diff --git a/python/pydantic_core/_pydantic_core.pyi b/python/pydantic_core/_pydantic_core.pyi
index a7b727f86..a6001f63c 100644
--- a/python/pydantic_core/_pydantic_core.pyi
+++ b/python/pydantic_core/_pydantic_core.pyi
@@ -73,7 +73,7 @@ class Some(Generic[_T]):
         Returns the value wrapped by `Some`.
         """
     @classmethod
-    def __class_getitem__(cls, __item: Any) -> Type[Self]: ...
+    def __class_getitem__(cls, item: Any, /) -> Type[Self]: ...
 
 @final
 class SchemaValidator:
diff --git a/python/pydantic_core/core_schema.py b/python/pydantic_core/core_schema.py
index bef04bc3a..ba8ce15ad 100644
--- a/python/pydantic_core/core_schema.py
+++ b/python/pydantic_core/core_schema.py
@@ -225,13 +225,13 @@ def simple_ser_schema(type: ExpectedSerializationTypes) -> SimpleSerSchema:
     return SimpleSerSchema(type=type)
 
 
-# (__input_value: Any) -> Any
+# (input_value: Any, /) -> Any
 GeneralPlainNoInfoSerializerFunction = Callable[[Any], Any]
-# (__input_value: Any, __info: FieldSerializationInfo) -> Any
+# (input_value: Any, info: FieldSerializationInfo, /) -> Any
 GeneralPlainInfoSerializerFunction = Callable[[Any, SerializationInfo], Any]
-# (__model: Any, __input_value: Any) -> Any
+# (model: Any, input_value: Any, /) -> Any
 FieldPlainNoInfoSerializerFunction = Callable[[Any, Any], Any]
-# (__model: Any, __input_value: Any, __info: FieldSerializationInfo) -> Any
+# (model: Any, input_value: Any, info: FieldSerializationInfo, /) -> Any
 FieldPlainInfoSerializerFunction = Callable[[Any, Any, FieldSerializationInfo], Any]
 SerializerFunction = Union[
     GeneralPlainNoInfoSerializerFunction,
@@ -275,7 +275,7 @@ def plain_serializer_function_ser_schema(
         function: The function to use for serialization
         is_field_serializer: Whether the serializer is for a field, e.g. takes `model` as the first argument,
             and `info` includes `field_name`
-        info_arg: Whether the function takes an `__info` argument
+        info_arg: Whether the function takes an `info` argument
         return_schema: Schema to use for serializing return value
         when_used: When the function should be called
     """
@@ -293,16 +293,16 @@ def plain_serializer_function_ser_schema(
 
 
 class SerializerFunctionWrapHandler(Protocol):  # pragma: no cover
-    def __call__(self, __input_value: Any, __index_key: int | str | None = None) -> Any: ...
+    def __call__(self, input_value: Any, index_key: int | str | None = None, /) -> Any: ...
 
 
-# (__input_value: Any, __serializer: SerializerFunctionWrapHandler) -> Any
+# (input_value: Any, serializer: SerializerFunctionWrapHandler, /) -> Any
 GeneralWrapNoInfoSerializerFunction = Callable[[Any, SerializerFunctionWrapHandler], Any]
-# (__input_value: Any, __serializer: SerializerFunctionWrapHandler, __info: SerializationInfo) -> Any
+# (input_value: Any, serializer: SerializerFunctionWrapHandler, info: SerializationInfo, /) -> Any
 GeneralWrapInfoSerializerFunction = Callable[[Any, SerializerFunctionWrapHandler, SerializationInfo], Any]
-# (__model: Any, __input_value: Any, __serializer: SerializerFunctionWrapHandler) -> Any
+# (model: Any, input_value: Any, serializer: SerializerFunctionWrapHandler, /) -> Any
 FieldWrapNoInfoSerializerFunction = Callable[[Any, Any, SerializerFunctionWrapHandler], Any]
-# (__model: Any, __input_value: Any, __serializer: SerializerFunctionWrapHandler, __info: FieldSerializationInfo) -> Any
+# (model: Any, input_value: Any, serializer: SerializerFunctionWrapHandler, info: FieldSerializationInfo, /) -> Any
 FieldWrapInfoSerializerFunction = Callable[[Any, Any, SerializerFunctionWrapHandler, FieldSerializationInfo], Any]
 WrapSerializerFunction = Union[
     GeneralWrapNoInfoSerializerFunction,
@@ -338,7 +338,7 @@ def wrap_serializer_function_ser_schema(
         function: The function to use for serialization
         is_field_serializer: Whether the serializer is for a field, e.g. takes `model` as the first argument,
             and `info` includes `field_name`
-        info_arg: Whether the function takes an `__info` argument
+        info_arg: Whether the function takes an `info` argument
         schema: The schema to use for the inner serialization
         return_schema: Schema to use for serializing return value
         when_used: When the function should be called
@@ -1767,7 +1767,7 @@ def dict_schema(
     )
 
