`!dataclasses` --- Data Classes dataclasses Source code: Lib/dataclasses.py This module provides a decorator and functions for automatically adding generated special methods such as ~object.__init__ and ~object.__repr__ to user-defined classes. It was originally described in 557. The member variables to use in these generated methods are defined using 526 type annotations. For example, this code: from dataclasses import dataclass @dataclass class InventoryItem: """Class for keeping track of an item in inventory.""" name: str unit_price: float quantity_on_hand: int = 0 def total_cost(self) -> float: return self.unit_price * self.quantity_on_hand will add, among other things, a !__init__ that looks like: def __init__(self, name: str, unit_price: float, quantity_on_hand: int = 0): self.name = name self.unit_price = unit_price self.quantity_on_hand = quantity_on_hand Note that this method is automatically added to the class: it is not directly specified in the !InventoryItem definition shown above. 3.7 Module contents dataclass(*, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False, weakref_slot=False) This function is a decorator that is used to add generated special methods to classes, as described below. The @dataclass decorator examines the class to find fields. A field is defined as a class variable that has a type annotation . With two exceptions described below, nothing in @dataclass examines the type specified in the variable annotation. The order of the fields in all of the generated methods is the order in which they appear in the class definition. The @dataclass decorator will add various "dunder" methods to the class, described below. If any of the added methods already exist in the class, the behavior depends on the parameter, as documented below. The decorator returns the same class that it is called on; no new class is created. If @dataclass is used just as a simple decorator with no parameters, it acts as if it has the default values documented in this signature. That is, these three uses of @dataclass are equivalent: @dataclass class C: ... @dataclass() class C: ... @dataclass(init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False, weakref_slot=False) class C: ... The parameters to @dataclass are: init: If true (the default), a ~object.__init__ method will be generated. If the class already defines !__init__, this parameter is ignored. repr: If true (the default), a ~object.__repr__ method will be generated. The generated repr string will have the class name and the name and repr of each field, in the order they are defined in the class. Fields that are marked as being excluded from the repr are not included. For example: InventoryItem(name='widget', unit_price=3.0, quantity_on_hand=10). If the class already defines !__repr__, this parameter is ignored. eq: If true (the default), an ~object.__eq__ method will be generated. This method compares the class by comparing each field in order. Both instances in the comparison must be of the identical type. If the class already defines !__eq__, this parameter is ignored. 3.13 The generated __eq__ method now compares each field individually (for example, self.a == other.a and self.b == other.b), rather than comparing tuples of fields as in previous versions. This change makes the comparison faster but it may alter results in cases where attributes compare equal by identity but not by value (such as float('nan')). In Python 3.12 and earlier, the comparison was performed by creating tuples of the fields and comparing them (for example, (self.a, self.b) == (other.a, other.b)). order: If true (the default is False), ~object.__lt__, ~object.__le__, ~object.__gt__, and ~object.__ge__ methods will be generated. These compare the class as if it were a tuple of its fields, in order. Both instances in the comparison must be of the identical type. If order is true and eq is false, a ValueError is raised. If the class already defines any of !__lt__, !__le__, !__gt__, or !__ge__, then TypeError is raised. unsafe_hash: If true, force dataclasses to create a ~object.__hash__ method, even though it may not be safe to do so. Otherwise, generate a ~object.__hash__ method according to how eq and frozen are set. The default value is False. !__hash__ is used by built-in hash, and when objects are added to hashed collections such as dictionaries and sets. Having a !__hash__ implies that instances of the class are immutable. Mutability is a complicated property that depends on the programmer's intent, the existence and behavior of !__eq__, and the values of the eq and frozen flags in the @dataclass decorator. By default, @dataclass will not implicitly add a ~object.__hash__ method unless it is safe to do so. Neither will it add or change an existing explicitly defined !__hash__ method. Setting the class attribute __hash__ = None has a specific meaning to Python, as described in the !__hash__ documentation. If !__hash__ is not explicitly defined, or if it is set to None, then @dataclass may add an implicit !__hash__ method. Although not recommended, you can force @dataclass to create a !__hash__ method with unsafe_hash=True. This might be the case if your class is logically immutable but can still be mutated. This is a specialized use case and should be considered carefully. Here are the rules governing implicit creation of a !__hash__ method. Note that you cannot both have an explicit !__hash__ method in your dataclass and set unsafe_hash=True; this will result in a TypeError. If eq and frozen are both true, by default @dataclass will generate a !__hash__ method for you. If eq is true and frozen is false, !