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How do you specify default values for dataclass fields, and what ordering restriction applies? Why can't you use a mutable default value (like an empty list) directly as a dataclass field default, and how do you use field(default_factory=...) to fix it? How do you make a dataclass immutable (read-only after creation) using frozen=True? How does typing.NamedTuple differ from collections.namedtuple in terms of syntax and type annotation support? How do you use dataclasses.field(compare=False) or field(repr=False) to exclude a specific field from the auto-generated __eq__ or __repr__? How do you implement custom ordering (<, <=, >, >=) for a dataclass using order=True, and what constraint does it place on field comparison? How do you convert a dataclass instance to and from a dictionary using dataclasses.asdict() and manual reconstruction? How would you implement inheritance between dataclasses, and what field-ordering rule must be respected across the hierarchy? How do you implement a custom __eq__ or __hash__ on a dataclass while still benefiting from its other auto-generated methods? How would you use dataclasses.replace() to create a modified copy of an immutable (frozen) dataclass instance?
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How do you add validation logic to a dataclass field using __post_init__?

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Answer

__post_init__ is automatically called by the auto-generated __init__ RIGHT AFTER all fields are assigned, letting you add custom validation or derived-field computation without having to write __init__ manually from scratch.
@dataclass
class User:
    name: str
    age: int

    def __post_init__(self):
        if self.age < 0:
            raise ValueError("Age cannot be negative")

# User("Sam", -5)  # raises ValueError, caught by __post_init__
Real-world example Validating that a dataclass's fields satisfy business rules (like a non-negative age or a valid email format) right after construction.

Common follow-ups: How would you compute a DERIVED field (not passed to __init__ directly) inside __post_init__?

Exception Handling
All 15 Dataclasses & NamedTuples questions