MediumBackendRefactor

Refactor Duplicate Validation and Decouple Database Access

`main.py` has three handler functions: `register_user`, `update_profile`, and `create_post`. They all work — but the code has two problems. First, the same username validation block is copy-pasted into all three handler...

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What you will practice

PythonREST APIs / RoutesRefactoringDRYDependency Injection

Requirements

`register_user`, `update_profile`, and `create_post` must return identical responses to the originals for all valid and invalid inputs.
The username validation logic must not be duplicated — it must live in exactly one place.
A `validators.py` file must exist and contain the extracted validation logic.
`main.py` must not import `Database` from `db.py`.
Each handler must accept its save mechanism as an external dependency so tests can pass a mock without touching the real database.

Starter files

main.pyEditable starter

db.pyReference starter

What the judge checks

Runs in the python environment with the python-pytest runner.
Uses a 5000ms judge budget.
Behavior rules include: Behavior Preserved, Validation Not Duplicated, Database Not Imported In Main, Validators Module Exists.

Constraints

Do not change the response shape or error messages.
Do not modify `db.py`.

Example behavior

Input

register_user({'username': 'alice', 'email': 'a@b.com'}, save=mock_save)

Output

{'status': 'ok', 'data': {'username': 'alice', 'email': 'a@b.com'}}

Input

register_user({'username': 'x'}, save=mock_save)

Output

{'error': 'username must be between 3 and 20 characters'}

Follow-up

If you needed to support multiple database backends (Postgres, SQLite, in-memory), what would you change about the interface you designed?