我目前正在尝试Python 3.7中引入的新数据类结构。我目前被困在试图做一些继承的父类。看起来参数的顺序被我当前的方法搞砸了,比如子类中的bool形参在其他形参之前传递。这将导致一个类型错误。

from dataclasses import dataclass

@dataclass
class Parent:
    name: str
    age: int
    ugly: bool = False

    def print_name(self):
        print(self.name)

    def print_age(self):
        print(self.age)

    def print_id(self):
        print(f'The Name is {self.name} and {self.name} is {self.age} year old')

@dataclass
class Child(Parent):
    school: str
    ugly: bool = True


jack = Parent('jack snr', 32, ugly=True)
jack_son = Child('jack jnr', 12, school = 'havard', ugly=True)

jack.print_id()
jack_son.print_id()

当我运行这段代码时,我得到这个TypeError:

TypeError: non-default argument 'school' follows default argument

我怎么解决这个问题?


当前回答

一个实验性但有趣的解决方案是使用元类。下面的解决方案允许使用带有简单继承的Python数据类,而完全不使用数据类装饰器。此外,它可以继承父基类的字段,而不必抱怨位置参数的顺序(非默认字段)。

from collections import OrderedDict
import typing as ty
import dataclasses
from itertools import takewhile

class DataClassTerm:
    def __new__(cls, *args, **kwargs):
        return super().__new__(cls)

class DataClassMeta(type):
    def __new__(cls, clsname, bases, clsdict):
        fields = {}

        # Get list of base classes including the class to be produced(initialized without its original base classes as those have already become dataclasses)
        bases_and_self = [dataclasses.dataclass(super().__new__(cls, clsname, (DataClassTerm,), clsdict))] + list(bases)

        # Whatever is a subclass of DataClassTerm will become a DataClassTerm. 
        # Following block will iterate and create individual dataclasses and collect their fields
        for base in bases_and_self[::-1]: # Ensure that last fields in last base is prioritized
            if issubclass(base, DataClassTerm):
                to_dc_bases = list(takewhile(lambda c: c is not DataClassTerm, base.__mro__))
                for dc_base in to_dc_bases[::-1]: # Ensure that last fields in last base in MRO is prioritized(same as in dataclasses)
                    if dataclasses.is_dataclass(dc_base):
                        valid_dc = dc_base
                    else:
                        valid_dc = dataclasses.dataclass(dc_base)
                    for field in dataclasses.fields(valid_dc):
                        fields[field.name] = (field.name, field.type, field)
        
        # Following block will reorder the fields so that fields without default values are first in order
        reordered_fields = OrderedDict()
        for n, t, f  in fields.values():
            if f.default is dataclasses.MISSING and f.default_factory is dataclasses.MISSING:
                reordered_fields[n] = (n, t, f)
        for n, t, f  in fields.values():
            if n not in reordered_fields.keys():
                reordered_fields[n] = (n, t, f)
        
        # Create a new dataclass using `dataclasses.make_dataclass`, which ultimately calls type.__new__, which is the same as super().__new__ in our case
        fields = list(reordered_fields.values())
        full_dc = dataclasses.make_dataclass(cls_name=clsname, fields=fields, init=True, bases=(DataClassTerm,))
        
        # Discard the created dataclass class and create new one using super but preserve the dataclass specific namespace.
        return super().__new__(cls, clsname, bases, {**full_dc.__dict__,**clsdict})
    
class DataClassCustom(DataClassTerm, metaclass=DataClassMeta):
    def __new__(cls, *args, **kwargs):
        if len(args)>0:
            raise RuntimeError("Do not use positional arguments for initialization.")
        return super().__new__(cls, *args, **kwargs)

现在让我们创建一个带有父数据类和混合类的样本数据类:

class DataClassCustomA(DataClassCustom):
    field_A_1: int = dataclasses.field()
    field_A_2: ty.AnyStr = dataclasses.field(default=None)

class SomeOtherClass:
    def methodA(self):
        print('print from SomeOtherClass().methodA')

class DataClassCustomB(DataClassCustomA,SomeOtherClass):
    field_B_1: int = dataclasses.field()
    field_B_2: ty.Dict = dataclasses.field(default_factory=dict)

结果是

result_b = DataClassCustomB(field_A_1=1, field_B_1=2)

result_b
# DataClassCustomB(field_A_1=1, field_B_1=2, field_A_2=None, field_B_2={})

result_b.methodA()
# print from SomeOtherClass().methodA

尝试在每个父类上使用@dataclass装饰器做同样的事情会在接下来的子类中引发一个异常,如TypeError(f'non-default argument <field-name)跟随默认参数')。上面的解决方案防止了这种情况的发生,因为字段首先被重新排序。然而,由于字段的顺序被修改了,在DataClassCustom中防止*args的使用。__new__是强制的,因为原来的顺序不再有效。

虽然在Python >=3.10中引入了kw_only特性,本质上使数据类中的继承更加可靠,但上面的示例仍然可以用作一种使数据类可继承的方法,而不需要使用@dataclass装饰器。

其他回答

如果将属性从init函数中排除,则可以在父类中使用带有默认值的属性。如果您需要覆盖init的默认值,请使用Praveen Kulkarni的答案扩展代码。

from dataclasses import dataclass, field

@dataclass
class Parent:
    name: str
    age: int
    ugly: bool = field(default=False, init=False)

