate_block(call_setattr)
         builder.gen_method_call(builder.self(), setattr.name, [attr_arg, value_arg], None, line)
         builder.add(Return(Integer(0, c_int_rprimitive), line))
 
@@ -514,6 +537,14 @@ def handle_ext_method(builder: IRBuilder, cdef: ClassDef, fdef: FuncDef) -> None
         generate_getattr_wrapper(builder, cdef, fdef)
     elif fdef.name == "__setattr__":
         generate_setattr_wrapper(builder, cdef, fdef)
+    elif fdef.name == "__delattr__":
+        setattr = cdef.info.get("__setattr__")
+        if not setattr or not setattr.node or setattr.node.fullname.startswith("builtins."):
+            builder.error(
+                '"__delattr__" supported only in classes that also override "__setattr__", '
+                + "or inherit from a native class that overrides it.",
+                fdef.line,
+            )
 
 
 def handle_non_ext_method(

mypyc/test-data/fixtures/ir.py

@@ -46,6 +46,7 @@ def __eq__(self, x: object) -> bool: pass
     def __ne__(self, x: object) -> bool: pass
     def __str__(self) -> str: pass
     def __setattr__(self, k: str, v: object) -> None: pass
+    def __delattr__(self, k: str) -> None: pass
 
 class type:
     def __init__(self, o: object) -> None: ...

mypyc/test-data/irbuild-classes.test

@@ -2222,18 +2222,41 @@ class AllowsInterpreted:
     def __setattr__(self, attr: str, val: object) -> None:  # E: "__setattr__" not supported in class "AllowsInterpreted" because it allows interpreted subclasses
         pass
 
+    def __delattr__(self, attr: str) -> None:
+        pass
+
 class InheritsInterpreted(dict):
     def __setattr__(self, attr: str, val: object) -> None:  # E: "__setattr__" not supported in class "InheritsInterpreted" because it inherits from a non-native class
         pass
 
+    def __delattr__(self, attr: str) -> None:
+        pass
+
 @mypyc_attr(native_class=False)
 class NonNative:
-    pass
+    def __setattr__(self, attr: str, val: object) -> None:
+        pass
 
 class InheritsNonNative(NonNative):
     def __setattr__(self, attr: str, val: object) -> None:  # E: "__setattr__" not supported in class "InheritsNonNative" because it inherits from a non-native class
         pass
 
+    def __delattr__(self, attr: str) -> None:
+        pass
+
+[case testUnsupportedDelAttr]
+class SetAttr:
+    def __setattr__(self, attr: str, val: object) -> None:
+        pass
+
+class NoSetAttr:
+    def __delattr__(self, attr: str) -> None:  # E: "__delattr__" supported only in classes that also override "__setattr__", or inherit from a native class that overrides it.
+        pass
+
+class InheritedSetAttr(SetAttr):
+    def __delattr__(self, attr: str) -> None:
+        pass
+
 [case testSetAttr]
 from typing import ClassVar
 class SetAttr:
@@ -2329,11 +2352,21 @@ L6:
 def SetAttr.__setattr____wrapper(__mypyc_self__, attr, value):
     __mypyc_self__ :: __main__.SetAttr
     attr, value :: object
-    r0 :: str
-    r1 :: None
+    r0 :: bit
+    r1 :: i32
+    r2 :: bit
+    r3 :: str
+    r4 :: None
 L0:
-    r0 = cast(str, attr)
-    r1 = __mypyc_self__.__setattr__(r0, value)
+    r0 = value == 0
+    if r0 goto L1 else goto L2 :: bool
+L1:
+    r1 = CPyObject_GenericSetAttr(__mypyc_self__, attr, 0)
+    r2 = r1 >= 0 :: signed
+    return 0
+L2:
+    r3 = cast(str, attr)
+    r4 = __mypyc_self__.__setattr__(r3, value)
     return 0
 def test(attr, val):
     attr :: str
@@ -2372,6 +2405,124 @@ L0:
     r14 = r13 >= 0 :: signed
     return 1
 
