Add a class attribute hook to the plugin system

docs/source/extending_mypy.rst

@@ -198,6 +198,10 @@ fields which already exist on the class. *Exception:* if :py:meth:`__getattr__ <
 :py:meth:`__getattribute__ <object.__getattribute__>` is a method on the class, the hook is called for all
 fields which do not refer to methods.
 
+**get_class_attribute_hook()** is similar to above, but for attributes on classes rather than instances.
+Unlike above, this does not have special casing for :py:meth:`__getattr__ <object.__getattr__>` or
+:py:meth:`__getattribute__ <object.__getattribute__>`.
+
 **get_class_decorator_hook()** can be used to update class definition for
 given class decorators. For example, you can add some attributes to the class
 to match runtime behaviour:

mypy/checkmember.py

@@ -703,10 +703,13 @@ def analyze_class_attribute_access(itype: Instance,
     if override_info:
         info = override_info
 
+    fullname = '{}.{}'.format(info.fullname, name)
+    hook = mx.chk.plugin.get_class_attribute_hook(fullname)
+
     node = info.get(name)
     if not node:
         if info.fallback_to_any:
-            return AnyType(TypeOfAny.special_form)
+            return apply_class_attr_hook(mx, hook, AnyType(TypeOfAny.special_form))
         return None
 
     is_decorated = isinstance(node.node, Decorator)
@@ -731,14 +734,16 @@ def analyze_class_attribute_access(itype: Instance,
     if info.is_enum and not (mx.is_lvalue or is_decorated or is_method):
         enum_class_attribute_type = analyze_enum_class_attribute_access(itype, name, mx)
         if enum_class_attribute_type:
-            return enum_class_attribute_type
+            return apply_class_attr_hook(mx, hook, enum_class_attribute_type)
 
     t = node.type
     if t:
         if isinstance(t, PartialType):
             symnode = node.node
             assert isinstance(symnode, Var)
-            return mx.chk.handle_partial_var_type(t, mx.is_lvalue, symnode, mx.context)
+            return apply_class_attr_hook(mx, hook,
+                                         mx.chk.handle_partial_var_type(t, mx.is_lvalue, symnode,
+                                                                        mx.context))
 
         # Find the class where method/variable was defined.
         if isinstance(node.node, Decorator):
@@ -789,7 +794,8 @@ def analyze_class_attribute_access(itype: Instance,
                                  mx.self_type, original_vars=original_vars)
         if not mx.is_lvalue:
             result = analyze_descriptor_access(result, mx)
-        return result
+
+        return apply_class_attr_hook(mx, hook, result)
     elif isinstance(node.node, Var):
         mx.not_ready_callback(name, mx.context)
         return AnyType(TypeOfAny.special_form)
@@ -813,7 +819,7 @@ def analyze_class_attribute_access(itype: Instance,
     if is_decorated:
         assert isinstance(node.node, Decorator)
         if node.node.type:
-            return node.node.type
+            return apply_class_attr_hook(mx, hook, node.node.type)
         else:
             mx.not_ready_callback(name, mx.context)
             return AnyType(TypeOfAny.from_error)
@@ -825,7 +831,17 @@ def analyze_class_attribute_access(itype: Instance,
         # unannotated implicit class methods we do this here.
         if node.node.is_class:
             typ = bind_self(typ, is_classmethod=True)
-        return typ
+        return apply_class_attr_hook(mx, hook, typ)
+
+
+def apply_class_attr_hook(mx: MemberContext,
+                          hook: Optional[Callable[[AttributeContext], Type]],
+                          result: Type,
+                          ) -> Optional[Type]:
+    if hook:
+        result = hook(AttributeContext(get_proper_type(mx.original_type),
+                                       result, mx.context, mx.chk))
+    return result
 
 
 def analyze_enum_class_attribute_access(itype: Instance,

mypy/plugin.py

@@ -632,10 +632,10 @@ def get_method_hook(self, fullname: str
 
     def get_attribute_hook(self, fullname: str
                            ) -> Optional[Callable[[AttributeContext], Type]]:
-        """Adjust type of a class attribute.
+        """Adjust type of an instance attribute.
 
-        This method is called with attribute full name using the class where the attribute was
-        defined (or Var.info.fullname for generated attributes).
+        This method is called with attribute full name using the class of the instance where
+        the attribute was defined (or Var.info.fullname for generated attributes).
 
