Add Class Decorator/Metaclass/Base Class plugin (#4328)

Such plugin kinds will be useful for projects like attrs, see #2088

mypy/plugin.py

@@ -4,7 +4,7 @@
 from abc import abstractmethod
 from typing import Callable, List, Tuple, Optional, NamedTuple, TypeVar
 
-from mypy.nodes import Expression, StrExpr, IntExpr, UnaryExpr, Context, DictExpr
+from mypy.nodes import Expression, StrExpr, IntExpr, UnaryExpr, Context, DictExpr, ClassDef
 from mypy.types import (
     Type, Instance, CallableType, TypedDictType, UnionType, NoneTyp, FunctionLike, TypeVarType,
     AnyType, TypeList, UnboundType, TypeOfAny
@@ -13,7 +13,7 @@
 from mypy.options import Options
 
 
-class AnalyzerPluginInterface:
+class TypeAnalyzerPluginInterface:
     """Interface for accessing semantic analyzer functionality in plugins."""
 
     @abstractmethod
@@ -40,7 +40,7 @@ def analyze_callable_args(self, arglist: TypeList) -> Optional[Tuple[List[Type],
     'AnalyzeTypeContext', [
         ('type', UnboundType),  # Type to analyze
         ('context', Context),
-        ('api', AnalyzerPluginInterface)])
+        ('api', TypeAnalyzerPluginInterface)])
 
 
 class CheckerPluginInterface:
@@ -53,6 +53,23 @@ def named_generic_type(self, name: str, args: List[Type]) -> Instance:
         raise NotImplementedError
 
 
+class SemanticAnalyzerPluginInterface:
+    """Interface for accessing semantic analyzer functionality in plugins."""
+
+    @abstractmethod
+    def named_type(self, qualified_name: str, args: Optional[List[Type]] = None) -> Instance:
+        raise NotImplementedError
+
+    @abstractmethod
+    def parse_bool(self, expr: Expression) -> Optional[bool]:
+        raise NotImplementedError
+
+    @abstractmethod
+    def fail(self, msg: str, ctx: Context, serious: bool = False, *,
+             blocker: bool = False) -> None:
+        raise NotImplementedError
+
+
 # A context for a function hook that infers the return type of a function with
 # a special signature.
 #
@@ -98,6 +115,14 @@ def named_generic_type(self, name: str, args: List[Type]) -> Instance:
         ('context', Context),
         ('api', CheckerPluginInterface)])
 
+# A context for a class hook that modifies the class definition.
+ClassDefContext = NamedTuple(
+    'ClassDecoratorContext', [
+        ('cls', ClassDef),       # The class definition
+        ('reason', Expression),  # The expression being applied (decorator, metaclass, base class)
+        ('api', SemanticAnalyzerPluginInterface)
+    ])
+
 
 class Plugin:
     """Base class of all type checker plugins.
@@ -136,7 +161,17 @@ def get_attribute_hook(self, fullname: str
                            ) -> Optional[Callable[[AttributeContext], Type]]:
         return None
 
-    # TODO: metaclass / class decorator hook
+    def get_class_decorator_hook(self, fullname: str
+                                 ) -> Optional[Callable[[ClassDefContext], None]]:
+        return None
+
+    def get_metaclass_hook(self, fullname: str
+                           ) -> Optional[Callable[[ClassDefContext], None]]:
+        return None
+
+    def get_base_class_hook(self, fullname: str
+                            ) -> Optional[Callable[[ClassDefContext], None]]:
+        return None
 
