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pythonbeginner

Dataclasses with Post-Init Processing

Use Python dataclasses with __post_init__ for computed fields, validation, and default factories.

python
from dataclasses import dataclass, field
from datetime import datetime
from uuid import uuid4

@dataclass
class Order:
    customer_id: str
    items: list[str]
    unit_price: float
    quantity: int = 1
    id: str = field(default_factory=lambda: str(uuid4()))
    created_at: datetime = field(default_factory=datetime.now)
    total: float = field(init=False)
    status: str = field(init=False, default="pending")

    def __post_init__(self):
        if self.quantity < 1:
            raise ValueError("Quantity must be at least 1")
        if self.unit_price < 0:
            raise ValueError("Price cannot be negative")
        self.total = round(self.unit_price * self.quantity, 2)

# Usage
order = Order(
    customer_id="cust_123",
    items=["Widget A"],
    unit_price=19.99,
    quantity=3,
)
print(f"Order {order.id}: ${order.total}")

Use Cases

  • Domain models
  • Configuration objects
  • DTO patterns

Tags

#dataclasses#python#models#validation

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