         For classes without __getattr__ or __getattribute__, this hook is only called for
         names of fields/properties (but not methods) that exist in the instance MRO.
@@ -662,6 +662,25 @@ class Derived(Base):
         """
         return None
 
+    def get_class_attribute_hook(self, fullname: str
+                                 ) -> Optional[Callable[[AttributeContext], Type]]:
+        """
+        Adjust type of a class attribute.
+
+        This method is called with attribute full name using the class where the attribute was
+        defined (or Var.info.fullname for generated attributes).
+
+        For example:
+
+            class Cls:
+                x: Any
+
+            Cls.x
+
+        get_class_attribute_hook is called with '__main__.Cls.x' as fullname.
+        """
+        return None
+
     def get_class_decorator_hook(self, fullname: str
                                  ) -> Optional[Callable[[ClassDefContext], None]]:
         """Update class definition for given class decorators.
@@ -783,6 +802,10 @@ def get_attribute_hook(self, fullname: str
                            ) -> Optional[Callable[[AttributeContext], Type]]:
         return self._find_hook(lambda plugin: plugin.get_attribute_hook(fullname))
 
+    def get_class_attribute_hook(self, fullname: str
+                                 ) -> Optional[Callable[[AttributeContext], Type]]:
+        return self._find_hook(lambda plugin: plugin.get_class_attribute_hook(fullname))
+
     def get_class_decorator_hook(self, fullname: str
                                  ) -> Optional[Callable[[ClassDefContext], None]]:
         return self._find_hook(lambda plugin: plugin.get_class_decorator_hook(fullname))

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

@@ -902,3 +902,92 @@ reveal_type(f())  # N: Revealed type is "builtins.str"
 [file mypy.ini]
 \[mypy]
 plugins=<ROOT>/test-data/unit/plugins/method_in_decorator.py
+
+[case testClassAttrPluginClassVar]
+# flags: --config-file tmp/mypy.ini
+
+from typing import Type
+
+class Cls:
+    attr = 'test'
+    unchanged = 'test'
+
+reveal_type(Cls().attr)  # N: Revealed type is "builtins.str"
+reveal_type(Cls.attr)  # N: Revealed type is "builtins.int"
+reveal_type(Cls.unchanged)  # N: Revealed type is "builtins.str"
+x: Type[Cls]
+reveal_type(x.attr)  # N: Revealed type is "builtins.int"
+[file mypy.ini]
+\[mypy]
+plugins=<ROOT>/test-data/unit/plugins/class_attr_hook.py
+
+[case testClassAttrPluginMethod]
+# flags: --config-file tmp/mypy.ini
+
+class Cls:
+    def attr(self) -> None:
+        pass
+
+reveal_type(Cls.attr)  # N: Revealed type is "builtins.int"
+[file mypy.ini]
+\[mypy]
+plugins=<ROOT>/test-data/unit/plugins/class_attr_hook.py
+
+[case testClassAttrPluginEnum]
+# flags: --config-file tmp/mypy.ini
+
+import enum
+
+class Cls(enum.Enum):
+    attr = 'test'
+
+reveal_type(Cls.attr)  # N: Revealed type is "builtins.int"
+[file mypy.ini]
+\[mypy]
+plugins=<ROOT>/test-data/unit/plugins/class_attr_hook.py
+
+[case testClassAttrPluginMetaclassAnyBase]
+# flags: --config-file tmp/mypy.ini
+
+from typing import Any, Type
+class M(type):
+    attr = 'test'
+
+B: Any
+class Cls(B, metaclass=M):
+    pass
+
+reveal_type(Cls.attr) # N: Revealed type is "builtins.int"
+[file mypy.ini]
+\[mypy]
+plugins=<ROOT>/test-data/unit/plugins/class_attr_hook.py
+
+[case testClassAttrPluginMetaclassRegularBase]
+# flags: --config-file tmp/mypy.ini
+
+from typing import Any, Type
+class M(type):
+    attr = 'test'
+
+class B:
+    attr = None
+
+class Cls(B, metaclass=M):
+    pass
+
+reveal_type(Cls.attr) # N: Revealed type is "builtins.int"
+[file mypy.ini]
+\[mypy]
+plugins=<ROOT>/test-data/unit/plugins/class_attr_hook.py
+
+[case testClassAttrPluginPartialType]
+# flags: --config-file tmp/mypy.ini
+
+class Cls:
+    attr = None
+    def f(self) -> int:
+        return Cls.attr + 1
+
+[file mypy.ini]
+\[mypy]
+plugins=<ROOT>/test-data/unit/plugins/class_attr_hook.py

test-data/unit/plugins/class_attr_hook.py

@@ -0,0 +1,20 @@
+from typing import Callable, Optional
+
+from mypy.plugin import AttributeContext, Plugin
+from mypy.types import Type as MypyType
+
+
+class ClassAttrPlugin(Plugin):
+    def get_class_attribute_hook(self, fullname: str
+                                 ) -> Optional[Callable[[AttributeContext], MypyType]]:
+        if fullname == '__main__.Cls.attr':
+            return my_hook
+        return None
+
+
+def my_hook(ctx: AttributeContext) -> MypyType:
+    return ctx.api.named_generic_type('builtins.int', [])
+
+
+def plugin(_version: str):
+    return ClassAttrPlugin