Dict Patterns

Dictionaries are the backbone of most Python applications.

1) Counting (classic)

text = "mississippi"
count = {}

for ch in text:
    count[ch] = count.get(ch, 0) + 1

print(count)

2) Grouping values by key

students = [
  ("Tom", "A"),
  ("Sarah", "A"),
  ("Mike", "B"),
]

groups = {}
for name, grade in students:
    groups.setdefault(grade, []).append(name)

print(groups)  # {'A': ['Tom','Sarah'], 'B': ['Mike']}

3) Merge dicts

a = {"x": 1, "y": 2}
b = {"y": 99, "z": 3}
merged = {**a, **b}
print(merged)

4) Invert a dict (careful with duplicates)

d = {"a": 1, "b": 2}
inv = {v: k for k, v in d.items()}
print(inv)

Graph: grouping

flowchart TD
  A[("Tom","A")] --> B[A group]
  C[("Sarah","A")] --> B
  D[("Mike","B")] --> E[B group]

Remember

• get() and setdefault() are your friends
• dict is best for fast lookup and grouping
• careful when inverting if values repeat