__hash__ will be set to None, marking it unhashable (which it is, since it is mutable). If eq is false, !__hash__ will be left untouched meaning the !__hash__ method of the superclass will be used (if the superclass is object, this means it will fall back to id-based hashing). frozen: If true (the default is False), assigning to fields will generate an exception. This emulates read-only frozen instances. See the discussion below. If ~object.__setattr__ or ~object.__delattr__ is defined in the class and frozen is true, then TypeError is raised. match_args: If true (the default is True), the ~object.__match_args__ tuple will be created from the list of non keyword-only parameters to the generated ~object.__init__ method (even if !__init__ is not generated, see above). If false, or if !__match_args__ is already defined in the class, then !__match_args__ will not be generated. 3.10 kw_only: If true (the default value is False), then all fields will be marked as keyword-only. If a field is marked as keyword-only, then the only effect is that the ~object.__init__ parameter generated from a keyword-only field must be specified with a keyword when !__init__ is called. See the parameter glossary entry for details. Also see the KW_ONLY section. Keyword-only fields are not included in !__match_args__. 3.10 slots: If true (the default is False), ~object.__slots__ attribute will be generated and new class will be returned instead of the original one. If !__slots__ is already defined in the class, then TypeError is raised. [!WARNING] Passing parameters to a base class ~object.__init_subclass__ when using slots=True will result in a TypeError. Either use __init_subclass__ with no parameters or use default values as a workaround. See 91126 for full details. 3.10 3.11 If a field name is already included in the !__slots__ of a base class, it will not be included in the generated !__slots__ to prevent overriding them . Therefore, do not use !__slots__ to retrieve the field names of a dataclass. Use fields instead. To be able to determine inherited slots, base class !__slots__ may be any iterable, but not an iterator. weakref_slot: If true (the default is False), add a slot named "__weakref__", which is required to make an instance weakref-able . It is an error to specify weakref_slot=True without also specifying slots=True. 3.11 fields may optionally specify a default value, using normal Python syntax: @dataclass class C: a: int # 'a' has no default value b: int = 0 # assign a default value for 'b' In this example, both !a and !b will be included in the added ~object.__init__ method, which will be defined as: def __init__(self, a: int, b: int = 0): TypeError will be raised if a field without a default value follows a field with a default value. This is true whether this occurs in a single class, or as a result of class inheritance. field(*, default=MISSING, default_factory=MISSING, init=True, repr=True, hash=None, compare=True, metadata=None, kw_only=MISSING, doc=None) For common and simple use cases, no other functionality is required. There are, however, some dataclass features that require additional per-field information. To satisfy this need for additional information, you can replace the default field value with a call to the provided !field function. For example: @dataclass class C: mylist: list[int] = field(default_factory=list) c = C() c.mylist += [1, 2, 3] As shown above, the MISSING value is a sentinel object used to detect if some parameters are provided by the user. This sentinel is used because None is a valid value for some parameters with a distinct meaning. No code should directly use the MISSING value. The parameters to !field are: default: If provided, this will be the default value for this field. This is needed because the !field call itself replaces the normal position of the default value. default_factory: If provided, it must be a zero-argument callable that will be called when a default value is needed for this field. Among other purposes, this can be used to specify fields with mutable default values, as discussed below. It is an error to specify both default and default_factory. init: If true (the default), this field is included as a parameter to the generated ~object.__init__ method. repr: If true (the default), this field is included in the string returned by the generated ~object.__repr__ method. hash: This can be a bool or None. If true, this field is included in the generated ~object.__hash__ method. If false, this field is excluded from the generated ~object.__hash__. If None (the default), use the value of compare: this would normally be the expected behavior, since a field should be included in the hash if it's used for comparisons. Setting this value to anything other than None is discouraged. One possible reason to set hash=False but compare=True would be if a field is expensive to compute a hash value for, that field is needed for equality testing, and there are other fields that contribute to the type's hash value. Even if a field is excluded from the hash, it will still be used for comparisons. compare: If true (the default), this field is included in the generated equality and comparison methods ( ~object.__eq__, ~object.__gt__, et al.). metadata: This can be a mapping or None. None is treated as an empty dict. This value is wrapped in ~types.MappingProxyType to make it read-only, and exposed on the Field object. It is not used at all by Data Classes, and is provided as a third-party extension mechanism. Multiple third-parties can each have their own key, to use as a namespace in the metadata. kw_only: If true, this field will be marked as keyword-only. This is used when the generated ~object.