PEP: 681 Title: Data Class Transforms Author: Erik De Bonte , Eric Traut Sponsor: Jelle Zijlstra Discussions-To: https://mail.python.org/archives/list/typing-sig@python.org/thread/EAALIHA3XEDFDNG2NRXTI3ERFPAD65Z4/ Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 02-Dec-2021 Python-Version: 3.11 Post-History: Abstract ======== :pep:`557` introduced the dataclass to the Python stdlib. Several popular libraries have behaviors that are similar to dataclasses, but these behaviors cannot be described using standard type annotations. Such projects include attrs, pydantic, and object relational mapper (ORM) packages such as SQLAlchemy and Django. Most type checkers, linters and language servers have full support for dataclasses. This proposal aims to generalize this functionality and provide a way for third-party libraries to indicate that certain decorator functions, classes, and metaclasses provide behaviors similar to dataclasses. These behaviors include: * Synthesizing an ``__init__`` method based on declared data fields. * Optionally synthesizing ``__eq__``, ``__ne__``, ``__lt__``, ``__le__``, ``__gt__`` and ``__ge__`` methods. * Supporting "frozen" classes, a way to enforce immutability during static type checking. * Supporting "field descriptors", which describe attributes of individual fields that a static type checker must be aware of, such as whether a default value is provided for the field. The full behavior of the stdlib dataclass is described in the `Python documentation <#dataclass-docs_>`_. This proposal does not affect CPython directly except for the addition of a ``dataclass_transform`` decorator in ``typing.py``. Motivation ========== There is no existing, standard way for libraries with dataclass-like semantics to declare their behavior to type checkers. To work around this limitation, Mypy custom plugins have been developed for many of these libraries, but these plugins don't work with other type checkers, linters or language servers. They are also costly to maintain for library authors, and they require that Python developers know about the existence of these plugins and download and configure them within their environment. Rationale ========= The intent of this proposal is not to support every feature of every library with dataclass-like semantics, but rather to make it possible to use the most common features of these libraries in a way that is compatible with static type checking. If a user values these libraries and also values static type checking, they may need to avoid using certain features or make small adjustments to the way they use them. That's already true for the Mypy custom plugins, which don't support every feature of every dataclass-like library. As new features are added to dataclass in the future, we intend, when appropriate, to add support for those features on ``dataclass_transform`` as well. Keeping these two feature sets in sync will make it easier for dataclass users to understand and use ``dataclass_transform`` and will simplify the maintenance of dataclass support in type checkers. Specification ============= The ``dataclass_transform`` decorator ------------------------------------- This specification introduces a new decorator function in the ``typing`` module named ``dataclass_transform``. This decorator can be applied to either a function that is itself a decorator, a class, or a metaclass. The presence of ``dataclass_transform`` tells a static type checker that the decorated function, class, or metaclass performs runtime "magic" that transforms a class, endowing it with dataclass-like behaviors. If ``dataclass_transform`` is applied to a function, using the decorated function as a decorator is assumed to apply dataclass-like semantics. If the function has overloads, the ``dataclass_transform`` decorator can be applied to the implementation of the function or any one, but not more than one, of the overloads. If ``dataclass_transform`` is applied to a class, dataclass-like semantics will be assumed for any class that derives from the decorated class or uses the decorated class as a metaclass. Examples of each approach are shown in the following sections. Each example creates a ``CustomerModel`` class with dataclass-like semantics. The implementation of the decorated objects is omitted for brevity, but we assume that they modify classes in the following ways: * They synthesize an ``__init__`` method using data fields declared within the class and its parent classes. * They synthesize ``__eq__`` and ``__ne__`` methods. Type checkers supporting this PEP will recognize that the ``CustomerModel`` class can be instantiated using the synthesized ``__init__`` method: .. code-block:: python # Using positional arguments c1 = CustomerModel(327, "John Smith") # Using keyword arguments c2 = CustomerModel(id=327, name="John Smith") # These calls will generate runtime errors and should be flagged as # errors by a static type checker. c3 = CustomerModel() c4 = CustomerModel(327, first_name="John") c5 = CustomerModel(327, "John Smith", 0) Decorator function example '''''''''''''''''''''''''' .. code-block:: python _T = TypeVar("_T") # The ``create_model`` decorator is defined by a library. # This could be in a type stub or inline. @typing.dataclass_transform() def create_model(cls: Type[_T]) -> Type[_T]: cls.