Dictionaries

Introduction

A dictionary stores data as key-value pairs. Keys must be unique and are used to look up values. Dictionaries are unordered in older Python versions, but in Python 3.7+ they preserve insertion order.

Creating a Dictionary

employee = {
    "firstname": "Alice",
    "lastname": "Smith",
    "department": "Engineering",
    "salary": 80000
}

Accessing Values

print(employee["firstname"])           # Alice
print(employee.get("salary"))          # 80000
print(employee.get("age", "unknown"))  # unknown — default if key missing

Use .get() when the key may not exist — accessing a missing key with [] raises a KeyError.

Adding and Updating

employee["age"] = 30          # add a new key
employee["salary"] = 85000    # update an existing key

Removing Items

employee.pop("age")       # remove by key, returns the value
del employee["lastname"]  # remove by key

Checking Keys

if "department" in employee:
    print(employee["department"])

Iterating Over a Dictionary

employee = {"firstname": "Alice", "department": "Engineering", "salary": 80000}

for key in employee:
    print(key)

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

print(list(employee.keys()))    # ['firstname', 'department', 'salary']
print(list(employee.values()))  # ['Alice', 'Engineering', 80000]

Nested Dictionaries

company = {
    "Alice": {"department": "Engineering", "salary": 80000},
    "Bob":   {"department": "Finance",     "salary": 65000},
}

print(company["Alice"]["salary"])  # 80000

for name, details in company.items():
    print(f"{name} — {details['department']}")

Dictionary Comprehensions

salaries = {"Alice": 80000, "Bob": 65000, "Charlie": 55000}

# Create a new dict with salaries above 60000
high_earners = {name: salary for name, salary in salaries.items() if salary > 60000}
print(high_earners)  # {'Alice': 80000, 'Bob': 65000}

Practice Exercises

• Create a dictionary representing a product with keys for name, price, and category. Print the price.
• Add a stock key to the dictionary. Then update the price.
• Loop over the dictionary and print each key and value on a separate line.
• Create a list of product dictionaries and loop over it to print each product name and price.
• Use .get() to safely access a key that does not exist, providing a default value.