Dictionaries & Sets

Dictionaries — Key-Value Storage

Dictionaries are unordered (Python 3.7+: insertion-ordered) mappings from keys to values. They enable O(1) lookups — essential for large datasets.

student = {
    "name": "Alice",
    "age": 22,
    "grades": [90, 85, 92],
    "enrolled": True
}

Accessing & Modifying

student["name"]              # "Alice"
student.get("gpa", 0.0)      # 0.0  (safe default)
student["email"] = "a@b.com" # add key
del student["age"]           # remove key
"name" in student            # True

Iterating

for key, value in student.items():
    print(f"{key}: {value}")

Data Science relevance: JSON APIs, configuration files, and Pandas' to_dict() all return dictionaries.