__init__ = ... cls.__eq__ = ... cls.__ne__ = ... return cls # The ``create_model`` decorator can now be used to create new model # classes, like this: @create_model class CustomerModel: id: int name: str Class example ''''''''''''' .. code-block:: python # The ``ModelBase`` class is defined by a library. This could be in # a type stub or inline. @typing.dataclass_transform() class ModelBase: ... # The ``ModelBase`` class can now be used to create new model # subclasses, like this: class CustomerModel(ModelBase): id: int name: str Metaclass example ''''''''''''''''' .. code-block:: python # The ``ModelMeta`` metaclass and ``ModelBase`` class are defined by # a library. This could be in a type stub or inline. @typing.dataclass_transform() class ModelMeta(type): ... class ModelBase(metaclass=ModelMeta): ... # The ``ModelBase`` class can now be used to create new model # subclasses, like this: class CustomerModel(ModelBase): id: int name: str Decorator function and class/metaclass parameters ------------------------------------------------- A decorator function, class, or metaclass that provides dataclass-like functionality may accept parameters that modify certain behaviors. This specification defines the following parameters that static type checkers must honor if they are used by a dataclass transform. Each of these parameters accepts a bool argument, and it must be possible for the bool value (``True`` or ``False``) to be statically evaluated. * ``eq``. ``order``, ``frozen``, ``init`` and ``unsafe_hash`` are parameters supported in the stdlib dataclass, with meanings defined in :pep:`PEP 557 <557#id7>`. * ``kw_only``, ``match_args`` and ``slots`` are parameters supported in the stdlib dataclass, first introduced in Python 3.10. ``dataclass_transform`` parameters ---------------------------------- Parameters to ``dataclass_transform`` allow for some basic customization of default behaviors: .. code-block:: python _T = TypeVar("_T") def dataclass_transform( *, eq_default: bool = True, order_default: bool = False, kw_only_default: bool = False, field_descriptors: tuple[type | Callable[..., Any], ...] = (), ) -> Callable[[_T], _T]: ... * ``eq_default`` indicates whether the ``eq`` parameter is assumed to be True or False if it is omitted by the caller. If not specified, ``eq_default`` will default to True (the default assumption for dataclass). * ``order_default`` indicates whether the ``order`` parameter is assumed to be True or False if it is omitted by the caller. If not specified, ``order_default`` will default to False (the default assumption for dataclass). * ``kw_only_default`` indicates whether the ``kw_only`` parameter is assumed to be True or False if it is omitted by the caller. If not specified, ``kw_only_default`` will default to False (the default assumption for dataclass). * ``field_descriptors`` specifies a static list of supported classes that describe fields. Some libraries also supply functions to allocate instances of field descriptors, and those functions may also be specified in this tuple. If not specified, ``field_descriptors`` will default to an empty tuple (no field descriptors supported). The standard dataclass behavior supports only one type of field descriptor called ``Field`` plus a helper function (``field``) that instantiates this class, so if we were describing the stdlib dataclass behavior, we would provide the tuple argument ``(dataclasses.Field, dataclasses.field)``. In the future, we may add additional parameters to ``dataclass_transform`` as needed to support common behaviors in user code. These additions will be made after reaching consensus on typing-sig rather than via additional PEPs. The following sections provide additional examples showing how these parameters are used. Decorator function example '''''''''''''''''''''''''' .. code-block:: python # Indicate that the ``create_model`` function assumes keyword-only # parameters for the synthesized ``__init__`` method unless it is # invoked with ``kw_only=False``. It always synthesizes order-related # methods and provides no way to override this behavior. @typing.dataclass_transform(kw_only_default=True, order_default=True) def create_model( *, frozen: bool = False, kw_only: bool = True, ) -> Callable[[Type[_T]], Type[_T]]: ... # Example of how this decorator would be used by code that imports # from this library: @create_model(frozen=True, kw_only=False) class CustomerModel: id: int name: str Class example ''''''''''''' .. code-block:: python # Indicate that classes that derive from this class default to # synthesizing comparison methods. @typing.dataclass_transform(eq_default=True, order_default=True) class ModelBase: def __init_subclass__( cls, *, init: bool = True, frozen: bool = False, eq: bool = True, order: bool = True, ): ... # Example of how this class would be used by code that imports # from this library: class