القيم الافتراضية وخيارات field()

تعيين قيم افتراضية للحقول

from dataclasses import dataclass

@dataclass
class Server:
    host: str
    port: int = 8080

print(Server('localhost'))
print(Server('example.com', 443))

الترتيب مهم

from dataclasses import dataclass

@dataclass
class User:
    name: str       # required
    active: bool = True   # optional

print(User('Alice'))

مشكلة القيمة الافتراضية القابلة للتغيير

from dataclasses import dataclass
from typing import List

# This raises ValueError at class definition time:
try:
    @dataclass
    class Bad:
        items: List[int] = []
except ValueError as e:
    print('Error:', e)

‏field(default_factory=...)

from dataclasses import dataclass, field
from typing import List

@dataclass
class Cart:
    items: List[str] = field(default_factory=list)

a = Cart()
a.items.append('apple')
b = Cart()
print(a.items, b.items)

‏default في مقابل default_factory

from dataclasses import dataclass, field

@dataclass
class Config:
    retries: int = field(default=3)
    tags: list = field(default_factory=list)

print(Config())

استبعاد حقل من repr

from dataclasses import dataclass, field

@dataclass
class Account:
    user: str
    token: str = field(repr=False, default='secret')

print(Account('alice'))

استبعاد حقل من المقارنة

from dataclasses import dataclass, field

@dataclass
class Event:
    name: str
    timestamp: float = field(compare=False, default=0.0)

print(Event('login', 1.0) == Event('login', 99.0))

حقول init=False

from dataclasses import dataclass, field

@dataclass
class Circle:
    radius: float
    area: float = field(init=False, default=0.0)

c = Circle(2.0)
print(c)

‏__post_init__ للقيم المشتقة

from dataclasses import dataclass, field

@dataclass
class Circle:
    radius: float
    area: float = field(init=False, default=0.0)

    def __post_init__(self):
        self.area = 3.14159 * self.radius ** 2

print(Circle(2.0))

تخزين البيانات الوصفية

from dataclasses import dataclass, field, fields

@dataclass
class Product:
    price: float = field(metadata={'unit': 'USD'})

for f in fields(Product):
    print(f.name, f.metadata)

دمج خيارات field()

from dataclasses import dataclass, field

@dataclass
class Session:
    user: str
    token: str = field(default='x', repr=False, compare=False)

print(Session('alice'))
print(Session('alice') == Session('alice', token='different'))

تحقق سريع

مراجعة