Dependencies with parameters in FastAPI - Mini Project: Build & Apply

FastAPI Dependencies with Parameters

📖 Scenario: You are building a simple FastAPI app that greets users differently based on a greeting style parameter.This is like choosing how you say hello to a friend: formally, casually, or excitedly.

🎯 Goal: Create a FastAPI app that uses a dependency with a parameter to customize the greeting message.The app will have one endpoint /greet/ that returns a greeting message based on the style passed as a query parameter.

📋 What You'll Learn

Create a FastAPI app instance called app.

Define a dependency function get_greeting_style that accepts a parameter style from the query string.

Use the dependency in the /greet/ endpoint to return a greeting message based on the style.

Support greeting styles: 'formal', 'casual', and 'excited'.

💡 Why This Matters

🌍 Real World

Using dependencies with parameters in FastAPI helps you write clean, reusable code that can customize behavior based on user input, like greeting styles or user roles.

💼 Career

Understanding FastAPI dependencies with parameters is essential for backend developers building scalable and maintainable APIs that adapt to different client needs.

1

Create the FastAPI app instance

Import FastAPI from fastapi and create an app instance called app.

FastAPI

# Import FastAPI and create app instance
# Your code here

Need a hint?

Use from fastapi import FastAPI and then app = FastAPI().

2

Define the dependency function with a parameter

Define a function called get_greeting_style that takes a parameter style of type str with a default value of "casual". Import Query from fastapi and use it to get style from the query parameters.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

# Define get_greeting_style dependency with style parameter
# Your code here

Need a hint?

Use def get_greeting_style(style: str = Query("casual")) and return style.

3

Create the /greet/ endpoint using the dependency

Create a GET endpoint /greet/ using @app.get("/greet/"). Define a function greet that takes a parameter style which uses the dependency get_greeting_style with Depends. Import Depends from fastapi. Inside the function, return a dictionary with key message and value based on style: if style is 'formal', return 'Good day to you.'; if 'casual', return 'Hey there!'; if 'excited', return 'Hello!!!'.

FastAPI

from fastapi import FastAPI, Query, Depends

app = FastAPI()

def get_greeting_style(style: str = Query("casual")) -> str:
    return style

# Create /greet/ endpoint using get_greeting_style dependency
# Your code here

Need a hint?

Use @app.get("/greet/") and def greet(style: str = Depends(get_greeting_style)). Return a dictionary with the correct message based on style.

4

Add type hints and import statements for completeness

Add the import statement for Depends from fastapi if missing. Ensure all functions have proper type hints: get_greeting_style returns str, greet returns a dictionary with str keys and values. This completes the FastAPI app with dependency injection using parameters.

FastAPI

from fastapi import FastAPI, Query, Depends

app = FastAPI()

def get_greeting_style(style: str = Query("casual")) -> str:
    return style

@app.get("/greet/")
def greet(style: str = Depends(get_greeting_style)) -> dict[str, str]:
    # Your code here

Need a hint?

Make sure to import Depends and add return type hints for both functions.