What is the dataclass __post_init__ method?

Question

Accepted Answer

**__post_init__** runs after **__init__** in dataclasses for custom initialization.

```python
from dataclasses import dataclass, field

@dataclass
class User:
    name: str
    email: str
    _id: int = field(init=False, repr=False)
    is_active: bool = field(init=False, default=True)

def __post_init__(self):
        # Called after __init__
        self._id = hash(self.name + self.email)
        self.name = self.name.title()  # Normalize
        self.email = self.email.lower()

user = User(name="alice", email="Alice@Example.COM")
print(user.name)    # "Alice" (title case)
print(user.email)   # "alice@example.com" (lowercase)
print(user._id)     # Hash computed

# Validation in __post_init__
@dataclass
class Age:
    value: int

def __post_init__(self):
        if self.value < 0 or self.value > 150:
            raise ValueError(f"Invalid age: {self.value}")

# With inheritance
@dataclass
class Person:
    name: str

def __post_init__(self):
        self.name = self.name.strip()

@dataclass
class Employee(Person):
    department: str

def __post_init__(self):
        super().__post_init__()  # Call parent
        self.department = self.department.upper()
```

**Use cases:**
- Validation.
- Computed fields.
- Data normalization.
- Derived attributes.

Python: What is the dataclass __post_init__ method?