# Dataclasses ### Create dataclass ```python from dataclasses import dataclass @dataclass class Position: name: str lon: float lat: float ``` 可以发现，主要起作用的是装饰符@dataclass ，需要注意，如果想要使用dataclass，需要Python 3.7或更高版本 使用dataclass的好处是可以节省书写\_\_init()\_\_等一些常用的实例方法 这里创建一个Position类，用来显示一个地点的位置 * name：地点的名字 * lon：经度 * lat：纬度 新建一个实例来看看: ```python >>> pos = Position('Oslo', 10.8, 59.9) >>> print(pos) Position(name='Oslo', lon=10.8, lat=59.9) >>> pos.lat 59.9 >>> print(f'{pos.name} is at {pos.lat}°N, {pos.lon}°E') Oslo is at 59.9°N, 10.8°E # 除了这种方法，还要一种类似创建namedtuple的方式也可以： from dataclasses import make_dataclass Position = make_dataclass('Position', ['name', 'lat', 'lon']) ``` ### Default Value ```python from dataclasses import dataclass @dataclass class Position: name: str lon: float = 0.0 lat: float = 0.0 # 效果和普通的类设定初始值的效果是一样的： >>> Position('Null Island') Position(name='Null Island', lon=0.0, lat=0.0) >>> Position('Greenwich', lat=51.8) Position(name='Greenwich', lon=0.0, lat=51.8) >>> Position('Vancouver', -123.1, 49.3) Position(name='Vancouver', lon=-123.1, lat=49.3) ``` ### Input hint 大家可以发现我们的Positon类规定了三个属性的类型： * name：str * lon：float * lat：float 现在如果想要开放限制，允许任意的数据类型，可以这么做: ```python from dataclasses import dataclass from typing import Any @dataclass class WithoutExplicitTypes: name: Any value: Any = 42 # 这样运行的时候不会报错，哪怕瞎传参： >>> Position(3.14, 'pi day', 2018) Position(name=3.14, lon='pi day', lat=2018) ``` ### Add a method ```python from dataclasses import dataclass from math import asin, cos, radians, sin, sqrt @dataclass class Position: name: str lon: float = 0.0 lat: float = 0.0 def distance_to(self, other): r = 6371 # Earth radius in kilometers lam_1, lam_2 = radians(self.lon), radians(other.lon) phi_1, phi_2 = radians(self.lat), radians(other.lat) h = (sin((phi_2 - phi_1) / 2)**2 + cos(phi_1) * cos(phi_2) * sin((lam_2 - lam_1) / 2)**2) return 2 * r * asin(sqrt(h)) >>> oslo = Position('Oslo', 10.8, 59.9) >>> vancouver = Position('Vancouver', -123.1, 49.3) >>> oslo.distance_to(vancouver) 7181.7841229421165 ``` ### More flexible use 目前为止我们已经看到了dataclass的基础用法，现在我们看看根据实际需要，有哪些其他灵活的应用方式。 现在创建两个类，纸牌类和牌库类，纸牌类的属性包括花色和数字，牌库是List类型，包含纸牌类的一些实例: ```python from dataclasses import dataclass from typing import List @dataclass class PlayingCard: rank: str suit: str @dataclass class Deck: cards: List[PlayingCard] # 向牌库添加红桃Q和黑桃A >>> queen_of_hearts = PlayingCard('Q', 'Hearts') >>> ace_of_spades = PlayingCard('A', 'Spades') >>> two_cards = Deck([queen_of_hearts, ace_of_spades]) Deck(cards=[PlayingCard(rank='Q', suit='Hearts'), PlayingCard(rank='A', suit='Spades')]) ``` 现在我们可以创建一套完整的扑克牌牌库，注意这里使用了符号表示花色，建议在实际环境中改换为字符串： ```python RANKS = '2 3 4 5 6 7 8 9 10 J Q K A'.split() SUITS = '♣ ♢ ♡ ♠'.split() from dataclasses import dataclass from typing import List @dataclass class Deck: # Will NOT work cards: List[PlayingCard] = make_french_deck() def make_french_deck(): return [PlayingCard(r, s) for s in SUITS for r in RANKS] >>> make_french_deck() [PlayingCard(rank='2', suit='♣'), PlayingCard(rank='3', suit='♣'), ... PlayingCard(rank='K', suit='♠'), PlayingCard(rank='A', suit='♠')] ``` 理论上，我们可以把这个方法作为Deck类的初始变量，但是根本不行，因为它可变： ```python from dataclasses import dataclass from typing import List @dataclass class Deck: # Will NOT work cards: List[PlayingCard] = make_french_deck() # 面对这种情况，dataclass的解决方案是使用default_factory函数作为field的参数来指明： from dataclasses import dataclass, field from typing import List @dataclass class Deck: cards: List[PlayingCard] = field(default_factory=make_french_deck) #现在就没有任何问题： >>> Deck() Deck(cards=[PlayingCard(rank='2', suit='♣'), PlayingCard(rank='3', suit='♣'), ... PlayingCard(rank='K', suit='♠'), PlayingCard(rank='A', suit='♠')]) ``` ### Field 现在简单总结一下dataclass中使用field涉及到的关键参数： * `default`: Default value of the field * `default_factory`: Function that returns the initial value of the field * `init`: Use field in `.