CustomerModel( ModelBase, init=False, frozen=True, eq=False, order=False, ): id: int name: str Metaclass example ''''''''''''''''' .. code-block:: python # Indicate that classes that use this metaclass default to # synthesizing comparison methods. @typing.dataclass_transform(eq_default=True, order_default=True) class ModelMeta(type): def __new__( cls, name, bases, namespace, *, init: bool = True, frozen: bool = False, eq: bool = True, order: bool = True, ): ... class ModelBase(metaclass=ModelMeta): ... # Example of how this class would be used by code that imports # from this library: class CustomerModel( ModelBase, init=False, frozen=True, eq=False, order=False, ): id: int name: str Field descriptors ----------------- Most libraries that support dataclass-like semantics provide one or more "field descriptor" types that allow a class definition to provide additional metadata about each field in the class. This metadata can describe, for example, default values, or indicate whether the field should be included in the synthesized ``__init__`` method. Field descriptors can be omitted in cases where additional metadata is not required: .. code-block:: python @dataclass class Employee: # Field with no descriptor name: str # Field that uses field descriptor class instance age: Optional[int] = field(default=None, init=False) # Field with type annotation and simple initializer to # describe default value is_paid_hourly: bool = True # Not a field (but rather a class variable) because type # annotation is not provided. office_number = "unassigned" Field descriptor parameters ''''''''''''''''''''''''''' Libraries that support dataclass-like semantics and support field descriptor classes typically use common parameter names to construct these field descriptors. This specification formalizes the names and meanings of the parameters that must be understood for static type checkers. These standardized parameters must be keyword-only. These parameters are a superset of those supported by ``dataclasses.field``, excluding those that do not have an impact on type checking such as ``compare`` and ``hash``. Field descriptor classes are allowed to use other parameters in their constructors, and those parameters can be positional and may use other names. * ``init`` is an optional bool parameter that indicates whether the field should be included in the synthesized ``__init__`` method. If unspecified, ``init`` defaults to True. Field descriptor functions can use overloads that implicitly specify the value of ``init`` using a literal bool value type (``Literal[False]`` or ``Literal[True]``). * ``default`` is an optional parameter that provides the default value for the field. * ``default_factory`` is an optional parameter that provides a runtime callback that returns the default value for the field. If neither ``default`` nor ``default_factory`` are specified, the field is assumed to have no default value and must be provided a value when the class is instantiated. * ``factory`` is an alias for ``default_factory``. Stdlib dataclasses use the name ``default_factory``, but attrs uses the name ``factory`` in many scenarios, so this alias is necessary for supporting attrs. * ``kw_only`` is an optional bool parameter that indicates whether the field should be marked as keyword-only. If true, the field will be keyword-only. If false, it will not be keyword-only. If unspecified, the value of the ``kw_only`` parameter on the object decorated with ``dataclass_transform`` will be used, or if that is unspecified, the value of ``kw_only_default`` on ``dataclass_transform`` will be used. * ``alias`` is an optional str parameter that provides an alternative name for the field. This alternative name is used in the synthesized ``__init__`` method. It is an error to specify more than one of ``default``, ``default_factory`` and ``factory``. This example demonstrates the above: .. code-block:: python # Library code (within type stub or inline) # In this library, passing a resolver means that init must be False, # and the overload with Literal[False] enforces that. @overload def model_field( *, default: Optional[Any] = ..., resolver: Callable[[], Any], init: Literal[False] = False, ) -> Any: ... @overload def model_field( *, default: Optional[Any] = ..., resolver: None = None, init: bool = True, ) -> Any: ... @typing.dataclass_transform( kw_only_default=True, field_descriptors=(model_field, )) def create_model( *, init: bool = True, ) -> Callable[[Type[_T]], Type[_T]]: ... # Code that imports this library: @create_model(init=False) class CustomerModel: id: int = model_field(resolver=lambda : 0) name: str Runtime behavior ---------------- At runtime, the ``dataclass_transform`` decorator's only effect is to set a string attribute named ``__dataclass_transform__`` on the decorated function or class to support introspection. The value of the attribute should be a dict mapping the names of the ``dataclass_transform`` parameters to their values. For example: .. code-block:: python { "eq_default": True, "order_default": False, "kw_only_default": False, "field_descriptors": (), } Dataclass semantics ------------------- Except