Class-based dependencies in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this FastAPI class-based dependency?

Consider this FastAPI dependency class and endpoint. What will be the JSON response when accessing /items/42?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

class ItemDependency:
    def __init__(self, item_id: int):
        self.item_id = item_id

    def get_item(self):
        return {"item_id": self.item_id, "name": f"Item {self.item_id}"}

@app.get("/items/{item_id}")
async def read_item(dep: ItemDependency = Depends(ItemDependency)):
    return dep.get_item()

A {"item_id": "42", "name": "Item 42"}

B TypeError: __init__() missing 1 required positional argument: 'item_id'

C {"item_id": 42}

D {"item_id": 42, "name": "Item 42"}

💡 Hint

Think about how FastAPI injects path parameters into class dependencies.

FastAPI cannot automatically pass the path parameter 'item_id' to the class constructor unless explicitly told. The dependency injection fails because the constructor requires 'item_id' but FastAPI does not know to provide it.

📝 Syntax

intermediate

Which option correctly defines a class-based dependency with a __call__ method in FastAPI?

Select the code snippet that correctly implements a class-based dependency with a __call__ method that accepts a path parameter user_id.

A

class UserDependency:
    def __call__(self, user_id: str):
        return {"user_id": int(user_id)}

@app.get("/users/{user_id}")
async def get_user(dep: UserDependency = Depends(UserDependency)):
    return dep

B

class UserDependency:
    def __call__(self):
        return {"user_id": user_id}

@app.get("/users/{user_id}")
async def get_user(dep: UserDependency = Depends(UserDependency)):
    return dep

C

class UserDependency:
    def __init__(self, user_id: int):
        self.user_id = user_id

@app.get("/users/{user_id}")
async def get_user(dep: UserDependency = Depends(UserDependency)):
    return {"user_id": dep.user_id}

D

class UserDependency:
    def __call__(self, user_id: int):
        return {"user_id": user_id}

@app.get("/users/{user_id}")
async def get_user(dep: UserDependency = Depends(UserDependency)):
    return dep

💡 Hint

The __call__ method must accept the parameter and return the expected data.

Option D correctly defines __call__ with user_id parameter typed as int, matching the path parameter. It returns a dictionary with user_id. FastAPI injects user_id automatically.

🔧 Debug

advanced

Why does this class-based dependency raise a runtime error?

Examine the code below. When calling /orders/5, it raises an error. What is the cause?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

class OrderDependency:
    def __init__(self, order_id: int):
        self.order_id = order_id

    def __call__(self):
        return {"order_id": self.order_id}

@app.get("/orders/{order_id}")
async def get_order(dep: OrderDependency = Depends(OrderDependency)):
    return dep()

A The endpoint returns the class instance instead of calling it, causing a TypeError.

B The __call__ method is missing the 'order_id' parameter, causing a runtime error.

C FastAPI cannot pass 'order_id' to the __init__ method automatically, causing a TypeError.

D The Depends function is used incorrectly without parentheses, causing a syntax error.

💡 Hint

Check how FastAPI injects parameters into class dependencies.

FastAPI does not automatically inject path parameters into the __init__ constructor of a class dependency. This causes a TypeError because order_id is required but not provided.

❓ state_output

advanced

What is the value of dep.value after this request?

Given the class-based dependency below, what will be the value of dep.value after a GET request to /calc/10?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

class Calculator:
    def __init__(self, number: int):
        self.value = number * 2

@app.get("/calc/{number}")
async def calculate(dep: Calculator = Depends(Calculator)):
    return {"result": dep.value}

A TypeError: __init__() missing 1 required positional argument: 'number'

B 20

C 10

D 0

💡 Hint

How does FastAPI pass path parameters to class constructors?

FastAPI cannot automatically pass the path parameter 'number' to the Calculator constructor, so it raises a TypeError.

🧠 Conceptual

expert

Which statement about FastAPI class-based dependencies is TRUE?

Select the correct statement about how FastAPI handles class-based dependencies with parameters.

A To receive parameters, a class-based dependency should implement a __call__ method with parameters matching the route parameters.

B FastAPI automatically injects path and query parameters into the __init__ constructor of dependency classes without extra configuration.

C Class-based dependencies cannot access request data unless explicitly passed via constructor parameters.

D Using class-based dependencies disables FastAPI's automatic validation of parameters.

💡 Hint

Think about how FastAPI calls class dependencies to inject parameters.

FastAPI calls the class instance if it has a __call__ method and injects parameters into that method. It does not inject parameters into the constructor automatically.