Practice: Phone Book

This exercise combines dictionary updates, get, and functions that share one dictionary.

Exercise

1. Define a function named add_contact with three parameters: book (a dictionary), name, and number. It stores the number under the name — adding or replacing — and returns nothing.
2. Define a function named lookup with two parameters: book and name. It returns the stored number, or the string "unknown" when the name is not in the book.
3. Define a function named contact_count with one parameter, book. It returns the number of contacts.

book = {}
add_contact(book, "Ada", "555-0101")
print(lookup(book, "Ada"))
print(lookup(book, "Grace"))

Output:

555-0101
unknown

Run your code to see the output, then press Submit.

Hints

• add_contact is one assignment; it needs no return.
• lookup is a single get with a fallback.

Tests

import unittest


class TestPhoneBook(unittest.TestCase):
    def test_added_contact_can_be_looked_up(self):
        book = {}
        add_contact(book, "Ada", "555-0101")
        self.assertEqual(lookup(book, "Ada"), "555-0101")

    def test_missing_contact_is_unknown(self):
        self.assertEqual(lookup({}, "Grace"), "unknown")

    def test_adding_again_replaces_the_number(self):
        book = {"Ada": "555-0101"}
        add_contact(book, "Ada", "555-0202")
        self.assertEqual(lookup(book, "Ada"), "555-0202")

    def test_contact_count(self):
        book = {}
        add_contact(book, "Ada", "1")
        add_contact(book, "Grace", "2")
        self.assertEqual(contact_count(book), 2)