Teach mypy plugin that validator methods are classmethods

changes/4102-DMRobertson.md

@@ -0,0 +1 @@
+Teach the mypy plugin that methods decorated by `@validator` are classmethods.

pydantic/mypy.py

@@ -185,6 +185,7 @@ def transform(self) -> None:
         ctx = self._ctx
         info = self._ctx.cls.info
 
+        self.adjust_validator_signatures()
         config = self.collect_config()
         fields = self.collect_fields(config)
         for field in fields:
@@ -200,6 +201,24 @@ def transform(self) -> None:
             'config': config.set_values_dict(),
         }
 
+    def adjust_validator_signatures(self) -> None:
+        """When we decorate a function `f` with `pydantic.validator(...), mypy sees
+        `f` as a regular method taking a `self` instance, even though pydantic
+        internally wraps `f` with `classmethod` if necessary.
+
+        Teach mypy this by marking any function whose outermost decorator is a
+        `validator()` call as a classmethod.
+        """
+        for name, sym in self._ctx.cls.info.names.items():
+            if isinstance(sym.node, Decorator):
+                first_dec = sym.node.original_decorators[0]
+                if (
+                    isinstance(first_dec, CallExpr)
+                    and isinstance(first_dec.callee, NameExpr)
+                    and first_dec.callee.fullname == 'pydantic.class_validators.validator'
+                ):
+                    sym.node.func.is_class = True
+
     def collect_config(self) -> 'ModelConfigData':
         """
         Collects the values of the config attributes that are used by the plugin, accounting for parent classes.

tests/mypy/modules/plugin_success.py

@@ -1,4 +1,4 @@
-from typing import ClassVar, Generic, Optional, TypeVar, Union
+from typing import ClassVar, Generic, Optional, Type, TypeVar, Union
 
 from pydantic import BaseModel, BaseSettings, Field, create_model, validator
 from pydantic.dataclasses import dataclass
@@ -193,3 +193,11 @@ class Response(GenericModel, Generic[T]):
 
 
 response = Response[Model](data=model, error=None)
+
+
+class ModelWithAnnotatedValidator(BaseModel):
+    name: str
+
+    @validator('name')
+    def noop_validator_with_annotations(cls: Type['ModelWithAnnotatedValidator'], name: str) -> str:
+        return name

tests/mypy/outputs/plugin_success.txt

@@ -1,3 +1,4 @@
 122: error: Unexpected keyword argument "name" for "AddProject"  [call-arg]
 122: error: Unexpected keyword argument "slug" for "AddProject"  [call-arg]
 122: error: Unexpected keyword argument "description" for "AddProject"  [call-arg]
+202: error: The erased type of self "Type[plugin_success.ModelWithAnnotatedValidator]" is not a supertype of its class "plugin_success.ModelWithAnnotatedValidator"  [misc]