Class-based dependencies in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class-based dependency in FastAPI.

FastAPI

from fastapi import Depends, FastAPI

class CommonQueryParams:
    def __init__(self, q: str = None):
        self.q = q

app = FastAPI()

@app.get("/items/")
async def read_items(commons: CommonQueryParams = Depends([1])):
    return {"q": commons.q}

Drag options to blanks, or click blank then click option'

ADepends(CommonQueryParams)

BCommonQueryParams()

CCommonQueryParams

DDepends(CommonQueryParams())

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an instance of the class instead of the class itself.

Wrapping Depends around an instance instead of the class.

In FastAPI, to use a class as a dependency, you pass the class itself (not an instance) to Depends(). FastAPI will create an instance automatically.

2fill in blank

medium

Complete the code to inject a class-based dependency into a path operation.

FastAPI

from fastapi import Depends, FastAPI

class User:
    def __init__(self, username: str):
        self.username = username

app = FastAPI()

@app.get("/users/me")
async def read_user(user: User = Depends([1])):
    return {"username": user.username}

Drag options to blanks, or click blank then click option'

AUser

BUser()

CDepends(User())

DDepends(User)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an instance instead of the class.

Using Depends with an instance instead of the class.

Pass the class name to Depends without parentheses. FastAPI will create the instance and inject it.

3fill in blank

hard

Fix the error in the class-based dependency usage.

FastAPI

from fastapi import Depends, FastAPI

class Settings:
    def __init__(self):
        self.debug = True

app = FastAPI()

@app.get("/settings")
async def get_settings(settings: Settings = Depends([1])):
    return {"debug": settings.debug}

Drag options to blanks, or click blank then click option'

ASettings

BSettings()

CDepends(Settings())

DDepends(Settings)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an instance instead of the class.

Wrapping Depends around an instance.

Pass the class name to Depends without parentheses. FastAPI will instantiate it automatically.

4fill in blank

hard

Fill both blanks to create a class-based dependency that accepts a query parameter and use it in a path operation.

FastAPI

from fastapi import Depends, FastAPI

class QueryParams:
    def __init__(self, q: str = None):
        self.q = q

app = FastAPI()

@app.get("/search")
async def search(params: QueryParams = Depends([1])):
    return {"query": params.[2]

Drag options to blanks, or click blank then click option'

AQueryParams

Bquery

Cq

DQueryParams()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an instance instead of the class.

Using the wrong attribute name for the query parameter.

Pass the class name to Depends. The instance has an attribute 'q' holding the query parameter.

5fill in blank

hard

Fill all three blanks to create a class-based dependency with two parameters and use them in a path operation.

FastAPI

from fastapi import Depends, FastAPI

class Pagination:
    def __init__(self, skip: int = 0, limit: int = 10):
        self.skip = skip
        self.limit = limit

app = FastAPI()

@app.get("/items")
async def list_items(pagination: Pagination = Depends([1])):
    return {"skip": pagination.[2], "limit": pagination.[3]

Drag options to blanks, or click blank then click option'

APagination

Bskip

Climit

DPagination()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an instance instead of the class.

Using incorrect attribute names for pagination.

Pass the class name to Depends. Access the 'skip' and 'limit' attributes from the instance.

Practice

1. What is the main purpose of using class-based dependencies in FastAPI?

easy

A. To automatically generate HTML forms

B. To replace all route functions with classes

C. To group related dependency logic in one reusable place

D. To handle database connections only

Solution

1. Step 1: Understand the role of class-based dependencies

   Class-based dependencies allow grouping related logic inside a class, making code cleaner and reusable.

2. Step 2: Compare options with this purpose

   Only To group related dependency logic in one reusable place correctly describes grouping related logic; others describe unrelated features.

3. Final Answer:

   To group related dependency logic in one reusable place -> Option C

4. Quick Check:

   Class-based dependencies = Group logic [OK]

Hint: Class-based dependencies group logic inside a class [OK]

Common Mistakes:

• Thinking class dependencies replace route functions
• Assuming they auto-generate HTML
• Believing they only handle databases

2. Which method must a class implement to be used as a dependency in FastAPI?

easy

A. __init__

B. __call__

C. dependency

D. run

Solution

1. Step 1: Recall FastAPI dependency requirements

   FastAPI requires the class to be callable, which means it must implement the __call__ method.

