Python Dictionaries: From Beginner to Advanced

What is a Dictionary?

A dictionary stores data as key-value pairs. Think of a real dictionary: the word is the key, the definition is the value.

student = {
    "name": "Chan Siu Ming",
    "age": 17,
    "class": "5A",
    "subjects": ["Chinese", "English", "Maths", "ICT"]
}

Creating Dictionaries

# Empty dict
d = {}

# With initial values
grades = {"Chan": "A", "Lee": "B", "Wong": "A"}

# Alternative syntax
grades = dict(Chan="A", Lee="B", Wong="A")

Accessing Values

grades = {"Chan": "A", "Lee": "B", "Wong": "A"}

print(grades["Chan"])   # "A"

# Missing key raises KeyError
print(grades["Ho"])     # ✗ KeyError

# Safe access with .get()
print(grades.get("Ho"))           # None
print(grades.get("Ho", "N/A"))    # "N/A" (default)

Adding and Updating

grades = {"Chan": "A", "Lee": "B"}

# Add new key
grades["Wong"] = "A"
# {"Chan": "A", "Lee": "B", "Wong": "A"}

# Update existing
grades["Chan"] = "A*"
# {"Chan": "A*", "Lee": "B", "Wong": "A"}

# Update multiple at once
grades.update({"Ho": "C", "Lee": "B*"})

Removing Items

grades = {"Chan": "A", "Lee": "B", "Wong": "A"}

# Delete by key
del grades["Lee"]

# Remove and return value
value = grades.pop("Chan")
print(value)   # "A"

# Clear everything
grades.clear()

Checking Membership

grades = {"Chan": "A", "Lee": "B"}

print("Chan" in grades)    # True
print("Ho" in grades)      # False
print("A" in grades.values())   # True

Iterating Over Dictionaries

grades = {"Chan": "A", "Lee": "B", "Wong": "A"}

# Keys (default)
for name in grades:
    print(name)

# Explicitly keys
for name in grades.keys():
    print(name)

# Values
for grade in grades.values():
    print(grade)

# Both
for name, grade in grades.items():
    print(f"{name}: {grade}")

Common Use Case: Counting

text = "hello world"
counts = {}

for char in text:
    if char in counts:
        counts[char] += 1
    else:
        counts[char] = 1

# Cleaner with .get()
counts = {}
for char in text:
    counts[char] = counts.get(char, 0) + 1

print(counts)
# {'h':1, 'e':1, 'l':3, 'o':2, ' ':1, 'w':1, 'r':1, 'd':1}

Common Use Case: Grouping Data

students = [
    ("Chan", "A"),
    ("Lee", "B"),
    ("Wong", "A"),
    ("Ho", "C"),
    ("Lam", "B"),
]

by_grade = {}
for name, grade in students:
    if grade not in by_grade:
        by_grade[grade] = []
    by_grade[grade].append(name)

print(by_grade)
# {'A': ['Chan', 'Wong'], 'B': ['Lee', 'Lam'], 'C': ['Ho']}

Nested Dictionaries

school = {
    "5A": {
        "Chan": 85,
        "Lee": 72,
    },
    "5B": {
        "Wong": 91,
        "Ho": 68,
    }
}

print(school["5A"]["Chan"])   # 85

# Iterate nested
for class_name, students in school.items():
    print(f"Class {class_name}:")
    for name, score in students.items():
        print(f"  {name}: {score}")

Dictionary vs List: When to Use Each

• List: ordered items, access by position (students[0])
• Dictionary: unordered pairs, access by meaningful key (grades["Chan"])

Limitations

• Keys must be immutable (strings, numbers, tuples — NOT lists)
• Keys must be unique (duplicates overwrite)