__init__ method's parameters are computed. Keyword-only fields are also not included in !__match_args__. 3.10 doc: optional docstring for this field. 3.14 If the default value of a field is specified by a call to !field, then the class attribute for this field will be replaced by the specified default value. If default is not provided, then the class attribute will be deleted. The intent is that after the dataclass decorator runs, the class attributes will all contain the default values for the fields, just as if the default value itself were specified. For example, after: @dataclass class C: x: int y: int = field(repr=False) z: int = field(repr=False, default=10) t: int = 20 The class attribute !C.z will be 10, the class attribute !C.t will be 20, and the class attributes !C.x and !C.y will not be set. 3.15 If metadata is None, use an empty frozendict, instead of a ~types.MappingProxyType of an empty dict. !Field objects describe each defined field. These objects are created internally, and are returned by the fields module-level method (see below). Users should never instantiate a !Field object directly. Its documented attributes are: !name: The name of the field. !type: The type of the field. !default, !default_factory, !init, !repr, !hash, !compare, !metadata, and !kw_only have the identical meaning and values as they do in the field function. Other attributes may exist, but they are private and must not be inspected or relied on. InitVar[T] type annotations describe variables that are init-only . Fields annotated with !InitVar are considered pseudo-fields, and thus are neither returned by the fields function nor used in any way except adding them as parameters to ~object.__init__ and an optional __post_init__. fields(class_or_instance) Returns a tuple of Field objects that define the fields for this dataclass. Accepts either a dataclass, or an instance of a dataclass. Raises TypeError if not passed a dataclass or instance of one. Does not return pseudo-fields which are ClassVar or InitVar. asdict(obj, *, dict_factory=dict) Converts the dataclass obj to a dict (by using the factory function dict_factory). Each dataclass is converted to a dict of its fields, as name: value pairs. dataclasses, dicts, frozendicts, lists, and tuples are recursed into. Other objects are copied with copy.deepcopy. Example of using !asdict on nested dataclasses: @dataclass class Point: x: int y: int @dataclass class C: mylist: list[Point] p = Point(10, 20) assert asdict(p) == {'x': 10, 'y': 20} c = C([Point(0, 0), Point(10, 4)]) assert asdict(c) == {'mylist': [{'x': 0, 'y': 0}, {'x': 10, 'y': 4}]} To create a shallow copy, the following workaround may be used: {field.name: getattr(obj, field.name) for field in fields(obj)} !asdict raises TypeError if obj is not a dataclass instance. astuple(obj, *, tuple_factory=tuple) Converts the dataclass obj to a tuple (by using the factory function tuple_factory). Each dataclass is converted to a tuple of its field values. dataclasses, dicts, frozendicts, lists, and tuples are recursed into. Other objects are copied with copy.deepcopy. Continuing from the previous example: assert astuple(p) == (10, 20) assert astuple(c) == ([(0, 0), (10, 4)],) To create a shallow copy, the following workaround may be used: tuple(getattr(obj, field.name) for field in dataclasses.fields(obj)) !astuple raises TypeError if obj is not a dataclass instance. make_dataclass(cls_name, fields, *, bases=(), namespace=None, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False, weakref_slot=False, module=None, qualname=None, decorator=dataclass) Creates a new dataclass with name cls_name, fields as defined in fields, base classes as given in bases, and initialized with a namespace as given in namespace. fields is an iterable whose elements are each either name, (name, type), or (name, type, Field). If just name is supplied, typing.Any is used for type. The values of init, repr, eq, order, unsafe_hash, frozen, match_args, kw_only, slots, and weakref_slot have the same meaning as they do in dataclass. If module is defined, the !__module__ attribute of the dataclass is set to that value. By default, it is set to the module name of the caller. If qualname is defined, the ~type.__qualname__ attribute of the dataclass is set to that value. By default, it is set to the value passed to cls_name. The decorator parameter is a callable that will be used to create the dataclass. It should take the class object as a first argument and the same keyword arguments as dataclass. By default, the dataclass function is used. This function is not strictly required, because any Python mechanism for creating a new class with ~object.__annotations__ can then apply the dataclass function to convert that class to a dataclass. This function is provided as a convenience. For example: C = make_dataclass('C', [('x', int), 'y', ('z', int, field(default=5))], namespace={'add_one': lambda self: self.x + 1}) Is equivalent to: @dataclass class C: x: int y: 'typing.Any' z: int = 5 def add_one(self): return self.x + 1 3.14 Added the decorator parameter. 3.16 Added the qualname parameter. replace(obj, /, **changes) Creates a new object of the same type as obj, replacing fields with values from changes. If obj is not a Data Class, raises TypeError. If keys in changes are not field names of the given dataclass, raises TypeError. The newly returned object is created by calling the ~object.