Uvicorn server basics in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run Uvicorn with default settings?

You start a FastAPI app using uvicorn main:app without extra options. What is the behavior of the server?

A The server runs on localhost at port 8000 but does not reload automatically on code changes.

B The server runs on all network interfaces at port 80 and reloads automatically on code changes.

C The server runs on localhost at port 8000 and reloads automatically on code changes.

D The server runs on all network interfaces at port 8000 but does not reload automatically.

💡 Hint

Think about the default host, port, and reload behavior of Uvicorn.

By default, Uvicorn runs on 127.0.0.1 (localhost) at port 8000. It does not reload automatically unless the --reload flag is used.

📝 Syntax

intermediate

Which command correctly starts Uvicorn with auto-reload and on all interfaces?

You want to run your FastAPI app with Uvicorn so it reloads on code changes and listens on all network interfaces. Which command is correct?

A uvicorn main:app --reload host=0.0.0.0

B uvicorn main:app --host 0.0.0.0 --reload

C uvicorn main:app --reload --host 0.0.0.0

D uvicorn main:app --host=localhost --reload

💡 Hint

Check the correct syntax for flags and their order.

The correct syntax uses double dashes for flags and spaces between flags and values. Both --reload and --host 0.0.0.0 are valid flags.

❓ state_output

advanced

What is the output when Uvicorn fails to bind to port 80?

You run uvicorn main:app --host 0.0.0.0 --port 80 without root privileges on Linux. What error message or behavior will you see?

A RuntimeError: Port 80 is already in use

B PermissionError: [Errno 13] Permission denied

C SyntaxError: invalid port number

D The server starts successfully on port 80

💡 Hint

Ports below 1024 require special permissions on Unix systems.

On Unix-like systems, binding to ports below 1024 requires root privileges. Without them, Uvicorn raises a PermissionError.

🔧 Debug

advanced

Why does Uvicorn not reload when using --reload with a custom app import path?

You run uvicorn myproject.app:app --reload but changes in your code do not trigger reload. What is the most likely cause?

A The server needs to be restarted manually to enable reload.

B The <code>--reload</code> flag only works with the default <code>main:app</code> import path.

C Uvicorn does not support reload on Windows systems.

D The import path is incorrect or the app is not detected properly.

💡 Hint

Check if the app path matches your project structure exactly.

If the import path is wrong or the app variable is not found, Uvicorn cannot watch files properly and reload won't work.

🧠 Conceptual

expert

How does Uvicorn handle multiple worker processes?

You start Uvicorn with --workers 4. What is true about how Uvicorn manages these workers?

A Uvicorn starts 4 separate processes, each running the app independently to handle requests concurrently.

B Uvicorn runs a single process with 4 threads to handle requests concurrently.

C Uvicorn runs 4 processes but shares the same memory space for app state.

D Uvicorn cannot run multiple workers; the flag is ignored.

💡 Hint

Think about how Python handles concurrency with processes vs threads.

Using --workers starts multiple independent processes. Each process runs its own instance of the app, allowing better CPU utilization.