Dictionaries and sets

Dictionary comprehensions

Like list comprehensions, but producing key-value pairs:

squares = {n: n * n for n in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

prices = {"apple": 1.2, "melon": 3.5, "fig": 0.4}
cheap = {name: p for name, p in prices.items() if p < 2}
# {'apple': 1.2, 'fig': 0.4}

Counter

Counting things is so common that the standard library has a dictionary built for it:

from collections import Counter

votes = ["red", "blue", "red", "green", "red", "blue"]
counts = Counter(votes)
counts["red"]            # 3
counts["purple"]         # 0, no KeyError
counts.most_common(2)    # [('red', 3), ('blue', 2)]

defaultdict

A defaultdict creates a starting value the first time you use a missing key, which removes a lot of "is it there yet?" checks:

from collections import defaultdict

words = ["apple", "avocado", "banana", "blueberry", "cherry"]
by_letter = defaultdict(list)
for w in words:
    by_letter[w[0]].append(w)

dict(by_letter)
# {'a': ['apple', 'avocado'], 'b': ['banana', 'blueberry'], 'c': ['cherry']}

Sets

A set is an unordered collection with no duplicates. Checking x in s is fast, unlike with a list, which has to look at every item.

seen = {"ada", "linus"}
seen.add("grace")
seen.add("ada")          # already there, nothing changes
len(seen)                # 3
"linus" in seen          # True

empty = set()            # {} would be an empty dict

Sets support the operations from maths:

a = {1, 2, 3, 4}
b = {3, 4, 5}
a | b    # union:        {1, 2, 3, 4, 5}
a & b    # intersection: {3, 4}
a - b    # difference:   {1, 2}
a ^ b    # in one only:  {1, 2, 5}

Nested data

Real data is often dictionaries inside lists inside dictionaries. It is the same shape as JSON, which you will meet in the next chapter.

course = {
    "code": "CS101",
    "students": [
        {"name": "Ada", "grade": 91},
        {"name": "Linus", "grade": 78},
    ],
}

for s in course["students"]:
    print(s["name"], s["grade"])

top = max(course["students"], key=lambda s: s["grade"])
top["name"]   # 'Ada'

What can be a key?

Dictionary keys and set items must be hashable, which in practice means they cannot change. Strings, numbers and tuples work. Lists, dictionaries and sets do not:

grid = {(0, 0): "start", (2, 3): "goal"}   # tuple keys: fine
bad = {[0, 0]: "start"}                     # TypeError: unhashable type: 'list'

If a key could change after being stored, the dictionary would look for it in the wrong place, so Python does not allow it.

Exercises

from collections import Counter
Counter("mississippi").most_common(3)
# [('i', 4), ('s', 4), ('p', 2)]

ada = {"Linus", "Grace", "Alan"}
linus = {"Grace", "Ada", "Ken"}
ada & linus    # {'Grace'}
ada - linus    # {'Linus', 'Alan'}

from collections import defaultdict

groups = defaultdict(list)
for w in "the quick brown fox jumps over the lazy dog".split():
    if w not in groups[len(w)]:
        groups[len(w)].append(w)

dict(groups)
# {3: ['the', 'fox', 'dog'], 5: ['quick', 'brown', 'jumps'], 4: ['over', 'lazy']}