编程►Python

Dataclasses 使用Demo

文章目录
  1. 1. 导入库
  2. 2. 原始实现
  3. 3. dataclasses

dataclasses会简化数据类的编码，period。在Python3.7中引入。

导入库

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# https://www.youtube.com/watch?v=vBH6GRJ1REM
import dataclasses
import inspect
from dataclasses import dataclass, field
from pprint import pprint
from typing import List
import attr

原始实现

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class ManualComment:
    def __init__(self, id: int, text: str):
        self.id: int = id
        self.text: str = text

    def __repr__(self):
        return "{}(id={}, text={})".format(self.__class__.__name__, self.id, self.text)

    def __eq__(self, other):
        if other.__class__ is self.__class__:
            return (self.id, self.text) == (other.id, other.text)
        else:
            return NotImplemented

    def __ne__(self, other):
        result = self.__eq__(other)
        if result is NotImplemented:
            return NotImplemented
        else:
            return not result

    def __hash__(self):
        return hash((self.__class__, self.id, self.text))

    def __lt__(self, other):
        if other.__class__ is self.__class__: #类型安全，没有比较不同的类
            return (self.id, self.text) < (other.id, other.text)
        else:
            return NotImplemented

    def __le__(self, other):
        if other.__class__ is self.__class__:
            return (self.id, self.text) <= (other.id, other.text)
        else:
            return NotImplemented

    def __gt__(self, other):
        if other.__class__ is self.__class__:
            return (self.id, self.text) > (other.id, other.text)
        else:
            return NotImplemented

    def __ge__(self, other):
        if other.__class__ is self.__class__:
            return (self.id, self.text) >= (other.id, other.text)
        else:
            return NotImplemented

可能会将上面这个类放在字典中，因此要实现__hash__方法；同时既然可哈了，它的内容最好不可变：通过将数据成员前面添加两个下划线self.__id

同时为了支持比较，也有必要实现__le__&　__gt__等方法；functools 提供total_ordering 装饰器，可以只实现__lt__,其会自动补全剩下的方法。但是会比较慢，因此自己实现会比较好。

  • ManualComment如果再增加一个数据成员 author，需要改动哪些地方？ 太复杂了。

dataclasses

  1. 直接指明有哪些数据成员.通过field函数可以指定成员如何初始化、参与哪些行为, etc
  2. 可为dataclasses.dataclass decorator传参的方式指定类有哪些行为。是否可改变(frozen=True)，是否支持排序(order=True)
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# frozen = True, immutable
# order = True, 

@dataclass(frozen=True, order=True)
class Comment:
    id: int
    text: str = "" # feild(default='')
    replies: List[int] = field(default_factory=list, repr=False, compare=False) # mutable default value?
        #compare=False, 不参与比较
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comment = Comment(1, "I just subscribed!")
print(comment)
Comment(id=1, text='I just subscribed!')

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# frozen=True
comment.id = 3  # can't immutable
---------------------------------------------------------------------------

FrozenInstanceError                       Traceback (most recent call last)

<ipython-input-12-429f24a37a2f> in <module>
----> 1 comment.id = 3


<string> in __setattr__(self, name, value)


FrozenInstanceError: cannot assign to field 'id'

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c1 = Comment(20, 'hello')
c2 = Comment(30, 'jack')
c1 < c2
True
  • default_factory=list #参数的默认值不能是可变类型，否则会因为多个instance共享一个实例产生难以追踪的错误。因此采用构造工厂的方式，每次初始化都新建一个list
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dataclasses.astuple(comment), dataclasses.asdict(comment)
((1, 'I just subscribed!', []),
 {'id': 1, 'text': 'I just subscribed!', 'replies': []})

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inspect.getmembers(Comment, inspect.isfunction)
[('__delattr__',
  <function __main__.__create_fn__.<locals>.__delattr__(self, name)>),
 ('__eq__', <function __main__.__create_fn__.<locals>.__eq__(self, other)>),
 ('__ge__', <function __main__.__create_fn__.<locals>.__ge__(self, other)>),
 ('__gt__', <function __main__.__create_fn__.<locals>.__gt__(self, other)>),
 ('__hash__', <function __main__.__create_fn__.<locals>.__hash__(self)>),
 ('__init__',
  <function __main__.__create_fn__.<locals>.__init__(self, id: int, text: str = '', replies: List[int] = <factory>) -> None>),
 ('__le__', <function __main__.__create_fn__.<locals>.__le__(self, other)>),
 ('__lt__', <function __main__.__create_fn__.<locals>.__lt__(self, other)>),
 ('__repr__', <function __main__.__create_fn__.<locals>.__repr__(self)>),
 ('__setattr__',
  <function __main__.__create_fn__.<locals>.__setattr__(self, name, value)>)]