Use __new__ instead of __init__ in serializers

rest_framework-stubs/serializers.pyi

@@ -1,5 +1,5 @@
 from collections.abc import Callable, Iterable, Iterator, Mapping, MutableMapping, Sequence
-from typing import Any, Generic, NoReturn, TypeVar
+from typing import Any, Generic, NoReturn, TypeVar, overload
 from _typeshed import Self
 
 from django.core.exceptions import ValidationError as DjangoValidationError
@@ -81,14 +81,33 @@ class BaseSerializer(Generic[_IN], Field[Any, Any, Any, _IN]):
  instance: _IN | None
  initial_data: Any
  _context: dict[str, Any]
- def __new__(cls: type[Self], *args: Any, **kwargs: Any) -> Self: ...
  def __class_getitem__(cls, *args, **kwargs): ...
- def __init__(
- self,
+ # When both __init__ and __new__ are present, mypy will prefer __init__
+ @overload
+ def __new__(
+ cls: type[Self],
+ instance: Iterable[_IN] | None,
+ many: Literal[True],
+ allow_empty: bool = ...,
+ context: dict[str, Any] = ...,
+ read_only: bool = ...,
+ write_only: bool = ...,
+ required: bool = ...,
+ default: Any = ...,
+ initial: Any = ...,
+ source: str = ...,
+ label: str = ...,
+ help_text: str = ...,
+ style: dict[str, Any] = ...,
+ error_messages: dict[str, str] = ...,
+ validators: Sequence[Validator[Any]] | None = ...,
+ allow_null: bool = ...,
+ ) -> ListSerializer[_IN]: ...
+ @overload
+ def __new__(
+ cls: type[Self],
  instance: _IN | None = ...,
- data: Any = ...,
- partial: bool = ...,
- many: bool = ...,
+ many: Literal[False] = ...,
  allow_empty: bool = ...,
  context: dict[str, Any] = ...,
  read_only: bool = ...,
@@ -103,7 +122,7 @@ class BaseSerializer(Generic[_IN], Field[Any, Any, Any, _IN]):
  error_messages: dict[str, str] = ...,
  validators: Sequence[Validator[Any]] | None = ...,
  allow_null: bool = ...,
- ): ...
+ ) -> Self: ...
  @classmethod
  def many_init(cls, *args: Any, **kwargs: Any) -> BaseSerializer: ...
  def is_valid(self, raise_exception: bool = ...) -> bool: ...
@@ -159,7 +178,7 @@ class ListSerializer(
  allow_empty: bool | None
  def __init__(
  self,
- instance: _IN | None = ...,
+ instance: Iterable[_IN] | None = ...,
  data: Any = ...,
  partial: bool = ...,
  context: dict[str, Any] = ...,
@@ -177,7 +196,7 @@ class ListSerializer(
  error_messages: dict[str, str] = ...,
  validators: Sequence[Validator[list[Any]]] | None = ...,
  allow_null: bool = ...,
- ): ...
+ ) -> None: ...
  def get_initial(self) -> list[Mapping[Any, Any]]: ...
  def validate(self, attrs: Any) -> Any: ...
  @property
@@ -203,27 +222,40 @@ class ModelSerializer(Serializer, BaseSerializer[_MT]):
  exclude: Sequence[str] | None
  depth: int | None
  extra_kwargs: dict[str, dict[str, Any]] # type: ignore[override]
- def __init__(
- self,
+ @overload
+ def __new__( # type: ignore[misc]
+ cls: type[Self],
+ instance: None | _MT | Sequence[_MT] | QuerySet[_MT] | Manager[_MT],
+ many: Literal[True],
+ ) -> ListSerializer[_MT]: ...
+ @overload
+ def __new__(
+ cls: type[Self],
+ instance: None | _MT | Sequence[_MT] | QuerySet[_MT] | Manager[_MT],
+ many: Literal[False],
+ ) -> Self: ...
+ @overload
+ def __new__(
+ cls: type[Self],
  instance: None | _MT | Sequence[_MT] | QuerySet[_MT] | Manager[_MT] = ...,
  data: Any = ...,
  partial: bool = ...,
  many: bool = ...,
+ allow_empty: bool = ...,
  context: dict[str, Any] = ...,
  read_only: bool = ...,
  write_only: bool = ...,
  required: bool = ...,
- default: _MT | Sequence[_MT] | Callable[[], _MT | Sequence[_MT]] = ...,
- initial: _MT | Sequence[_MT] | Callable[[], _MT | Sequence[_MT]] = ...,
+ default: Any = ...,
+ initial: Any = ...,
  source: str = ...,
  label: str = ...,
  help_text: str = ...,
  style: dict[str, Any] = ...,
  error_messages: dict[str, str] = ...,
- validators: Sequence[Validator[_MT]] | None = ...,
+ validators: Sequence[Validator[Any]] | None = ...,
  allow_null: bool = ...,
- allow_empty: bool = ...,
- ): ...
+ ) -> Self: ...
  def update(self, instance: _MT, validated_data: Any) -> _MT: ... # type: ignore[override]
  def create(self, validated_data: Any) -> _MT: ... # type: ignore[override]
  def save(self, **kwargs: Any) -> _MT: ... # type: ignore[override]

tests/typecheck/test_serializers.yml

@@ -75,3 +75,21 @@
  @cached_property
  def fields(self) -> BindingDict:
  return super().fields
+- case: test_serializer_many_equals_false
+ main: |
+ from rest_framework import serializers
+
+ class TestSerializer(serializers.Serializer[int]):
+ pass
+
+ test_serializer = TestSerializer(1)
+ reveal_type(test_serializer) # N: Revealed type is "main.TestSerializer"
+- case: test_serializer_many_equals_true
+ main: |
+ from rest_framework import serializers
+
+ class TestSerializer(serializers.Serializer[int]):
+ pass
+
+ test_serializer = TestSerializer(instance=[1, 2], many=True)
+ reveal_type(test_serializer) # N: Revealed type is "rest_framework.serializers.ListSerializer[builtins.int]"