• weakref_slot ：若 True (默认为 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 .

Added in version 3.11.

field s 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'
			

在此范例中，两者 a and b will be included in the added __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.

dataclasses. field ( * , default = MISSING , default_factory = MISSING , init = True , repr = True , hash = None , compare = True , metadata = None , kw_only = MISSING ) ¶

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 值。

参数用于 field() 是：

• 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 __init__() 方法。

• repr : If true (the default), this field is included in the string returned by the generated __repr__() 方法。

• hash : This can be a bool or None . If true, this field is included in the generated __hash__() 方法。若 None (the default), use the value of compare : this would normally be the expected behavior. A field should be considered 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 ( __eq__() , __gt__() , et al.).

• metadata : This can be a mapping or None . None is treated as an empty dict. This value is wrapped in 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 __init__() method’s parameters are computed.

Added in version 3.10.

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
			

类属性 C.z 将是 10 ，类属性 C.t 将是 20 ，和类属性 C.x and C.y will not be set.

class dataclasses. 字段 ¶

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 ：字段的名称。

• type ：字段的类型。

• default , default_factory , init , repr , hash , compare , metadata ，和 kw_only have the identical meaning and values as they do in the field() 函数。

Other attributes may exist, but they are private and must not be inspected or relied on.

dataclasses. 字段 ( 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 .

dataclasses. asdict ( obj , * , dict_factory = dict ) ¶

转换 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, 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() 引发 TypeError if obj 不是 dataclass 实例。

dataclasses. astuple ( obj , * , tuple_factory = tuple ) ¶

转换 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, 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() 引发 TypeError if obj 不是 dataclass 实例。

dataclasses. make_dataclass ( cls_name , 字段 , * , 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 , 模块 = None ) ¶

创建新 dataclass 采用名称 cls_name , fields as defined in 字段 , base classes as given in bases , and initialized with a namespace as given in namespace . 字段 is an iterable whose elements are each either name , (name, type) ，或 (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 ，和 weakref_slot have the same meaning as they do in @dataclass .

若 模块 有定义， __module__ attribute of the dataclass is set to that value. By default, it is set to the module name of the caller.

This function is not strictly required, because any Python mechanism for creating a new class with __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})
			

相当于：

@dataclass
class C:
    x: int
    y: 'typing.Any'
    z: int = 5
    def add_one(self):
        return self.x + 1
											

dataclasses. replace ( obj , / , ** changes ) ¶

创建相同类型的新对象如 obj , replacing fields with values from changes 。若 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 __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 。 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.

dataclasses. is_dataclass ( obj ) ¶

返回 True if its parameter is a dataclass (including subclasses of a dataclass) 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)
											

dataclasses. MISSING ¶

A sentinel value signifying a missing default or default_factory.

dataclasses. KW_ONLY ¶

A sentinel value 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 __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 .

Added in version 3.10.

exception dataclasses. FrozenInstanceError ¶

被引发当隐式定义 __setattr__() or __delattr__() is called on a dataclass which was defined with frozen=True 。它是子类化的 AttributeError .

初始化后处理 ¶

dataclasses. __post_init__ ( ) ¶

When defined on the class, it will be called by the generated __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 __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__() 方法：

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() 处理 init=False 字段。

类变量 ¶

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 PEP 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() 函数。

仅初始变量 ¶

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 dataclasses.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 __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)
			

在此情况下， fields() 将返回 Field 对象为 i and j , but not for database .

冻结实例 ¶

创建真正的不可变 Python 对象是不可能的。不管怎样，通过传递 frozen=True 到 @dataclass decorator you can emulate immutability. In that case, dataclasses will add __setattr__() and __delattr__() methods to the class. These methods will raise a FrozenInstanceError when invoked.

There is a tiny performance penalty when using frozen=True : __init__() cannot use simple assignment to initialize fields, and must use object.__setattr__() .

继承 ¶

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 .

生成的 __init__() 方法对于 C 将看起来像：

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 __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.

在此范例中， Base.y , Base.w ，和 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)
			

生成的 __init__() 方法对于 D 将看起来像：

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.

默认工厂函数 ¶

若 field() 指定 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)
			

若字段被排除从 __init__() (使用 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.

可变默认值 ¶

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.

使用 dataclasses， if 此代码有效：

@dataclass
class D:
    x: list = []      # This code raises ValueError
    def add(self, element):
        self.x.append(element)
			

将生成的代码类似于：

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 版改变： Instead of looking for and disallowing objects of type list , dict ，或 set , unhashable objects are now not allowed as default values. Unhashability is used to approximate mutability.

类型化描述符字段 ¶

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 __init__() method is passed to the descriptor’s __set__() method rather than overwriting the descriptor object.

• Similarly, when getting or setting the field, the descriptor’s __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.

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