Dictionaries and key-value data

A dictionary connects keys to values, which is useful when positions would be hard to remember.

What you will learn

Create a dictionary
Read and update a key
Use get for optional keys

Look up by meaning

A dictionary stores key-value pairs inside braces. A key such as 'name' expresses what its value represents, unlike a list index whose meaning depends on position. Keys must be unique and hashable; strings are a common choice. Values can be different kinds of data, including lists or other dictionaries.

Read and change entries

Square brackets retrieve a value for a known key, but a missing key raises KeyError. Use get when a key may be absent and a default is appropriate. Assigning to an existing key replaces its value; assigning to a new key adds an entry.

Process fields intentionally

A record-like dictionary can describe one learner, product, or setting. Read only the fields your operation needs, and choose names that make their meaning clear. Avoid depending on an accidental display order when the relationship between a key and its value is the important part.

Iterate over pairs

The items method yields key-value pairs, which a for loop can unpack into two names. This is useful for printing a summary or applying a transformation to every field. If a program requires a predictable display order, choose and document that order rather than assuming a particular arrangement.

Membership checks the keys

Use the in operator to ask whether a dictionary contains a key. It does not search through every value. This distinction matters when a record contains the same text in an unrelated field. Use get when a default is meaningful, but avoid a default that makes missing data look like a valid fact.

course = {'name': 'Python', 'lessons': 12}
course['lessons'] += 1
print(course.get('name'))
print(course['lessons'])

Expected output

Python
13

Practice exercise

Create a dictionary for a book with title and pages, then print a sentence using both values.

Show a hint

Read each value by its key.

Show one possible solution

book = {'title': 'Small Steps', 'pages': 120}; print(f"{book['title']} has {book['pages']} pages.") prints a sentence from the record.

Quick check

What happens when dictionary[key] uses a missing key? Python raises KeyError Python automatically creates the key

Frequently asked questions

When should I use a dictionary instead of a list?

<p>Use a dictionary when values are naturally looked up by meaningful keys. Use a list when order and position matter, or when you need to process a sequence of similar items.</p>