Class-based dependencies in FastAPI

Introduction

Class-based dependencies help organize related logic in one place. They make your code cleaner and easier to reuse.

When you want to group multiple related dependency methods together.

When you need to maintain some state or configuration inside the dependency.

When you want to reuse the same dependency logic in many routes.

When your dependency requires initialization or setup steps.

When you want to keep your code modular and easier to test.

Syntax

FastAPI

from fastapi import Depends, FastAPI

app = FastAPI()

class MyDependency:
    def __init__(self, param: str):
        self.param = param

    def __call__(self):
        return f"Value is {self.param}"

@app.get("/items/")
async def read_items(dep=Depends(MyDependency("example"))):
    return {"result": dep}

The class must implement the __call__ method to be used as a dependency.

You can pass parameters to the class constructor to customize the dependency.

Examples

This example shows a class that stores a user agent string and returns it when called.

FastAPI

from fastapi import Depends, FastAPI

app = FastAPI()

class CommonHeaders:
    def __init__(self, user_agent: str = "unknown"):
        self.user_agent = user_agent

    def __call__(self):
        return {"User-Agent": self.user_agent}

@app.get("/headers")
async def get_headers(headers=Depends(CommonHeaders("my-agent"))):
    return headers

This example shows a class that counts how many times the dependency is called.

FastAPI

from fastapi import Depends, FastAPI

app = FastAPI()

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

    def __call__(self):
        self.count += 1
        return self.count

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

Sample Program

This program defines a class-based dependency that returns a greeting message. The route uses this dependency to greet the user named Alice.

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

class Greeting:
    def __init__(self, name: str):
        self.name = name

    def __call__(self):
        return f"Hello, {self.name}!"

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

Important Notes

Class-based dependencies can hold state, but be careful with mutable state in async environments.

FastAPI creates a new instance of the class for each request by default.

You can combine class-based dependencies with other dependencies for flexible designs.

Summary

Class-based dependencies group related logic in one place.

They require a __call__ method to work as dependencies.

They help keep your FastAPI code clean and reusable.

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