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Python Tutorial · Data Structures · Lesson 18 of 42

Python Dictionaries

Key-value pairs: create, read safely, update, loop and nest dictionaries.

Creating dictionaries

A dictionary maps keys to values. Keys must be unique and immutable (strings, numbers, tuples).

Example 1Try it yourself
user = {"name": "Ada", "age": 36, "langs": ["Python", "SQL"]}
print(user["name"])
user["age"] = 37
user["city"] = "London"
del user["langs"]
print(user)
print(len(user))

Output

Ada
{'name': 'Ada', 'age': 37, 'city': 'London'}
3

Reading values safely

d[key] raises KeyError when the key is missing; d.get(key, default) returns a default instead.

Example 2Try it yourself
config = {"debug": True}
print(config.get("debug"))
print(config.get("timeout", 30))
print("timeout" in config)

Output

True
30
False

Looping

Example 3Try it yourself
prices = {"apple": 1.2, "pear": 0.9, "kiwi": 2.5}
for fruit, price in prices.items():
    print(f"{fruit}: {price}")
print(list(prices.keys()))
cheap = {k: v for k, v in prices.items() if v < 2}
print(cheap)

Output

apple: 1.2
pear: 0.9
kiwi: 2.5
['apple', 'pear', 'kiwi']
{'apple': 1.2, 'pear': 0.9}

Counting and grouping

Dictionaries are perfect for counting. collections.Counter and defaultdict make it even shorter:

Example 4Try it yourself
from collections import Counter, defaultdict
words = "the cat and the hat and the bat".split()
print(Counter(words).most_common(2))
by_letter = defaultdict(list)
for w in words:
    by_letter[w[0]].append(w)
print(dict(by_letter))

Output

[('the', 3), ('and', 2)]
{'t': ['the', 'the', 'the'], 'c': ['cat'], 'a': ['and', 'and'], 'h': ['hat'], 'b': ['bat']}

Test yourself

What does {"a": 1}.get("b", 0) return?

  1. None
  2. KeyError
  3. 0
  4. "b"
Show answer

Answer: 0

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