__init__()` method? (Default is `True`.) * `repr`: Use field in `repr` of the object? (Default is `True`.) * `compare`: Include the field in comparisons? (Default is `True`.) * `hash`: Include the field when calculating `hash()`? (Default is to use the same as for `compare`.) * `metadata`: A mapping with information about the field 最后这个metadata有点像前端h5中的那个，就是可以为类的一个属性添加一个额外的描述信息： ```python from dataclasses import dataclass, field @dataclass class Position: name: str lon: float = field(default=0.0, metadata={'unit': 'degrees'}) lat: float = field(default=0.0, metadata={'unit': 'degrees'}) # 可以发现，传递的是一个dict，现在可以使用fields来查看一个属性的附加信息了： >>> fields(Position) (Field(name='name',type=,...,metadata={}), Field(name='lon',type=,...,metadata={'unit': 'degrees'}), Field(name='lat',type=,...,metadata={'unit': 'degrees'})) >>> lat_unit = fields(Position)[2].metadata['unit'] >>> lat_unit 'degrees' ``` ### \_\_str\_\_,\_\_repr\_\_ ```python from dataclasses import dataclass @dataclass class PlayingCard: rank: str suit: str def __str__(self): return f'{self.suit}{self.rank}' >>> ace_of_spades = PlayingCard('A', '♠') >>> ace_of_spades PlayingCard(rank='A', suit='♠') >>> print(ace_of_spades) ♠A # 现在再自定义下牌库类Deck的描述 from dataclasses import dataclass, field from typing import List @dataclass class Deck: cards: List[PlayingCard] = field(default_factory=make_french_deck) def __repr__(self): cards = ', '.join(f'{c!s}' for c in self.cards) return f'{self.__class__.__name__}({cards})' >>> Deck() Deck(♣2, ♣3, ♣4, ♣5, ♣6, ♣7, ♣8, ♣9, ♣10, ♣J, ♣Q, ♣K, ♣A, ♢2, ♢3, ♢4, ♢5, ♢6, ♢7, ♢8, ♢9, ♢10, ♢J, ♢Q, ♢K, ♢A, ♡2, ♡3, ♡4, ♡5, ♡6, ♡7, ♡8, ♡9, ♡10, ♡J, ♡Q, ♡K, ♡A, ♠2, ♠3, ♠4, ♠5, ♠6, ♠7, ♠8, ♠9, ♠10, ♠J, ♠Q, ♠K, ♠A) ``` ### Frozen-dataclass ```python from dataclasses import dataclass @dataclass(frozen=True) class Position: name: str lon: float = 0.0 lat: float = 0.0 >>> pos = Position('Oslo', 10.8, 59.9) >>> pos.name 'Oslo' >>> pos.name = 'Stockholm' dataclasses.FrozenInstanceError: cannot assign to field 'name' ``` ### **Inheritance** 使用dataclass的继承也比较简单，和普通类的继承没有太大区别： ```python from dataclasses import dataclass @dataclass class Position: name: str lon: float lat: float @dataclass class Capital(Position): country: str ``` ```python >>> Capital('Oslo', 10.8, 59.9, 'Norway') Capital(name='Oslo', lon=10.8, lat=59.9, country='Norway') ``` 这里可以发现，创建Capital类的实例时会自动继承了父类的属性，我们只需要额外加入country这个新属性就可以了 假如父类初始化时有默认值： ```python from dataclasses import dataclass @dataclass class Position: name: str lon: float = 0.0 lat: float = 0.0 @dataclass class Capital(Position): country: str = 'Unknown' lat: float = 40.0 ``` ### Optimization 可以用我之前提到的slot方法进行优化： ```python from dataclasses import dataclass @dataclass class SimplePosition: name: str lon: float lat: float @dataclass class SlotPosition: __slots__ = ['name', 'lon', 'lat'] name: str lon: float lat: float ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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