+[case testSetAttrAndDelAttr]
+from typing import ClassVar
+class SetAttr:
+    _attributes: dict[str, object]
+    regular_attr: int
+    class_var: ClassVar[str] = "x"
+
+    def __init__(self, regular_attr: int, extra_attrs: dict[str, object], new_attr: str, new_val: object) -> None:
+        super().__setattr__("_attributes", extra_attrs)
+        object.__setattr__(self, "regular_attr", regular_attr)
+
+        super().__setattr__(new_attr, new_val)
+        object.__setattr__(self, new_attr, new_val)
+
+    def __setattr__(self, key: str, val: object) -> None:
+        if key == "regular_attr":
+            super().__setattr__("regular_attr", val)
+        elif key == "class_var":
+            raise AttributeError()
+        else:
+            self._attributes[key] = val
+
+    def __delattr__(self, key: str) -> None:
+        del self._attributes[key]
+
+[typing fixtures/typing-full.pyi]
+[out]
+def SetAttr.__init__(self, regular_attr, extra_attrs, new_attr, new_val):
+    self :: __main__.SetAttr
+    regular_attr :: int
+    extra_attrs :: dict
+    new_attr :: str
+    new_val :: object
+    r0 :: i32
+    r1 :: bit
+    r2 :: i32
+    r3 :: bit
+L0:
+    self._attributes = extra_attrs
+    self.regular_attr = regular_attr
+    r0 = CPyObject_GenericSetAttr(self, new_attr, new_val)
+    r1 = r0 >= 0 :: signed
+    r2 = CPyObject_GenericSetAttr(self, new_attr, new_val)
+    r3 = r2 >= 0 :: signed
+    return 1
+def SetAttr.__setattr__(self, key, val):
+    self :: __main__.SetAttr
+    key :: str
+    val :: object
+    r0 :: str
+    r1 :: bool
+    r2 :: int
+    r3 :: bool
+    r4 :: str
+    r5 :: bool
+    r6 :: object
+    r7 :: str
+    r8, r9 :: object
+    r10 :: dict
+    r11 :: i32
+    r12 :: bit
+L0:
+    r0 = 'regular_attr'
+    r1 = CPyStr_Equal(key, r0)
+    if r1 goto L1 else goto L2 :: bool
+L1:
+    r2 = unbox(int, val)
+    self.regular_attr = r2; r3 = is_error
+    goto L6
+L2:
+    r4 = 'class_var'
+    r5 = CPyStr_Equal(key, r4)
+    if r5 goto L3 else goto L4 :: bool
+L3:
+    r6 = builtins :: module
+    r7 = 'AttributeError'
+    r8 = CPyObject_GetAttr(r6, r7)
+    r9 = PyObject_Vectorcall(r8, 0, 0, 0)
+    CPy_Raise(r9)
+    unreachable
+L4:
+    r10 = self._attributes
+    r11 = CPyDict_SetItem(r10, key, val)
+    r12 = r11 >= 0 :: signed
+L5:
+L6:
+    return 1
+def SetAttr.__setattr____wrapper(__mypyc_self__, attr, value):
+    __mypyc_self__ :: __main__.SetAttr
+    attr, value :: object
+    r0 :: bit
+    r1 :: str
+    r2 :: None
+    r3 :: str
+    r4 :: None
+L0:
+    r0 = value == 0
+    if r0 goto L1 else goto L2 :: bool
+L1:
+    r1 = cast(str, attr)
+    r2 = __mypyc_self__.__delattr__(r1)
+    return 0
+L2:
+    r3 = cast(str, attr)
+    r4 = __mypyc_self__.__setattr__(r3, value)
+    return 0
+def SetAttr.__delattr__(self, key):
+    self :: __main__.SetAttr
+    key :: str
+    r0 :: dict
+    r1 :: i32
+    r2 :: bit
+L0:
+    r0 = self._attributes
+    r1 = PyObject_DelItem(r0, key)
+    r2 = r1 >= 0 :: signed
+    return 1
+
 [case testUntransformedSetAttr_64bit]
 from mypy_extensions import mypyc_attr