Course 4.2Lesson 1 of 5

Project 1: a contact book

A contact book has four jobs: add a contact, find a number, remove a contact and list everybody, and a dictionary is the right tool because we look things up by name. Two rules make it safe: the key is always the name in small letters so that Ravi and ravi are one contact, and a number must be digits only, which add enforces by raising a ValueError. Each job is a small function with a note that says what it does, and the main program catches the error so that it does not stop. We plan the cases and the expected book by hand, test the edges, and extend the book with a search by the start of a name.

about 30 min1 simulation2 practice questions

Lesson 1 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

design a small tool around a dictionary,
keep the data safe with a tidy key and a check on the input,
split a tool into functions that do one job each, and
search a dictionary by the start of a key.

Plan first

A contact book must do four jobs: add a contact, find a number, remove a contact and list everybody. A dictionary is the right tool, because we look things up by name.

A diagram of a contact book. In the middle is a dictionary called contacts with two pairs: asha is 98765 and ravi is 91234. Four boxes point to it: add of name and number, which sets contacts at name to the number; find of name, which looks up contacts at name or gives None; remove of name, which deletes contacts at name; and list, which uses sorted of contacts. A box at the bottom says that try and except keep it safe: a number with letters raises a ValueError, and an unknown name is answered with not in the book.

Two rules make it safe:

The key is always the name in small letters, so Ravi, ravi and RAVI are the same contact.
A number must have digits only. A number with a letter is refused.

Step 1: the expected result

The book starts with Asha (98765) and Ravi (91234). Six commands are run. Work out the answers by hand.

Command	What should happen
add Meera 99887	added
add Kabir 12ab	refused: the number has a letter
find Ravi	91234
find Zoya	not in the book
remove Asha	removed
remove Asha again	nobody called Asha

At the end the book holds Meera and Ravi, in alphabetical order.

Step 2: the program

Predict first, then run it. Compare with your table.

Look at the design. Each job is a function, with a short note (a docstring) that says what it does. The commands are in a list at the bottom, so the program does not need a person to type, and you can change the list to test other cases.

- add checks the number and raises a ValueError when it is wrong. The check is in the function, so that it protects the book whoever calls it. The function does not print anything: it either changes the book or raises an error.

- find uses contacts.get(...), which gives None for a missing name. No try is needed.

- remove checks with in before it uses del, because del of a missing key is a KeyError. It gives True or False so that the caller can say what happened.

- The main loop catches the ValueError from add, and prints a message instead of stopping. This is the pattern of the course on errors: the function raises, and the caller decides what to do.

- sorted(contacts) gives the names in order, and name.title() puts a capital letter back for show.

Step 3: test the edges

Test	Expected
a name in capitals, `RAVI`	finds Ravi
a number with spaces, `98 765`	refused (a space is not a digit)
an empty number	refused (`"".isdigit()` is `False`)
adding a name that exists	the number is replaced
find in an empty book	`None`
remove from an empty book	`False`

The fourth row is a choice: replace silently, or refuse? A real book might ask. Decide, and write the rule in a comment.

The same idea makes the search kind to the user: look up name.lower(), and any spelling finds the contact.

Step 4: search the book

A book is more useful if it can find a name from its first letters. Two small functions: all the names that start with some letters, and all the contacts whose number starts with some digits.

Extend it

Pick one or two, and do them without looking at a solution. Write the expected answers first.

A shorter number. Refuse a number with fewer than 5 digits.
A duplicate. Refuse a name that is already in the book, and say so.
A count. count() gives the number of contacts.
Rename. rename(old, new) moves a number to a new name, and refuses if the new name exists.
Several numbers. A contact can have a list of numbers. What changes?

Common mistakes

Not making the key tidy. Ravi and ravi become two contacts.
contacts[name] for a missing name. A KeyError.
A function that prints and does not return. You cannot test it.
A bare except: in the main loop. A typing mistake would be hidden.
Checking the number only in the main program. The next caller can break the book.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

Warm-up. Open the lab Contact book tools. You write add, find and remove, and the checker tries them.
Build. Open the lab Search the contact book and the problem A safe add.
Stretch. Open the problem Find without a crash and the problem When the guess fails.

Remember these

A dictionary is the right tool when you look up by a name.
Make the key tidy (lower()), so that small differences do not make two contacts.
A function raises an error. The caller decides what to do with it.
An empty prefix matches everybody.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

Why do we use name.lower() for the key?

What does contacts.get("zoya") give if there is no Zoya?

Why does remove check with in before del?

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Problems (3)

Write add(name, number) for the contact book.Easy · High School (9–10) · by StemPy Lab

The book has asha and ravi. What happens with contacts["zoya"]?Easy · High School (9–10) · by StemPy Lab

The book is {"meera": "99887", "asha": "98765", "ravi": "91234", "kabir": "90000"}.Medium · High School (9–10) · by StemPy Lab

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