标准库: dataclasses
该模块提供了一个装饰器和函数，可以根据用户定义的类，自动生成__init__ 、__repr__等方法。
dataclasses模块中dataclass函数的实现：
def dataclass(cls=None, /, *, init=True, repr=True, eq=True, order=False,
unsafe_hash=False, frozen=False, match_args=True,
kw_only=False, slots=False):
"""Returns the same class as was passed in, with dunder methods
added based on the fields defined in the class.
Examines PEP 526 __annotations__ to determine fields.
If init is true, an __init__() method is added to the class. If
repr is true, a __repr__() method is added. If order is true, rich
comparison dunder methods are added. If unsafe_hash is true, a
__hash__() method function is added. If frozen is true, fields may
not be assigned to after instance creation. If match_args is true,
the __match_args__ tuple is added. If kw_only is true, then by
default all fields are keyword-only. If slots is true, an
__slots__ attribute is added.
"""
(...)
PEP 526规定的注释方式，被注释的简单名称会放到__annotations__ 映射中。
利用被注释的成员变量（field）去生成变量和对应方法，减少劳动。
这也意味着没有被类型注释的成员变量不会被考虑。
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
相当于生成以下方法：
def __init__(self, name: str, unit_price: float, quantity_on_hand: int = 0) -> None:
self.name = name
self.unit_price = unit_price
self.quantity_on_hand = quantity_on_hand
def __repr__(self):
return f'InventoryItem(name={self.name!r}, unit_price={self.unit_price!r}, quantity_on_hand={self.quantity_on_hand!r})'
def __eq__(self, other):
if other.__class__ is self.__class__:
return (self.name, self.unit_price, self.quantity_on_hand) == (other.name, other.unit_price, other.quantity_on_hand)
return NotImplemented
def __ne__(self, other):
if other.__class__ is self.__class__:
return (self.name, self.unit_price, self.quantity_on_hand) != (other.name, other.unit_price, other.quantity_on_hand)
return NotImplemented
def __lt__(self, other):
if other.__class__ is self.__class__:
return (self.name, self.unit_price, self.quantity_on_hand) < (other.name, other.unit_price, other.quantity_on_hand)
return NotImplemented
def __le__(self, other):
if other.__class__ is self.__class__:
return (self.name, self.unit_price, self.quantity_on_hand) <= (other.name, other.unit_price, other.quantity_on_hand)
return NotImplemented
def __gt__(self, other):
if other.__class__ is self.__class__:
return (self.name, self.unit_price, self.quantity_on_hand) > (other.name, other.unit_price, other.quantity_on_hand)
return NotImplemented
def __ge__(self, other):
if other.__class__ is self.__class__:
return (self.name, self.unit_price, self.quantity_on_hand) >= (other.name, other.unit_price, other.quantity_on_hand)
return NotImplemented
包含了构造方法、repr方法、比较方法、hash方法等；
field函数
dataclasses.field 方法提供接口，允许用户定制每个field的创建和相关设置，比如设置默认值，不将某个field作为实例变量等。
重要的参数：
- default：默认值；
- defualt_factory: 默认工厂函数，必须是0参数的函数，用于每次创建默认值；
- init：指定该field是否被构造函数初始化；
- repr：指定该field是否包含在repr方法里；
- compare：指定该filed是否用于顺序比较；
注意存在dataclasses.Field 对象，该对象不会直接实例化，而通过fields() 函数返回。
@dataclass
class C:
x: int
y: int = field(repr=False)
z: int = field(repr=False, default=10)
t: int = 20
可变对象的默认值
因为引用的关系，当类成员的默认值是不可变对象时，所有实例的该类成员指向同一个不可变对象。
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] # Pass
assert o1.x is o2.x # Pass
当使用dataclass 装饰器生成类时，同样会面临该问题。
@dataclass
class D:
x: List = []
def add(self, element):
self.x += element
相当于
class D:
x = []
def __init__(self, x=x): # def __init__(self, x=[]):
self.x = x
def add(self, element):
self.x += element
assert D().x is D().x # Pass
同时这也是提醒我们，方法的默认参数不应该是可变参数。
为了避免该情况，使用field 的默认方法创建默认值。
@dataclass
class D:
x: list = field(default_factory=list)
assert D().x is not D().x