数据类高级特性：字段工厂函数与继承，打造灵活的数据模型

            from dataclasses import dataclass

@dataclass
class User:
    user_id: int
    username: str
    is_active: bool = True

# Usage
user1 = User(user_id=101, username="alice")
user2 = User(user_id=102, username="bob", is_active=False)

print(user1)  # Output: User(user_id=101, username='alice', is_active=True)
print(user1 == User(user_id=101, username="alice")) # Output: True

            

              
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            # Problematic approach with standard classes (and dataclasses without factories)
class ShoppingCart:
    def __init__(self):
        self.items = [] # All instances will share this same list!

cart1 = ShoppingCart()
cart2 = ShoppingCart()
cart1.items.append("apple")

print(cart2.items) # Output: ['apple'] - unexpected!

            

              
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            from dataclasses import dataclass

@dataclass
class ProductInventory:
    product_name: str
    # WRONG: mutable default
    # stock_levels: dict = {}

# stock1 = ProductInventory(product_name="Laptop")
# stock2 = ProductInventory(product_name="Mouse")
# stock1.stock_levels["warehouse_A"] = 100
# print(stock2.stock_levels) # {'warehouse_A': 100} - unexpected!

            

              
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            from dataclasses import dataclass, field

@dataclass
class ProductInventory:
    product_name: str
    # Correct approach: use a factory function for the mutable dict
    stock_levels: dict = field(default_factory=dict)

# Usage
stock1 = ProductInventory(product_name="Laptop")
stock2 = ProductInventory(product_name="Mouse")

stock1.stock_levels["warehouse_A"] = 100
stock1.stock_levels["warehouse_B"] = 50
stock2.stock_levels["warehouse_A"] = 200

print(f"Laptop stock: {stock1.stock_levels}")
# Output: Laptop stock: {'warehouse_A': 100, 'warehouse_B': 50}
print(f"Mouse stock: {stock2.stock_levels}")
# Output: Mouse stock: {'warehouse_A': 200}

# Each instance gets its own distinct dictionary
assert stock1.stock_levels is not stock2.stock_levels

            

              
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            from dataclasses import dataclass, field
from datetime import datetime

@dataclass
class EventLog:
    event_id: int
    description: str
    # Factory for current timestamp
    timestamp: datetime = field(default_factory=datetime.now)

# Usage
event1 = EventLog(event_id=1, description="User logged in")
# A small delay to see timestamp differences
import time
time.sleep(0.01)
event2 = EventLog(event_id=2, description="Data processed")

print(f"Event 1 timestamp: {event1.timestamp}")
print(f"Event 2 timestamp: {event2.timestamp}")
# Notice the timestamps will be slightly different
assert event1.timestamp != event2.timestamp

            

              
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            from dataclasses import dataclass, field

def create_default_settings():
    # In a global app, these might be loaded from a config file based on locale
    return {"theme": "light", "language": "en", "notifications": True}

@dataclass
class UserProfile:
    user_id: int
    username: str
    settings: dict = field(default_factory=create_default_settings)

user_profile1 = UserProfile(user_id=201, username="charlie")
user_profile2 = UserProfile(user_id=202, username="david")

# Modify settings for user1 without affecting user2
user_profile1.settings["theme"] = "dark"

print(f"Charlie's settings: {user_profile1.settings}")
print(f"David's settings: {user_profile2.settings}")

            

              
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            from dataclasses import dataclass

@dataclass
class Resource:
    resource_id: str
    name: str
    owner: str

@dataclass
class Server(Resource):
    ip_address: str
    os_type: str

@dataclass
class Database(Resource):
    db_type: str
    version: str

# Usage
server1 = Server(resource_id="srv-001", name="webserver-prod", owner="ops_team", ip_address="192.168.1.10", os_type="Linux")
db1 = Database(resource_id="db-005", name="customer_db", owner="db_admins", db_type="PostgreSQL", version="14.2")

print(server1)
# Output: Server(resource_id='srv-001', name='webserver-prod', owner='ops_team', ip_address='192.168.1.10', os_type='Linux')
print(db1)
# Output: Database(resource_id='db-005', name='customer_db', owner='db_admins', db_type='PostgreSQL', version='14.2')

            

              
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            # The __init__ signature for Server would conceptually be:
# def __init__(self, resource_id: str, name: str, owner: str, ip_address: str, os_type: str): ...

# Initialization order matters:
# This would fail because Server expects parent fields first
# invalid_server = Server(ip_address="10.0.0.5", resource_id="srv-002", name="appserver", owner="devs", os_type="Windows")

            

              
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            from dataclasses import dataclass, field
from datetime import datetime

@dataclass
class Auditable:
    created_at: datetime = field(default_factory=datetime.now)
    created_by: str = "system"

@dataclass
class User(Auditable):
    user_id: int
    username: str
    is_admin: bool = False

# Usage
user1 = User(user_id=301, username="eve")
# We can override defaults
user2 = User(user_id=302, username="frank", created_by="admin_user_1", is_admin=True)

print(user1)
# Output: User(user_id=301, username='eve', is_admin=False, created_at=datetime.datetime(2023, 10, 27, 10, 0, 0, ...), created_by='system')
print(user2)
# Output: User(user_id=302, username='frank', is_admin=True, created_at=datetime.datetime(2023, 10, 27, 10, 0, 1, ...), created_by='admin_user_1')

            

              
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            from dataclasses import dataclass, field
from datetime import datetime
import uuid

@dataclass
class BaseNotification:
    notification_id: str = field(default_factory=lambda: str(uuid.uuid4()))
    recipient_id: str
    sent_at: datetime = field(default_factory=datetime.now)
    message: str
    read: bool = False

@dataclass
class EmailNotification(BaseNotification):
    subject: str
    sender_email: str
    # Override parent's message with a more specific default if subject exists
    message: str = field(init=False, default="") # Will be populated in __post_init__ or by other means

    def __post_init__(self):
        if not self.message: # If message wasn't explicitly set
            self.message = f"{self.subject} - [Sent from {self.sender_email}]"

@dataclass
class SMSNotification(BaseNotification):
    phone_number: str
    sms_provider: str = "Twilio"

# Usage
email_notif = EmailNotification(recipient_id="user@example.com", subject="Your Order Shipped", sender_email="noreply@company.com")
sms_notif = SMSNotification(recipient_id="user123", phone_number="+15551234", message="Your package is out for delivery.")

print(f"Email: {email_notif}")
# Output will show a generated notification_id and sent_at, plus the auto-generated message
print(f"SMS: {sms_notif}")
# Output will show a generated notification_id and sent_at, with explicit message and sms_provider

            

              
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