Dictionaries in Python

Lists and tuples store values in order, and you access each one by its position: index 0, index 1, and so on. But sometimes position isn’t a natural way to think about your data. If you’re storing a person’s details- name, age, city- you don’t want to remember that age is “item number 1.” You want to just ask for age directly. That’s what a dictionary is built for.

What a dictionary actually is

A dictionary stores data as key-value pairs. Each key acts like a label, and it points to a value. You create one with curly braces.

student = {"name": "Sujan", "age": 16, "city": "Kathmandu"}
print(student)

"""Output:
{'name': 'Sujan', 'age': 16, 'city': 'Kathmandu'}"""

Accessing values by key

No index numbers here: you access a value using its key instead.

student = {"name": "Sujan", "age": 16, "city": "Kathmandu"}
print(student["name"])   # Sujan
print(student["age"])    # 16

Try to access a key that doesn’t exist, and Python throws an error.

print(student["grade"])
"""Output
KeyError: 'grade' """

A safer way to access a value is with.get(), which returns None instead of crashing if the key isn’t there.

print(student.get("grade"))          # None
print(student.get("grade", "N/A"))   # N/A

Adding or updating a value

student = {"name": "Sujan", "age": 16}
student["city"] = "Kathmandu"      # adds a new key
student["age"] = 17                # updates an existing key
print(student)
"""{'name': 'Sujan', 'age': 17, 'city': 'Kathmandu'}"""

If the key already exists, Python updates it. If it doesn’t, Python adds it as new. Same syntax either way.

Removing a key

student = {"name": "Sujan", "age": 16, "city": "Kathmandu"}
del student["city"]
print(student)   # {'name': 'Sujan', 'age': 16}

You can also use .pop(), which removes the key and returns its value, similar to how pop() worked on a list.

age = student.pop("age")
print(age)        # 16
print(student)    # {'name': 'Sujan'}

Looping through a dictionary

student = {"name": "Sujan", "age": 16, "city": "Kathmandu"}

for key in student:
    print(key, ":", student[key])
    
""" Output
    name : Sujan
    age : 16
    city : Kathmandu"""

A cleaner way to do the same thing uses .items(), which gives you both the key and value directly.

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

Checking if a key exists

student = {"name": "Sujan", "age": 16}
print("name" in student)     # True
print("grade" in student)    # False

Same in operator you’ve already used on strings, lists, and tuples; it checks keys here, not values.

keys(), values(), and len()

student = {"name": "Sujan", "age": 16, "city": "Kathmandu"}
print(student.keys())     # dict_keys(['name', 'age', 'city'])
print(student.values())   # dict_values(['Sujan', 16, 'Kathmandu'])
print(len(student))       # 3

Python Dictionary Practice Questions

1. Create a dictionary representing a book with keys for title, author, and year. Print the author.
2. What is the output of the following code?
   Copy

   info = {"a": 1, "b": 2}
   print(info.get("c", "not found"))

3. Write code that adds a new key “grade” with value “A” to an existing dictionary.
4. Write a loop that prints every key and value in a dictionary using .items().
5. What error occurs if you try to access a key that doesn’t exist using square brackets? How would you avoid it?
6. Write a program that removes a key called “age” from a dictionary and prints its value before removing it.