__init__ method of the dataclass. This ensures that __post_init__, if present, is also called. Init-only variables without default values, if any exist, must be specified on the call to !replace so that they can be passed to !__init__ and __post_init__. It is an error for changes to contain any fields that are defined as having init=False. A ValueError will be raised in this case. Be forewarned about how init=False fields work during a call to !replace. They are not copied from the source object, but rather are initialized in __post_init__, if they're initialized at all. It is expected that init=False fields will be rarely and judiciously used. If they are used, it might be wise to have alternate class constructors, or perhaps a custom !replace (or similarly named) method which handles instance copying. Dataclass instances are also supported by generic function copy.replace. is_dataclass(obj) Return True if its parameter is a dataclass (including subclasses of a dataclass, but not including generic aliases ) or an instance of one, otherwise return False. If you need to know if a class is an instance of a dataclass (and not a dataclass itself), then add a further check for not isinstance(obj, type): def is_dataclass_instance(obj): return is_dataclass(obj) and not isinstance(obj, type) MISSING A sentinel object signifying a missing default or default_factory. 3.15 !MISSING is now a sentinel object. KW_ONLY A sentinel object used as a type annotation. Any fields after a pseudo-field with the type of !KW_ONLY are marked as keyword-only fields. Note that a pseudo-field of type !KW_ONLY is otherwise completely ignored. This includes the name of such a field. By convention, a name of _ is used for a !KW_ONLY field. Keyword-only fields signify ~object.__init__ parameters that must be specified as keywords when the class is instantiated. In this example, the fields y and z will be marked as keyword-only fields: @dataclass class Point: x: float _: KW_ONLY y: float z: float p = Point(0, y=1.5, z=2.0) In a single dataclass, it is an error to specify more than one field whose type is !KW_ONLY. 3.10 3.15 !KW_ONLY is now a sentinel object. FrozenInstanceError Raised when an implicitly defined ~object.__setattr__ or ~object.__delattr__ is called on a dataclass which was defined with frozen=True. It is a subclass of AttributeError. Post-init processing __post_init__() When defined on the class, it will be called by the generated ~object.__init__, normally as !self.__post_init__. However, if any InitVar fields are defined, they will also be passed to !__post_init__ in the order they were defined in the class. If no !__init__ method is generated, then !__post_init__ will not automatically be called. Among other uses, this allows for initializing field values that depend on one or more other fields. For example: @dataclass class C: a: float b: float c: float = field(init=False) def __post_init__(self): self.c = self.a + self.b The ~object.__init__ method generated by dataclass does not call base class !__init__ methods. If the base class has an !__init__ method that has to be called, it is common to call this method in a __post_init__ method: class Rectangle: def __init__(self, height, width): self.height = height self.width = width @dataclass class Square(Rectangle): side: float def __post_init__(self): super().__init__(self.side, self.side) Note, however, that in general the dataclass-generated !__init__ methods don't need to be called, since the derived dataclass will take care of initializing all fields of any base class that is a dataclass itself. See the section below on init-only variables for ways to pass parameters to !__post_init__. Also see the warning about how replace handles init=False fields. Class variables One of the few places where dataclass actually inspects the type of a field is to determine if a field is a class variable as defined in 526. It does this by checking if the type of the field is typing.ClassVar. If a field is a ClassVar, it is excluded from consideration as a field and is ignored by the dataclass mechanisms. Such ClassVar pseudo-fields are not returned by the module-level fields function. Init-only variables Another place where dataclass inspects a type annotation is to determine if a field is an init-only variable. It does this by seeing if the type of a field is of type InitVar. If a field is an InitVar, it is considered a pseudo-field called an init-only field. As it is not a true field, it is not returned by the module-level fields function. Init-only fields are added as parameters to the generated ~object.__init__ method, and are passed to the optional __post_init__ method. They are not otherwise used by dataclasses. For example, suppose a field will be initialized from a database, if a value is not provided when creating the class: @dataclass class C: i: int j: int | None = None database: InitVar[DatabaseType | None] = None def __post_init__(self, database): if self.j is None and database is not None: self.j = database.lookup('j') c = C(10, database=my_database) In this case, fields will return Field objects for !i and !j, but not for !database. Frozen instances It is not possible to create truly immutable Python objects. However, by passing frozen=True to the dataclass decorator you can emulate immutability. In that case, dataclasses will add ~object.__setattr__ and ~object.__delattr__ methods to the class. These methods will raise a FrozenInstanceError when invoked. There is a tiny performance penalty when using frozen=True: ~object.__init__ cannot use simple assignment to initialize fields, and must use !object.