where stated otherwise in this PEP, classes impacted by ``dataclass_transform``, either by inheriting from a class that is decorated with ``dataclass_transform`` or by being decorated with a function decorated with ``dataclass_transform``, are assumed to behave like stdlib ``dataclass``. This includes, but is not limited to, the following semantics: * Frozen dataclasses cannot inherit from non-frozen dataclasses. A class that has been decorated with ``dataclass_transform`` is considered neither frozen nor non-frozen, thus allowing frozen classes to inherit from it. Similarly, a class that directly specifies a metaclass that is decorated with ``dataclass_transform`` is considered neither frozen nor non-frozen. Consider these class examples: .. code-block:: python # ModelBase is not considered either "frozen" or "non-frozen" # because it is decorated with ``dataclass_transform`` @typing.dataclass_transform() class ModelBase(): ... # Vehicle is considered non-frozen because it does not specify # "frozen=True". class Vehicle(ModelBase): name: str # Car is a frozen class that derives from Vehicle, which is a # non-frozen class. This is an error. class Car(Vehicle, frozen=True): wheel_count: int And these similar metaclass examples: .. code-block:: python @typing.dataclass_transform() class ModelMeta(type): ... # ModelBase is not considered either "frozen" or "non-frozen" # because it directly specifies ModelMeta as its metaclass. class ModelBase(metaclass=ModelMeta): ... # Vehicle is considered non-frozen because it does not specify # "frozen=True". class Vehicle(ModelBase): name: str # Car is a frozen class that derives from Vehicle, which is a # non-frozen class. This is an error. class Car(Vehicle, frozen=True): wheel_count: int * Field ordering and inheritance is assumed to follow the rules specified in :pep:`557 <557#inheritance>`. This includes the effects of overrides (redefining a field in a child class that has already been defined in a parent class). * :pep:`PEP 557 indicates <557#post-init-parameters>` that all fields without default values must appear before fields with default values. Although not explicitly stated in PEP 557, this rule is ignored when ``init=False``, and this specification likewise ignores this requirement in that situation. Likewise, there is no need to enforce this ordering when keyword-only parameters are used for ``__init__``, so the rule is not enforced if ``kw_only`` semantics are in effect. * As with dataclass, method synthesis is skipped if it would overwrite a method that is explicitly declared within the class. For example, if a class declares an ``__init__`` method explicitly, an ``__init__`` method will not be synthesized for that class. * KW_ONLY sentinel values are supported as described in `the Python docs <#kw-only-docs_>`_ and `bpo-43532 <#kw-only-issue_>`_. * ClassVar attributes are not considered dataclass fields and are `ignored by dataclass mechanisms <#class-var_>`_. Undefined behavior ------------------ If multiple ``dataclass_transform`` decorators are found, either on a single function (including its overloads), a single class, or within a class hierarchy, the resulting behavior is undefined. Library authors should avoid these scenarios. Reference Implementation ======================== `Pyright <#pyright_>`_ contains the reference implementation of type checker support for ``dataclass_transform``. Pyright's ``dataClasses.ts`` `source file <#pyright-impl_>`_ would be a good starting point for understanding the implementation. The `attrs <#attrs-usage_>`_ and `pydantic <#pydantic-usage_>`_ libraries are using ``dataclass_transform`` and serve as real-world examples of its usage. Rejected Ideas ============== ``auto_attribs`` parameter -------------------------- The attrs library supports an ``auto_attribs`` parameter that indicates whether class members decorated with :pep:`526` variable annotations but with no assignment should be treated as data fields. We considered supporting ``auto_attribs`` and a corresponding ``auto_attribs_default`` parameter, but decided against this because it is specific to attrs and appears to be a legacy behavior. Instead of supporting this in the new standard, we recommend that the maintainers of attrs move away from the legacy semantics and adopt ``auto_attribs`` behaviors by default. Django does not support declaring fields using type annotations only, so Django users who leverage ``dataclass_transform`` should be aware that they should always supply assigned values. ``cmp`` parameter ----------------- The attrs library supports a bool parameter ``cmp`` that is equivalent to setting both ``eq`` and ``order`` to True. We chose not to support a ``cmp`` parameter, since it only applies to attrs. Attrs users should use the dataclass-standard ``eq`` and ``order`` parameter names instead. Automatic field name aliasing ----------------------------- The attrs library performs `automatic aliasing <#attrs-aliasing_>`_ of field names that start with a single underscore, stripping the underscore from the name of the corresponding ``__init__`` parameter. This proposal omits