Sub-dependencies in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a sub-dependency in FastAPI?

A sub-dependency is a dependency that is used inside another dependency. It helps break down complex dependencies into smaller, reusable parts.

beginner

How do you declare a sub-dependency in FastAPI?

You declare a sub-dependency by adding a dependency parameter inside another dependency function using Depends().

intermediate

Why use sub-dependencies in FastAPI?

Sub-dependencies help organize code better, avoid repetition, and make testing easier by isolating smaller parts of logic.

intermediate

Example: What does this code do?

```python
from fastapi import Depends

def get_token_header():
    return "token"

def get_current_user(token: str = Depends(get_token_header)):
    return f"User with {token}"
```

The function get_current_user depends on get_token_header. FastAPI will first run get_token_header to get the token, then pass it to get_current_user.

intermediate

How does FastAPI handle errors in sub-dependencies?

If a sub-dependency raises an error, FastAPI stops and returns that error response immediately without running the main dependency or endpoint.

What is the purpose of sub-dependencies in FastAPI?

A To replace middleware

B To make the app slower

C To break down dependencies into smaller reusable parts

D To avoid using Depends()

Sub-dependencies help organize code by breaking dependencies into smaller parts that can be reused.

How do you include a sub-dependency inside another dependency?

A By using Depends() inside the main dependency's parameters

B By calling the sub-dependency function directly

C By importing the sub-dependency globally

D By using async/await keywords

You use Depends() in the main dependency's parameters to include a sub-dependency.

If a sub-dependency raises an error, what happens?

A FastAPI retries the sub-dependency

B FastAPI stops and returns the error response immediately

C FastAPI ignores the error and continues

D FastAPI logs the error but runs the endpoint

FastAPI stops processing and returns the error response immediately if a sub-dependency fails.

Which FastAPI function is used to declare dependencies and sub-dependencies?

A Inject()

B DependsOn()

C Require()

D Depends()

Depends() is the FastAPI function used to declare dependencies and sub-dependencies.

What is a benefit of using sub-dependencies?

A Avoids code repetition and improves testing

B Makes code harder to read

C Slows down the app

D Removes the need for routers

Sub-dependencies help avoid repeating code and make testing smaller parts easier.

Explain how sub-dependencies work in FastAPI and why they are useful.

Think about how smaller parts can be combined to build bigger features.

Describe what happens when a sub-dependency raises an error in FastAPI.

Consider how FastAPI handles errors to keep the app safe and predictable.