A registration system stores users in order, retrieves a user by ID and tracks unique teams.
What You Need to Understand
Lists store ordered elements, dictionaries map keys to values, and sets store distinct hashable elements for membership and set operations. Choosing according to the question you need to answer often matters more than memorizing every method.
When to Use It
A registration system stores users in order, retrieves a user by ID and tracks unique teams.
Step-by-Step Python Example
usernames = ["maya", "noa", "maya"]
profiles = {"maya": "editor", "noa": "viewer"}
unique_users = set(usernames)
print(usernames)
print(profiles["maya"])
print(len(unique_users))How the Example Works
The list retains the repeated name, the dictionary looks up a role by key, and the set deduplicates names.
Example Output
['maya', 'noa', 'maya']
editor
2
UnknownA Second Example
Compare this smaller variation with the main example, then experiment with your own values.
users_by_id = {101: "Maya", 102: "Noa"}
print(users_by_id.get(103, "Unknown"))Important Behavior and Edge Cases
A set is unordered and cannot be indexed. Dictionary keys must be hashable, and a dictionary is not designed to store duplicate keys.
Try It Yourself
Choose a container for a chronological activity log and one for fast lookup by product code.
Common Beginner Mistakes
- A set is unordered and cannot be indexed. Dictionary keys must be hashable, and a dictionary is not designed to store duplicate keys.
- Treating missing dictionary keys, replacement precedence, and shallow copies as if they had identical behavior in every context.
Key Takeaways
- Lists store ordered elements, dictionaries map keys to values, and sets store distinct hashable elements for membership and set operations.
- The list retains the repeated name, the dictionary looks up a role by key, and the set deduplicates names.
- A set is unordered and cannot be indexed.