Class-based dependencies in FastAPI - Mini Project: Build & Apply

Class-based dependencies in FastAPI

📖 Scenario: You are building a simple FastAPI app that uses a class to manage a counter. This counter will be shared as a dependency in your API endpoints.

🎯 Goal: Create a FastAPI app with a class-based dependency that tracks a counter value. Use this dependency in an endpoint to return the current count.

📋 What You'll Learn

Create a class called Counter with an attribute count initialized to 0

Add a method increment in Counter that increases count by 1

Create a FastAPI app instance called app

Use Depends to inject the Counter class as a dependency in an endpoint

Create a GET endpoint /count that calls increment and returns the current count

💡 Why This Matters

🌍 Real World

Class-based dependencies help organize and reuse logic that needs to maintain state or configuration in web APIs.

💼 Career

Understanding class-based dependencies is important for building scalable and maintainable FastAPI applications in professional backend development.

1

Create the Counter class

Create a class called Counter with an attribute count set to 0 inside the __init__ method.

FastAPI

class Counter:
    # Your code here

Need a hint?

Define a class with class Counter:. Inside, create def __init__(self): and set self.count = 0.

2

Add increment method to Counter

Add a method called increment inside the Counter class that increases self.count by 1.

FastAPI

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

    # Your code here

Need a hint?

Define def increment(self): and inside it add self.count += 1.

3

Create FastAPI app and dependency

Import FastAPI and Depends from fastapi. Create an app instance called app. Define a function get_counter that returns a new Counter instance.

FastAPI

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

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

# Your code here

Need a hint?

Import FastAPI and Depends. Create app = FastAPI(). Define get_counter() that returns Counter().

4

Create endpoint using class-based dependency

Create a GET endpoint /count using @app.get("/count"). Add a parameter counter with type Counter and default value Depends(get_counter). Inside the function, call counter.increment() and return a dictionary with key count and value counter.count.

FastAPI

from fastapi import FastAPI, Depends

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

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

app = FastAPI()

def get_counter():
    return Counter()

# Your code here

Need a hint?

Use @app.get("/count") to create the endpoint. Add counter: Counter = Depends(get_counter) as parameter. Call counter.increment() and return {"count": counter.count}.