@dataclass
class Child(Parent):
    school: str

jack = Parent('jack snr', 32)
jack_son = Child('jack jnr', 12, school = 'havard')
jack_son.ugly = True

甚至

@dataclass
class Child(Parent):
    school: str
    ugly = True
    # This does not work
    # ugly: bool = True

jack_son = Child('jack jnr', 12, school = 'havard')
assert jack_son.ugly

如何像这样定义丑陋的字段,而不是默认的方式?

ugly: bool = field(metadata=dict(required=False, missing=False))

当您使用Python继承创建数据类时,不能保证所有具有默认值的字段将出现在所有没有默认值的字段之后。

一个简单的解决方案是避免使用多重继承来构造“合并”数据类。相反,我们可以通过对父数据类的字段进行过滤和排序来构建合并的数据类。

试试这个merge_dataclasses()函数:

import dataclasses
import functools
from typing import Iterable, Type


def merge_dataclasses(
    cls_name: str,
    *,
    merge_from: Iterable[Type],
    **kwargs,
):
    """
    Construct a dataclass by merging the fields
    from an arbitrary number of dataclasses.

    Args:
        cls_name: The name of the constructed dataclass.

        merge_from: An iterable of dataclasses
            whose fields should be merged.

        **kwargs: Keyword arguments are passed to
            :py:func:`dataclasses.make_dataclass`.

    Returns:
        Returns a new dataclass
    """
    # Merge the fields from the dataclasses,
    # with field names from later dataclasses overwriting
    # any conflicting predecessor field names.
    each_base_fields = [d.__dataclass_fields__ for d in merge_from]
    merged_fields = functools.reduce(
        lambda x, y: {**x, **y}, each_base_fields
    )

    # We have to reorder all of the fields from all of the dataclasses
    # so that *all* of the fields without defaults appear
    # in the merged dataclass *before* all of the fields with defaults.
    fields_without_defaults = [
        (f.name, f.type, f)
        for f in merged_fields.values()
        if isinstance(f.default, dataclasses._MISSING_TYPE)
    ]
    fields_with_defaults = [
        (f.name, f.type, f)
        for f in merged_fields.values()
        if not isinstance(f.default, dataclasses._MISSING_TYPE)
    ]
    fields = [*fields_without_defaults, *fields_with_defaults]

    return dataclasses.make_dataclass(
        cls_name=cls_name,
        fields=fields,
        **kwargs,
    )

然后,您可以按照如下方式合并数据类。注意,我们可以合并A和B,默认字段B和d被移动到合并的数据类的末尾。

@dataclasses.dataclass
class A:
    a: int
    b: int = 0


@dataclasses.dataclass
class B:
    c: int
    d: int = 0


C = merge_dataclasses(
    "C",
    merge_from=[A, B],
)

# Note that 
print(C(a=1, d=1).__dict__)
# {'a': 1, 'd': 1, 'b': 0, 'c': 0}

当然,这种解决方案的缺陷是C实际上不继承A和B,这意味着您不能使用isinstance()或其他类型断言来验证C的亲本。

一种可行的解决方法是使用monkey-patch来附加父字段

import dataclasses as dc

def add_args(parent): 
    def decorator(orig):
        "Append parent's fields AFTER orig's fields"

        # Aggregate fields
        ff  = [(f.name, f.type, f) for f in dc.fields(dc.dataclass(orig))]
        ff += [(f.name, f.type, f) for f in dc.fields(dc.dataclass(parent))]

        new = dc.make_dataclass(orig.__name__, ff)
        new.__doc__ = orig.__doc__

        return new
    return decorator

class Animal:
    age: int = 0 

@add_args(Animal)
class Dog:
    name: str
    noise: str = "Woof!"

@add_args(Animal)
class Bird:
    name: str
    can_fly: bool = True

Dog("Dusty", 2)               # --> Dog(name='Dusty', noise=2, age=0)
b = Bird("Donald", False, 40) # --> Bird(name='Donald', can_fly=False, age=40)

也可以预先添加非默认字段, 通过检查f.default是否为dc。失踪, 但这可能太脏了。

虽然猴子补丁缺乏遗传的一些特征, 它仍然可以用于向所有伪子类添加方法。

对于更细粒度的控制,请设置默认值 使用直流。字段(compare=False, repr=True,…)

在发现数据类可能会获得一个允许字段重新排序的装饰器参数后,我回到了这个问题。这无疑是一个有希望的发展,尽管这一功能的进展似乎有些停滞。

现在,您可以通过使用dataclassy(我对数据类的重新实现,克服了这种挫折)来获得这种行为,以及其他一些细节。在原始示例中使用from dataclassy来代替from dataclassy意味着它运行时没有错误。

使用inspect打印Child的签名使正在发生的事情变得清晰;结果是(name: str, age: int, school: str, ugly: bool = True)。字段总是重新排序,以便在初始化式的参数中,具有默认值的字段位于不具有默认值的字段之后。两个列表(没有默认值的字段和有默认值的字段)仍然按照定义顺序排序。

面对这个问题是促使我编写数据类替代品的因素之一。这里详细介绍的变通方法虽然很有用,但要求将代码扭曲到完全否定数据类的简单方法(即字段顺序可以简单地预测)所提供的可读性优势的程度。