Add is_lvalue attribute to AttributeContext (#17881)

Refs #17878

Author: brianschubert

docs/source/extending_mypy.rst

@@ -179,7 +179,7 @@ mypy will call ``get_method_signature_hook("ctypes.Array.__setitem__")``
 so that the plugin can mimic the :py:mod:`ctypes` auto-convert behavior.
 
 **get_attribute_hook()** overrides instance member field lookups and property
-access (not assignments, and not method calls). This hook is only called for
+access (not method calls). This hook is only called for
 fields which already exist on the class. *Exception:* if :py:meth:`__getattr__ <object.__getattr__>` or
 :py:meth:`__getattribute__ <object.__getattribute__>` is a method on the class, the hook is called for all
 fields which do not refer to methods.

mypy/checkmember.py

@@ -572,7 +572,11 @@ def analyze_member_var_access(
                     if hook:
                         result = hook(
                             AttributeContext(
-                                get_proper_type(mx.original_type), result, mx.context, mx.chk
+                                get_proper_type(mx.original_type),
+                                result,
+                                mx.is_lvalue,
+                                mx.context,
+                                mx.chk,
                             )
                         )
                     return result
@@ -829,7 +833,9 @@ def analyze_var(
         result = analyze_descriptor_access(result, mx)
     if hook:
         result = hook(
-            AttributeContext(get_proper_type(mx.original_type), result, mx.context, mx.chk)
+            AttributeContext(
+                get_proper_type(mx.original_type), result, mx.is_lvalue, mx.context, mx.chk
+            )
         )
     return result
 
@@ -1148,7 +1154,9 @@ def apply_class_attr_hook(
 ) -> Type | None:
     if hook:
         result = hook(
-            AttributeContext(get_proper_type(mx.original_type), result, mx.context, mx.chk)
+            AttributeContext(
+                get_proper_type(mx.original_type), result, mx.is_lvalue, mx.context, mx.chk
+            )
         )
     return result
 

mypy/plugin.py

@@ -495,6 +495,7 @@ class MethodContext(NamedTuple):
 class AttributeContext(NamedTuple):
     type: ProperType  # Type of object with attribute
     default_attr_type: Type  # Original attribute type
+    is_lvalue: bool  # Whether the attribute is the target of an assignment
     context: Context  # Relevant location context (e.g. for error messages)
     api: CheckerPluginInterface
 

test-data/unit/check-custom-plugin.test

@@ -232,6 +232,8 @@ reveal_type(magic.non_magic_method())  # N: Revealed type is "builtins.int"
 reveal_type(magic.non_magic_field)  # N: Revealed type is "builtins.int"
 magic.nonexistent_field  # E: Field does not exist
 reveal_type(magic.fallback_example)  # N: Revealed type is "Any"
+reveal_type(magic.no_assignment_field)  # N: Revealed type is "builtins.float"
+magic.no_assignment_field = "bad"  # E: Cannot assign to field
 
 derived = DerivedMagic()
 reveal_type(derived.magic_field)  # N: Revealed type is "builtins.str"
@@ -250,6 +252,7 @@ class Magic:
     def __getattr__(self, x: Any) -> Any: ...
     def non_magic_method(self) -> int: ...
     non_magic_field: int
+    no_assignment_field: float
 
 class DerivedMagic(Magic): ...
 [file mypy.ini]

test-data/unit/plugins/attrhook2.py

@@ -12,6 +12,8 @@ def get_attribute_hook(self, fullname: str) -> Callable[[AttributeContext], Type
             return magic_field_callback
         if fullname == "m.Magic.nonexistent_field":
             return nonexistent_field_callback
+        if fullname == "m.Magic.no_assignment_field":
+            return no_assignment_field_callback
         return None
 
 
@@ -24,5 +26,12 @@ def nonexistent_field_callback(ctx: AttributeContext) -> Type:
     return AnyType(TypeOfAny.from_error)
 
 
+def no_assignment_field_callback(ctx: AttributeContext) -> Type:
+    if ctx.is_lvalue:
+        ctx.api.fail(f"Cannot assign to field", ctx.context)
+        return AnyType(TypeOfAny.from_error)
+    return ctx.default_attr_type
+
+
 def plugin(version: str) -> type[AttrPlugin]:
     return AttrPlugin