Dicts and Sets

Dicts — fast key-value lookup

A dict maps hashable keys to arbitrary values. Since Python 3.7 the insertion order of keys is guaranteed and preserved.

scores = {'alice': 95, 'bob': 87}
scores['carol'] = 92         # add a new key
print(scores['alice'])       # 95
del scores['bob']            # remove a key
print(scores)                # {'alice': 95, 'carol': 92}

Safe access: `get` and `setdefault`

Accessing a missing key with [] raises KeyError. Use .get() for a safe read with an optional default, or .setdefault() to insert a default value when the key is absent.

scores = {'alice': 95, 'bob': 87}
unknown = scores.get('dave', 0)      # returns 0 — no KeyError
scores.setdefault('eve', 88)         # inserts eve=88 only if absent
print(scores)
# {'alice': 95, 'bob': 87, 'eve': 88}

Iterating with `.items()`

.items() yields (key, value) pairs, which you can unpack directly in a for loop.

for name, score in scores.items():
    print(f'{name}: {score}')

You can also iterate over .keys() (default) or .values() independently.

Dict comprehensions

Like list comprehensions, dict comprehensions build a new dict in one expression.

squared = {n: n**2 for n in range(1, 6)}
# {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Sets — unique elements and set algebra

A set is an unordered collection of unique hashable elements. It is optimised for in membership tests (O(1) average) and supports mathematical set operations.

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}

print(a | b)   # union:            {1, 2, 3, 4, 5, 6}
print(a & b)   # intersection:     {3, 4}
print(a - b)   # difference:       {1, 2}
print(a ^ b)   # symmetric diff:   {1, 2, 5, 6}

Use frozenset for an immutable set (hashable, usable as a dict key or set element).

Full runnable demo

Open in an online REPL

# --- dict safe access ---
scores = {'alice': 95, 'bob': 87}
scores['carol'] = 92
print('scores:', scores)

unknown = scores.get('dave', 0)
print('dave score:', unknown)

scores.setdefault('eve', 88)
print('after setdefault:', scores)

for name, score in scores.items():
    print(f'  {name}: {score}')

# --- dict comprehension ---
squared = {n: n**2 for n in range(1, 6)}
print('squared:', squared)

# --- set algebra ---
a = {1, 2, 3, 4}
b = {3, 4, 5, 6}
print('union:', sorted(a | b))
print('intersection:', sorted(a & b))
print('difference:', sorted(a - b))
print('symmetric diff:', sorted(a ^ b))