Dictionaries

Think of the contacts on your phone: you look up “Mum” and see the number at once — you don't need to know which position the contact has in the list. A paper dictionary works the same way: you find a word, and its meaning is written next to it. In Python this kind of data is stored in a dictionary (dict): every value has its own key.

Creating a dictionary

student = {'name': 'Leyla', 'age': 14, 'city': 'Ganja'}
print(student)
print(student['name'])
print(len(student))
print(student.get('email'))
print(student.get('email', 'no email'))

{'name': 'Leyla', 'age': 14, 'city': 'Ganja'}
Leyla
3
None
no email

If you read a key that is not in the dictionary, like student['email'], Python raises a KeyError. The get() method does not: if the key is missing, it returns None (Python's special value meaning “nothing”) or whatever you pass as the second argument.

Changing a dictionary

Adding a new pair and changing an existing value are written the same way: student['age'] = 15. If the key exists, the value is updated; if not, a new pair is created. del removes a pair, and in checks whether a key is in the dictionary.

student = {'name': 'Leyla', 'age': 14}
student['age'] = 15
student['grade'] = 9
print(student)

del student['grade']
print('grade' in student, 'name' in student)

{'name': 'Leyla', 'age': 15, 'grade': 9}
False True

Looping over a dictionary

keys() gives all the keys, values() all the values and items() the (key, value) pairs. In a loop over items() you can unpack each pair straight into two variables — just like in the lesson on tuples.

prices = {'bread': 0.8, 'milk': 1.9, 'cheese': 6.5}
for item, price in prices.items():
    print(f'{item}: {price:.2f} AZN')

print('Items:', list(prices.keys()))
print(f'Total: {sum(prices.values()):.2f} AZN')

bread: 0.80 AZN
milk: 1.90 AZN
cheese: 6.50 AZN
Items: ['bread', 'milk', 'cheese']
Total: 9.20 AZN

Counting with a dictionary

One of the most useful jobs for a dictionary is counting how many times something appears. For each word we write counts.get(word, 0) + 1: when a word appears for the first time, get returns 0; after that, 1 is added to the old count.

text = 'the cat and the dog and the bird'
counts = {}
for word in text.split():
    counts[word] = counts.get(word, 0) + 1
print(counts)
print('the:', counts['the'])

{'the': 3, 'cat': 1, 'and': 2, 'dog': 1, 'bird': 1}
the: 3

Nested structures

A dictionary value can be of any type — for example, a list. With this structure you can build a simple class register: the key is a student's name and the value is the list of their grades.

journal = {
    'Aysel': [5, 4, 5],
    'Murad': [4, 3, 4],
    'Nigar': [5, 5, 5],
}
for name, grades in journal.items():
    avg = sum(grades) / len(grades)
    print(f'{name}: {avg:.1f}')

journal['Murad'].append(5)
print(journal['Murad'])

Aysel: 4.7
Murad: 3.7
Nigar: 5.0
[4, 3, 4, 5]

A dictionary is especially handy for:

• looking values up quickly by a name or code: contacts, product prices;
• counting: how often words, votes or grades appear;
• grouping: each class with its own list of students;
• describing the properties of one thing: name, age, city.

import json

student = {'name': 'Leyla', 'age': 14, 'city': 'Ganja'}
text = json.dumps(student)
print(text)

back = json.loads(text)
print(back['city'])

{"name": "Leyla", "age": 14, "city": "Ganja"}
Ganja

prices = {'apple': 1.5, 'bread': 0.8, 'milk': 1.9}
# 1) add tea for 3.2 and change bread to 1.0
# 2) print each item like: apple: 1.50
# 3) print Total: and the sum of all prices

apple: 1.50
bread: 1.00
milk: 1.90
tea: 3.20
Total: 7.60

votes = ['plov', 'dolma', 'plov', 'kebab', 'dolma', 'plov']
counts = {}
# 1) count the votes for each dish
# 2) print the dishes in alphabetical order like: dolma: 2
# 3) print Winner: and the dish with the most votes

dolma: 2
kebab: 1
plov: 3
Winner: plov