Dictionaries

A dictionary stores key-value pairs, letting you look up values by a unique key instead of by position.

Creating Dictionaries

# Empty dictionary
empty = {}

# Dictionary with values
person = {
    "name": "Alice",
    "age": 20,
    "major": "Computer Science"
}

Accessing Values

person = {"name": "Alice", "age": 20}

# Access by key
print(person["name"])

Output:

Alice

Use .get() to avoid errors when a key might not exist:

person = {"name": "Alice"}

print(person.get("name"))      # Alice
print(person.get("age"))       # None
print(person.get("age", 0))    # 0 (default value)

Adding and Updating Values

person = {"name": "Alice"}

# Add a new key
person["age"] = 20

# Update an existing key
person["age"] = 21

print(person)

Output:

{'name': 'Alice', 'age': 21}

Removing Items

person = {"name": "Alice", "age": 20, "major": "CS"}

# Remove and get the value
age = person.pop("age")
print(age)  # 20

# Remove without getting the value
del person["major"]

print(person)

Output:

{'name': 'Alice'}

Checking if a Key Exists

person = {"name": "Alice", "age": 20}

if "name" in person:
    print("Name found!")

if "email" not in person:
    print("No email on file.")

Output:

Name found!
No email on file.

Looping Through Dictionaries

person = {"name": "Alice", "age": 20}

# Loop over keys
for key in person:
    print(key)

# Loop over values
for value in person.values():
    print(value)

# Loop over both (most common)
for key, value in person.items():
    print(key + ": " + str(value))

Output:

name
age
Alice
20
name: Alice
age: 20

Nested Dictionaries

Dictionaries can contain other dictionaries:

students = {
    "alice": {"age": 20, "grade": "A"},
    "bob": {"age": 21, "grade": "B"}
}

print(students["alice"]["grade"])

Output:

Counting with Dictionaries

A common pattern is counting how many times items appear:

words = ["apple", "banana", "apple", "cherry", "banana", "apple"]
counts = {}

for word in words:
    if word in counts:
        counts[word] = counts[word] + 1
    else:
        counts[word] = 1

print(counts)

Output:

{'apple': 3, 'banana': 2, 'cherry': 1}

Common Mistakes

Using a key that doesn't exist

person = {"name": "Alice"}

# Wrong - causes KeyError
print(person["age"])

# Right - use .get() for safety
print(person.get("age"))

Forgetting quotes around string keys

# Wrong
person = {name: "Alice"}

# Right
person = {"name": "Alice"}

Using a list as a key

# Wrong - lists can't be keys
data = {[1, 2]: "value"}

# Right - use a tuple instead
data = {(1, 2): "value"}