2. Step 2: Match method names to this requirement

   Only __call__ makes the class instance callable; __init__ is for initialization, others are invalid.

3. Final Answer:

   __call__ -> Option B

4. Quick Check:

   Callable class = __call__ method [OK]

Hint: Class must be callable via __call__ method [OK]

Common Mistakes:

• Choosing __init__ instead of __call__
• Using random method names like 'run'
• Confusing dependency with method name

3. Given this class-based dependency, what will be the output when accessing the endpoint?

from fastapi import FastAPI, Depends

app = FastAPI()

class Greeting:
    def __init__(self, name: str = "Guest"):
        self.name = name
    def __call__(self):
        return f"Hello, {self.name}!"

@app.get("/hello")
async def hello(greet: str = Depends(Greeting)):
    return {"message": greet}

medium

A. {"message": "Hello!"}

B. {"message": "Hello, name!"}

C. TypeError at runtime

D. {"message": "Hello, Guest!"}

Solution

1. Step 1: Analyze the Greeting class behavior

   The class sets name to "Guest" by default and __call__ returns "Hello, Guest!" string.

2. Step 2: Understand dependency injection in endpoint

   Depends(Greeting) creates an instance with default name, so greet is "Hello, Guest!" string.

3. Final Answer:

   {"message": "Hello, Guest!"} -> Option D

4. Quick Check:

   Default name used = Hello, Guest! [OK]

Hint: Default parameter used if no argument passed [OK]

Common Mistakes:

• Expecting 'name' literal instead of variable value
• Assuming runtime error without cause
• Ignoring default parameter in __init__

4. Identify the error in this class-based dependency usage:

class Counter:
    def __init__(self):
        self.count = 0

    def increment(self):
        self.count += 1

@app.get("/count")
async def get_count(counter: Counter = Depends(Counter)):
    counter.increment()
    return {"count": counter.count}

medium

A. count attribute should be a class variable

B. Counter class lacks a __call__ method

C. increment method should be async

D. Depends() cannot accept classes

Solution

1. Step 1: Trace the dependency execution flow

   Depends(Counter) creates a new instance each request; self.count = 0, increment() sets to 1, returns {"count": 1}. Count resets every request.

2. Step 2: Pinpoint the logical error

   self.count is an instance attribute (per-request); for persistent counting across requests, count must be a class attribute.

3. Final Answer:

   count attribute should be a class variable -> Option A

4. Quick Check:

   Instance attr = resets per request [OK]

Hint: Use class variables for shared state across requests [OK]

Common Mistakes:

• Thinking Depends can't accept classes
• Assuming async needed for increment
• Confusing instance and class variables

5. How can you modify this class-based dependency to accept a dynamic parameter from the request query?

class UserInfo:
    def __init__(self, user_id: int):
        self.user_id = user_id
    def __call__(self):
        return f"User ID is {self.user_id}"

@app.get("/user")
async def user(info: str = Depends(UserInfo)):
    return {"info": info}

Choose the correct way to pass user_id from query parameters.

hard

A. Use __init__(self, user_id: int = Query(...)) and import Query

B. Add user_id parameter to __call__ method instead

C. Pass user_id directly in Depends(UserInfo(user_id))

D. Use global variable for user_id inside UserInfo

Solution

1. Step 1: Understand how FastAPI injects parameters

   FastAPI injects parameters into __init__ if they have default values with Query or Body.

2. Step 2: Use Query to declare user_id in __init__

   Adding user_id: int = Query(...) in __init__ allows FastAPI to get it from query parameters.

3. Final Answer:

   Use __init__(self, user_id: int = Query(...)) and import Query -> Option A

4. Quick Check:

   Query param in __init__ = dynamic dependency [OK]

Hint: Use Query in __init__ to get query params [OK]

Common Mistakes:

• Trying to pass parameters in __call__
• Passing instance in Depends directly
• Using global variables instead of parameters