that behavior since it is specific to attrs. Users can manually alias these fields using the ``alias`` parameter. Alternate field ordering algorithms ----------------------------------- The attrs library currently supports two approaches to ordering the fields within a class: * Dataclass order: The same ordering used by dataclasses. This is the default behavior of the older APIs (e.g. ``attr.s``). * Method Resolution Order (MRO): This is the default behavior of the newer APIs (e.g. define, mutable, frozen). Older APIs (e.g. ``attr.s``) can opt into this behavior by specifying ``collect_by_mro=True``. The resulting field orderings can differ in certain diamond-shaped multiple inheritance scenarios. For simplicity, this proposal does not support any field ordering other than that used by dataclasses. Fields redeclared in subclasses ------------------------------- The attrs library differs from stdlib dataclasses in how it handles inherited fields that are redeclared in subclasses. The dataclass specification preserves the original order, but attrs defines a new order based on subclasses. For simplicity, we chose to only support the dataclass behavior. Users of attrs who rely on the attrs-specific ordering will not see the expected order of parameters in the synthesized ``__init__`` method. Django primary and foreign keys ------------------------------- Django applies `additional logic for primary and foreign keys <#django-ids_>`_. For example, it automatically adds an ``id`` field (and ``__init__`` parameter) if there is no field designated as a primary key. As this is not broadly applicable to dataclass libraries, this additional logic is not accommodated with this proposal, so users of Django would need to explicitly declare the ``id`` field. Class-wide default values ------------------------- SQLAlchemy requested that we expose a way to specify that the default value of all fields in the transformed class is None. It is typical that all of their fields are optional, and None indicates that the field is not set. We chose not to support this feature, since it is specific to SQLAlchemy. Users can manually set ``default=None`` on these fields instead. Descriptor-typed field support ------------------------------ We considered adding a boolean parameter on ``dataclass_transform`` to enable better support for fields with descriptor types, which is common in SQLAlchemy. When enabled, the type of each parameter on the synthesized ``__init__`` method corresponding to a descriptor-typed field would be the type of the value parameter to the descriptor's ``__set__`` method rather than the descriptor type itself. Similarly, when setting the field, the ``__set__`` value type would be expected. And when getting the value of the field, its type would be expected to match the return type of ``__get__``. This idea was based on the belief that ``dataclass`` did not properly support descriptor-typed fields. In fact it does, but type checkers (at least mypy and pyright) did not reflect the runtime behavior which led to our misunderstanding. For more details, see the `Pyright bug <#pyright-descriptor-bug_>`__. ``converter`` field descriptor parameter ---------------------------------------- The attrs library supports a ``converter`` field descriptor parameter, which is a ``Callable`` that is called by the generated ``__init__`` method to convert the supplied value to some other desired value. This is tricky to support since the parameter type in the synthesized ``__init__`` method needs to accept uncovered values, but the resulting field is typed according to the output of the converter. Some aspects of this issue are detailed in a `Pyright discussion <#converters_>`_. There may be no good way to support this because there's not enough information to derive the type of the input parameter. One possible solution would be to add support for a ``converter`` field descriptor parameter but then use the ``Any`` type for the corresponding parameter in the ``__init__`` method. We chose not to support this feature and recommend that attrs users avoid converters. References ========== .. _#dataclass-docs: https://docs.python.org/3.11/library/dataclasses.html .. _#pyright: https://github.com/Microsoft/pyright .. _#pyright-impl: https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/dataClasses.ts .. _#attrs-usage: https://github.com/python-attrs/attrs/pull/796 .. _#pydantic-usage: https://github.com/samuelcolvin/pydantic/pull/2721 .. _#attrs-aliasing: https://www.attrs.org/en/stable/init.html#private-attributes .. _#django-ids: https://docs.djangoproject.com/en/4.0/topics/db/models/#automatic-primary-key-fields .. _#converters: https://github.com/microsoft/pyright/discussions/1782?sort=old#discussioncomment-653909 .. _#kw-only-docs: https://docs.python.org/3/library/dataclasses.html#dataclasses.KW_ONLY .. _#kw-only-issue: https://bugs.python.org/issue43532 .. _#class-var: https://docs.python.org/3/library/dataclasses.html#class-variables .. _#pyright-descriptor-bug: https://github.com/microsoft/pyright/issues/3245 Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.