Dataclasses Validation

Introduction

Dataclasses provide a way to define classes primarily to store data. Using post_init and field validators, we can add validation and computed fields.

Basic Dataclass

from dataclasses import dataclass, field

@dataclass
class Point:
    x: float
    y: float
    
    def distance_from_origin(self):
        return (self.x ** 2 + self.y ** 2) ** 0.5

p = Point(3, 4)
print(p.distance_from_origin())  # 5.0

Post Init Validation

from dataclasses import dataclass

@dataclass
class Person:
    name: str
    age: int
    
    def __post_init__(self):
        if self.age < 0:
            raise ValueError("Age cannot be negative")
        if not self.name:
            raise ValueError("Name cannot be empty")

person = Person("Alice", 30)

Field Validators

from dataclasses import dataclass, field
from typing import List

@dataclass
class Team:
    name: str
    members: List[str] = field(default_factory=list)
    
    def add_member(self, member: str):
        if member not in self.members:
            self.members.append(member)

Field with Validator

from dataclasses import dataclass, field
from typing import Any

def validate_positive(instance, field_obj, value):
    if value < 0:
        raise ValueError(f"{field_obj.name} must be positive")
    return value

@dataclass
class Product:
    name: str
    price: float = field(default=0, metadata={"validator": validate_positive})
    quantity: int = 0

Complex Validation

from dataclasses import dataclass, field
from datetime import datetime

@dataclass
class Event:
    name: str
    start_date: datetime
    end_date: datetime
    
    def __post_init__(self):
        if self.end_date <= self.start_date:
            raise ValueError("End date must be after start date")
    
    @property
    def duration_days(self):
        return (self.end_date - self.start_date).days

Practice Problems

1. Validate email format in dataclass
2. Create post-init validation for date ranges
3. Use field default_factory for mutable defaults
4. Implement computed properties
5. Build nested dataclass with validation