Exercise 1: Build a notification builder

Now practice the basic Builder idea with a small existing class.

You are given this final object. Do not change it.

from dataclasses import dataclass


@dataclass(frozen=True)
class Notification:
    recipient: str
    title: str
    body: str
    channel: str
    priority: str
    retry_count: int
    send_after_minutes: int | None

Your task is to implement a NotificationBuilder that creates a valid Notification.

The final usage should look like this:

notification = (
    NotificationBuilder()
    .to("alice@example.com")
    .titled("Payment received")
    .with_body("Your payment was successfully processed.")
    .via_email()
    .high_priority()
    .retrying(3)
    .send_after(10)
    .build()
)

The builder should support these methods:

.to(recipient)
.titled(title)
.with_body(body)
.via_email()
.via_sms()
.low_priority()
.normal_priority()
.high_priority()
.retrying(count)
.send_after(minutes)
.build()

Use these defaults:

Field	Default
body	""
channel	"email"
priority	"normal"
retry_count	0
send_after_minutes	None

Validation rules:

• recipient is required.
• title is required.
• channel must be either "email" or "sms".
• priority must be "low", "normal", or "high".
• retry_count cannot be negative.
• send_after_minutes cannot be negative.

This should fail because there is no recipient:

notification = (
    NotificationBuilder()
    .titled("Hello")
    .build()
)

This should fail because the retry count is negative:

notification = (
    NotificationBuilder()
    .to("alice@example.com")
    .titled("Hello")
    .retrying(-1)
    .build()
)

Start with this skeleton:

class NotificationBuilder:
    def __init__(self):
        self._recipient = None
        self._title = None
        self._body = ""
        self._channel = "email"
        self._priority = "normal"
        self._retry_count = 0
        self._send_after_minutes = None

    def to(self, recipient):
        # TODO
        return self

    def titled(self, title):
        # TODO
        return self

    def with_body(self, body):
        # TODO
        return self

    def via_email(self):
        # TODO
        return self

    def via_sms(self):
        # TODO
        return self

    def low_priority(self):
        # TODO
        return self

    def normal_priority(self):
        # TODO
        return self

    def high_priority(self):
        # TODO
        return self

    def retrying(self, count):
        # TODO
        return self

    def send_after(self, minutes):
        # TODO
        return self

    def build(self):
        # TODO: validate required fields
        # TODO: validate invalid values
        # TODO: return Notification(...)
        pass

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