Dictionaries, Tuples and Sets: Quick Revision

A brief note on Dictionaries, Tuples and Sets. Read it before a test. Try each question at the end before you open its answer.

Dictionaries: find a value by its key

• A dictionary stores key: value pairs: fees = {"chess": 300, "dance": 450}. Look up with fees["dance"].
• len(fees) counts the pairs. "chess" in fees asks about the keys, not the values.
• A key appears only once, and it must be fixed: text, a number or a tuple. A list cannot be a key.
• fees["art"] = 200 adds a new pair, or replaces the old value if the key is already there.
• del fees["art"] removes a pair. fees.pop("art") removes it and gives the value back.
• fees.get("drama", 0) gives 0 for a missing key, instead of a KeyError.
• keys(), values() and items() show the whole dictionary. for key, value in fees.items(): gives both.

Counting and grouping

• Counting: counts[x] = counts.get(x, 0) + 1. The key is the thing you count; the value is how many times.
• Grouping: the value is a list. If the key is new, make an empty list first, then append.
• A value can itself be a dictionary, a record with named parts. Read a deep lookup from left to right, one key at a time: pupils["Zoya"]["marks"][0].
• To find the biggest value, keep the best key and the best value so far, and update them in a loop.

Tuples: a row that stays fixed

• A tuple uses round brackets, and the commas make it: seat = (2, 5). It cannot be changed after it is made.
• Unpacking: row, column = seat. Swapping: a, b = b, a.
• A function can return two values as a tuple, and the caller unpacks them.
• A tuple can be a dictionary key, so (2, 5) can stand for row 2, column 5.

Sets: each value only once

• A set keeps each value once and has no order, so it has no positions. set(names) removes the repeats.
• add puts a value in; discard takes one out, with no error if it is missing. x in s is a quick test.
• set() is an empty set. {} is an empty dictionary.
• For two sets A and B: A & B is in both, A | B is in either, A - B is only in A, and A ^ B is in exactly one.
• A - B and B - A are different. Use sorted(s) to show a set in order.

Choosing a container

Ask three questions: position or name? Fixed or changing? Do the order and the repeats matter?

A worked example

A class votes for the fruit of the school fair. What does this program show?

Follow the dictionary after each vote.

1. A new fruit. counts.get(fruit, 0) gives 0, so the new key starts at 1. No KeyError happens.
2. A fruit seen before. get gives the old count, and the line stores one more.
3. The order. A dictionary keeps the order in which the keys were first added.

The program shows {'mango': 3, 'apple': 2, 'kiwi': 1}.

Practice with new numbers

A program to try

A reading club notes each book that a member reads, as a tuple of a name and a book. The program groups the books of each reader in a set, counts the readings of each book, and finds the book read most.

Mistakes to avoid

• Looking up a missing key. fees["drama"] stops with KeyError: 'drama'. Check with in, or use get with a default.
• A list as a key. seats[[2, 5]] = "Imran" stops with TypeError: unhashable type: 'list'. Use the tuple (2, 5).
• Changing a tuple. seat[0] = 3 stops with TypeError: 'tuple' object does not support item assignment.
• {} for an empty set. It makes an empty dictionary. Write set().
• A position in a set. colours[0] stops with TypeError: 'set' object is not subscriptable. A set has no order.
• in for a value. in looks at the keys of a dictionary. To search the values, use value in d.values().

Check yourself

1. A stationery shop keeps stock = {"pens": 40, "files": 12}. It receives 15 more files. Write one line that updates the stock, and give the new number of files.

2. Tenzin stores the name of each pupil by seat, such as row 2, column 5. Should the key be [2, 5] or (2, 5), and why?

3. A teacher writes down the roll number of each answer sheet handed in: [4, 7, 4, 9, 7, 7]. Which short expression gives the number of different pupils who handed in, and what is it?

4. Priya writes empty = {} and then empty.add(5). Python stops with AttributeError: 'dict' object has no attribute 'add'. What went wrong?

5. Choose a container for each: the seven days of the week, in order, never changing; the phone number of each friend, found by name; the different cities that visitors to a museum came from.