 
-# (__input_value: Any) -> Any
+# (input_value: Any, /) -> Any
 NoInfoValidatorFunction = Callable[[Any], Any]
 
 
@@ -1776,7 +1776,7 @@ class NoInfoValidatorFunctionSchema(TypedDict):
     function: NoInfoValidatorFunction
 
 
-# (__input_value: Any, __info: ValidationInfo) -> Any
+# (input_value: Any, info: ValidationInfo, /) -> Any
 WithInfoValidatorFunction = Callable[[Any, ValidationInfo], Any]
 
 
@@ -1990,7 +1990,7 @@ def __call__(self, input_value: Any, outer_location: str | int | None = None) ->
         ...
 
 
-# (__input_value: Any, __validator: ValidatorFunctionWrapHandler) -> Any
+# (input_value: Any, validator: ValidatorFunctionWrapHandler, /) -> Any
 NoInfoWrapValidatorFunction = Callable[[Any, ValidatorFunctionWrapHandler], Any]
 
 
@@ -1999,7 +1999,7 @@ class NoInfoWrapValidatorFunctionSchema(TypedDict):
     function: NoInfoWrapValidatorFunction
 
 
-# (__input_value: Any, __validator: ValidatorFunctionWrapHandler, __info: ValidationInfo) -> Any
+# (input_value: Any, validator: ValidatorFunctionWrapHandler, info: ValidationInfo, /) -> Any
 WithInfoWrapValidatorFunction = Callable[[Any, ValidatorFunctionWrapHandler, ValidationInfo], Any]
 
 
diff --git a/tests/test_typing.py b/tests/test_typing.py
index 00f58e31f..63d17a790 100644
--- a/tests/test_typing.py
+++ b/tests/test_typing.py
@@ -221,8 +221,8 @@ def test_type_error():
 
 
 def test_ser_function_plain():
-    def f(__input: Any, __info: core_schema.SerializationInfo) -> str:
-        return str(__info)
+    def f(input: Any, info: core_schema.SerializationInfo, /) -> str:
+        return str(info)
 
     s = SchemaSerializer(
         core_schema.any_schema(
@@ -239,9 +239,9 @@ def f(__input: Any, __info: core_schema.SerializationInfo) -> str:
 
 def test_ser_function_wrap():
     def f(
-        __input: Any, __serialize: core_schema.SerializerFunctionWrapHandler, __info: core_schema.SerializationInfo
+        input: Any, serialize: core_schema.SerializerFunctionWrapHandler, info: core_schema.SerializationInfo, /
     ) -> str:
-        return f'{__serialize} {__info}'
+        return f'{serialize} {info}'
 
     s = SchemaSerializer(
         core_schema.any_schema(
diff --git a/tests/validators/test_model_fields.py b/tests/validators/test_model_fields.py
index e21f50008..8a22d96c3 100644
--- a/tests/validators/test_model_fields.py
+++ b/tests/validators/test_model_fields.py
@@ -32,8 +32,8 @@ def __iter__(self):
     def __len__(self) -> int:
         return len(self._d)
 
-    def __getitem__(self, __k):
-        return self._d[__k]
+    def __getitem__(self, k, /):
+        return self._d[k]
 
     def __repr__(self):
         return 'Map({})'.format(', '.join(f'{k}={v!r}' for k, v in self._d.items()))
@@ -1188,9 +1188,9 @@ class Source:
         a = 1
         b = 2
 
-        def __getattribute__(self, __name: str) -> Any:
-            accessed.append(__name)
-            return super().__getattribute__(__name)
+        def __getattribute__(self, name: str, /) -> Any:
+            accessed.append(name)
+            return super().__getattribute__(name)
 
     assert v.validate_python(Source()) == ({'a': 1}, None, {'a'})
     assert 'a' in accessed and 'b' not in accessed
diff --git a/tests/validators/test_typed_dict.py b/tests/validators/test_typed_dict.py
index 8fb25cff6..dc18cd86e 100644
--- a/tests/validators/test_typed_dict.py
+++ b/tests/validators/test_typed_dict.py
@@ -32,8 +32,8 @@ def __iter__(self):
     def __len__(self) -> int:
         return len(self._d)
 
-    def __getitem__(self, __k):
-        return self._d[__k]
+    def __getitem__(self, k, /):
+        return self._d[k]
 
     def __repr__(self):
         return 'Map({})'.format(', '.join(f'{k}={v!r}' for k, v in self._d.items()))