 
 T = TypeVar('T')
@@ -182,6 +217,18 @@ 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_decorator_hook(self, fullname: str
+                                 ) -> Optional[Callable[[ClassDefContext], None]]:
+        return self._find_hook(lambda plugin: plugin.get_class_decorator_hook(fullname))
+
+    def get_metaclass_hook(self, fullname: str
+                           ) -> Optional[Callable[[ClassDefContext], None]]:
+        return self._find_hook(lambda plugin: plugin.get_metaclass_hook(fullname))
+
+    def get_base_class_hook(self, fullname: str
+                            ) -> Optional[Callable[[ClassDefContext], None]]:
+        return self._find_hook(lambda plugin: plugin.get_base_class_hook(fullname))
+
     def _find_hook(self, lookup: Callable[[Plugin], T]) -> Optional[T]:
         for plugin in self._plugins:
             hook = lookup(plugin)

mypy/semanal.py

@@ -81,7 +81,7 @@
 from mypy.sametypes import is_same_type
 from mypy.options import Options
 from mypy import experiments
-from mypy.plugin import Plugin
+from mypy.plugin import Plugin, ClassDefContext, SemanticAnalyzerPluginInterface
 from mypy import join
 from mypy.util import get_prefix
 
@@ -172,7 +172,7 @@
 }
 
 
-class SemanticAnalyzerPass2(NodeVisitor[None]):
+class SemanticAnalyzerPass2(NodeVisitor[None], SemanticAnalyzerPluginInterface):
     """Semantically analyze parsed mypy files.
 
     The analyzer binds names and does various consistency checks for a
@@ -715,9 +715,48 @@ def analyze_class_body(self, defn: ClassDef) -> Iterator[bool]:
                 yield True
                 self.calculate_abstract_status(defn.info)
                 self.setup_type_promotion(defn)
-
+                self.apply_class_plugin_hooks(defn)
                 self.leave_class()
 
+    def apply_class_plugin_hooks(self, defn: ClassDef) -> None:
+        """Apply a plugin hook that may infer a more precise definition for a class."""
+        def get_fullname(expr: Expression) -> Optional[str]:
+            if isinstance(expr, CallExpr):
+                return get_fullname(expr.callee)
+            elif isinstance(expr, IndexExpr):
+                return get_fullname(expr.base)
+            elif isinstance(expr, RefExpr):
+                if expr.fullname:
+                    return expr.fullname
+                # If we don't have a fullname look it up. This happens because base classes are
+                # analyzed in a different manner (see exprtotype.py) and therefore those AST
+                # nodes will not have full names.
+                sym = self.lookup_type_node(expr)
+                if sym:
+                    return sym.fullname
+            return None
+
+        for decorator in defn.decorators:
+            decorator_name = get_fullname(decorator)
+            if decorator_name:
+                hook = self.plugin.get_class_decorator_hook(decorator_name)
+                if hook:
+                    hook(ClassDefContext(defn, decorator, self))
+
+        if defn.metaclass:
+            metaclass_name = get_fullname(defn.metaclass)
+            if metaclass_name:
+                hook = self.plugin.get_metaclass_hook(metaclass_name)
+                if hook:
+                    hook(ClassDefContext(defn, defn.metaclass, self))
+
+        for base_expr in defn.base_type_exprs:
+            base_name = get_fullname(base_expr)
+            if base_name:
+                hook = self.plugin.get_base_class_hook(base_name)
+                if hook:
+                    hook(ClassDefContext(defn, base_expr, self))
+
     def analyze_class_keywords(self, defn: ClassDef) -> None:
         for value in defn.keywords.values():
             value.accept(self)

mypy/typeanal.py

@@ -27,7 +27,7 @@
 from mypy.sametypes import is_same_type
 from mypy.exprtotype import expr_to_unanalyzed_type, TypeTranslationError
 from mypy.subtypes import is_subtype
-from mypy.plugin import Plugin, AnalyzerPluginInterface, AnalyzeTypeContext
+from mypy.plugin import Plugin, TypeAnalyzerPluginInterface, AnalyzeTypeContext
 from mypy import nodes, messages
 
 
@@ -132,7 +132,7 @@ def no_subscript_builtin_alias(name: str, propose_alt: bool = True) -> str:
     return msg
 
 
-class TypeAnalyser(SyntheticTypeVisitor[Type], AnalyzerPluginInterface):
+class TypeAnalyser(SyntheticTypeVisitor[Type], TypeAnalyzerPluginInterface):
     """Semantic analyzer for types (semantic analysis pass 2).
 
     Converts unbound types into bound types.