__setattr__. Inheritance When the dataclass is being created by the dataclass decorator, it looks through all of the class's base classes in reverse MRO (that is, starting at object) and, for each dataclass that it finds, adds the fields from that base class to an ordered mapping of fields. After all of the base class fields are added, it adds its own fields to the ordered mapping. All of the generated methods will use this combined, calculated ordered mapping of fields. Because the fields are in insertion order, derived classes override base classes. An example: @dataclass class Base: x: Any = 15.0 y: int = 0 @dataclass class C(Base): z: int = 10 x: int = 15 The final list of fields is, in order, !x, !y, !z. The final type of !x is int, as specified in class !C. The generated ~object.__init__ method for !C will look like: def __init__(self, x: int = 15, y: int = 0, z: int = 10): Re-ordering of keyword-only parameters in !__init__ After the parameters needed for ~object.__init__ are computed, any keyword-only parameters are moved to come after all regular (non-keyword-only) parameters. This is a requirement of how keyword-only parameters are implemented in Python: they must come after non-keyword-only parameters. In this example, !Base.y, !Base.w, and !D.t are keyword-only fields, and !Base.x and !D.z are regular fields: @dataclass class Base: x: Any = 15.0 _: KW_ONLY y: int = 0 w: int = 1 @dataclass class D(Base): z: int = 10 t: int = field(kw_only=True, default=0) The generated !__init__ method for !D will look like: def __init__(self, x: Any = 15.0, z: int = 10, *, y: int = 0, w: int = 1, t: int = 0): Note that the parameters have been re-ordered from how they appear in the list of fields: parameters derived from regular fields are followed by parameters derived from keyword-only fields. The relative ordering of keyword-only parameters is maintained in the re-ordered !__init__ parameter list. Default factory functions If a field specifies a default_factory, it is called with zero arguments when a default value for the field is needed. For example, to create a new instance of a list, use: mylist: list = field(default_factory=list) If a field is excluded from ~object.__init__ (using init=False) and the field also specifies default_factory, then the default factory function will always be called from the generated !__init__ function. This happens because there is no other way to give the field an initial value. Mutable default values Python stores default member variable values in class attributes. Consider this example, not using dataclasses: class C: x = [] def add(self, element): self.x.append(element) o1 = C() o2 = C() o1.add(1) o2.add(2) assert o1.x == [1, 2] assert o1.x is o2.x Note that the two instances of class !C share the same class variable !x, as expected. Using dataclasses, if this code was valid: @dataclass class D: x: list = [] # This code raises ValueError def add(self, element): self.x.append(element) it would generate code similar to: class D: x = [] def __init__(self, x=x): self.x = x def add(self, element): self.x.append(element) assert D().x is D().x This has the same issue as the original example using class !C. That is, two instances of class !D that do not specify a value for !x when creating a class instance will share the same copy of !x. Because dataclasses just use normal Python class creation they also share this behavior. There is no general way for Data Classes to detect this condition. Instead, the dataclass decorator will raise a ValueError if it detects an unhashable default parameter. The assumption is that if a value is unhashable, it is mutable. This is a partial solution, but it does protect against many common errors. Using default factory functions is a way to create new instances of mutable types as default values for fields: @dataclass class D: x: list = field(default_factory=list) assert D().x is not D().x 3.11 Instead of looking for and disallowing objects of type list, dict, or set, unhashable objects are now not allowed as default values. Unhashability is used to approximate mutability. Descriptor-typed fields Fields that are assigned descriptor objects as their default value have the following special behaviors: The value for the field passed to the dataclass's ~object.__init__ method is passed to the descriptor's ~object.__set__ method rather than overwriting the descriptor object. Similarly, when getting or setting the field, the descriptor's ~object.__get__ or !__set__ method is called rather than returning or overwriting the descriptor object. To determine whether a field contains a default value, dataclass will call the descriptor's !__get__ method using its class access form: descriptor.__get__(obj=None, type=cls). If the descriptor returns a value in this case, it will be used as the field's default. On the other hand, if the descriptor raises AttributeError in this situation, no default value will be provided for the field. class IntConversionDescriptor: def __init__(self, *, default): self._default = default def __set_name__(self, owner, name): self._name = "_" + name def __get__(self, obj, type): if obj is None: return self._default return getattr(obj, self._name, self._default) def __set__(self, obj, value): setattr(obj, self._name, int(value)) @dataclass class InventoryItem: quantity_on_hand: IntConversionDescriptor = IntConversionDescriptor(default=100) i = InventoryItem() print(i.quantity_on_hand) # 100 i.quantity_on_hand = 2.5 # calls __set__ with 2.5 print(i.quantity_on_hand) # 2 Note that if a field is annotated with a descriptor type, but is not assigned a descriptor object as its default value, the field will act like a normal field.