Dictionaries snippets

Small patterns for dictionaries that come up often in the exercises. Each one is short on purpose: the case it solves, the syntax, and the result.

Read a key with a fallback

When a key might be missing and you want a default instead of a KeyError.

counts = {"a": 1}
counts.get("a", 0)     # → 1   key exists
counts.get("z", 0)     # → 0   key missing, returns the default

get never raises, so it is the safe way to read a key you are not sure about.

Group items with defaultdict

When you build lists (or counts) keyed by something, without checking whether the key exists first.

from collections import defaultdict

groups = defaultdict(list)
for word in ["ant", "ape", "bee"]:
    groups[word[0]].append(word)       # missing key auto-creates an empty list

dict(groups)   # → {"a": ["ant", "ape"], "b": ["bee"]}

defaultdict(list) creates an empty list the first time a key is touched, so the loop body stays one line. Convert back with dict(...) before returning, so callers get a plain dict.

Count occurrences with Counter

When you need how many times each item appears.

from collections import Counter

Counter("banana")                  # → Counter({"a": 3, "n": 2, "b": 1})
Counter("banana").most_common(2)   # → [("a", 3), ("n", 2)]   top two

Counter is a dict subclass, so you can read it like a normal dict. Missing keys return 0 instead of raising.

Build a dict in one expression

When you want a dict from an iterable, the same idea as a list comprehension.

nums = [1, 2, 3, 4]
{x: x**2 for x in nums}                 # → {1: 1, 2: 4, 3: 9, 4: 16}

# swap keys and values
d = {"a": 1, "b": 2}
{v: k for k, v in d.items()}            # → {1: "a", 2: "b"}

Iterate over keys, values, or both

When you need to loop through a dict. items() gives you both at once.

prices = {"pen": 2, "book": 5}

for key in prices:                 # keys by default
    print(key)                     # pen / book

for name, price in prices.items():
    print(name, price)             # pen 2 / book 5

Merge two dicts

When you want to combine dicts. On a key clash the right-hand value wins.

a = {"x": 1, "y": 2}
b = {"y": 9, "z": 3}
a | b                              # → {"x": 1, "y": 9, "z": 3}

The | merge operator was added in Python 3.9. On older versions use {**a, **b}, which does the same thing.