Python · Quick Reference

Lesson 9 — Dictionaries & Sets Cheat Sheet

Python

In one line: A dictionary stores data as key-value pairs. Instead of accessing items by numeric index like a list, you access them by a meaningful key — a label you define. This makes dicts ...

Key Ideas

1What is a Dictionary?. A dictionary stores data as key-value pairs. Instead of accessing items by numeric index like a list, you access them by a meaningful key — a label you define. This ma...

2Iterating Dictionaries. Three views let you loop over different aspects of a dictionary. Each returns a special view object that always reflects the current state of the dict.

3Nested Dicts & Real Patterns. Dictionaries are the building block of almost all Python data — JSON APIs, database records, configuration files, and objects are all represented as nested dicts in pr...

4Sets — Unique Unordered Collections. A set is an unordered collection of unique items. Duplicate values are automatically discarded. Sets are incredibly fast for membership testing and are the go-to tool ...

5Choosing the Right Collection. Python's four main collection types each have a distinct purpose. Picking the right one for your data makes code cleaner, faster, and less error-prone.

Code Examples

# Creating a dictionary
student = {
    "name"   : "Fatima Malik",
    "age"    : 20,
    "gpa"    : 3.92,
    "active" : True
}

# Accessing values by key
print(student["name"])    # Fatima Malik
print(student["gpa"])     # 3.92

# .get() — safe access (no...

scores = {"Ali": 88, "Sara": 95, "Zara": 72}

# Loop over keys
for name in scores:
    print(name, end=" ")   # Ali Sara Zara

# Loop over values
for score in scores.values():
    print(score, end=" ")  # 88 95 72

# Loop over key-value pairs — most common
...

# ── Nested dict — user record ─────────────────
user = {
    "id"      : 1001,
    "name"    : "Ali Hassan",
    "address" : {
        "city"   : "Lahore",
        "country": "Pakistan"
    },
    "courses" : ["Python", "Web Dev"]
}
print(user["address"]["...