Validate Dataclass Fields in __post_init__

from dataclasses import dataclass


class ValidationError(ValueError):
    def __init__(self, errors: dict[str, str]) -> None:
        self.errors = errors
        message = "; ".join(f"{field}: {text}" for field, text in errors.items())
        super().__init__(message)


@dataclass
class Signup:
    email: str
    age: int
    username: str

    def __post_init__(self) -> None:
        errors: dict[str, str] = {}

        email = self.email.strip().lower()
        if email.count("@") != 1 or email.startswith("@") or email.endswith("@"):
            errors["email"] = "must contain one @ with local and domain parts"
        else:
            self.email = email

        if isinstance(self.age, bool) or not isinstance(self.age, int):
            errors["age"] = "must be an integer"
        elif not 13 <= self.age <= 120:
            errors["age"] = "must be between 13 and 120"

        username = self.username.strip()
        if not 3 <= len(username) <= 20:
            errors["username"] = "must be 3-20 characters"
        elif not username.replace("_", "").isalnum():
            errors["username"] = "may contain only letters, digits, and underscores"
        else:
            self.username = username

        if errors:
            raise ValidationError(errors)


def main() -> None:
    print(Signup(email="  [email protected] ", age=36, username=" ada_1 "))
    try:
        Signup(email="nope", age=True, username="x")
    except ValidationError as error:
        print(error)
        print(error.errors)


if __name__ == "__main__":
    main()