Boolean query parameters in FastAPI - Mini Project: Build & Apply

Boolean Query Parameters in FastAPI

📖 Scenario: You are building a simple web API using FastAPI. Your API will have an endpoint that returns a greeting message. You want to add a boolean query parameter to control whether the greeting is formal or casual.

🎯 Goal: Create a FastAPI app with a GET endpoint /greet that accepts a boolean query parameter called formal. If formal is true, the endpoint returns a formal greeting. If formal is false or missing, it returns a casual greeting.

📋 What You'll Learn

Create a FastAPI app instance named app

Add a GET endpoint /greet with a boolean query parameter formal

Use formal: bool = False to set the default value

Return {"message": "Good evening, sir."} if formal is true

Return {"message": "Hey there!"} if formal is false

💡 Why This Matters

🌍 Real World

Boolean query parameters are common in APIs to toggle features or options, such as filtering results or changing response styles.

💼 Career

Understanding how to handle boolean query parameters in FastAPI is essential for backend developers building flexible and user-friendly APIs.

1

Create FastAPI app instance

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

FastAPI

from fastapi import FastAPI

# Create FastAPI app instance called app
# Your code here

Need a hint?

Use app = FastAPI() to create the app instance.

2

Add GET endpoint with boolean query parameter

Add a GET endpoint /greet to app. Define a function greet that takes a boolean query parameter formal with default value False.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Add GET endpoint /greet with boolean query parameter formal defaulting to False
# Your code here

Need a hint?

Use @app.get("/greet") decorator and define formal: bool = False in the function parameters.

3

Implement conditional greeting logic

Inside the greet function, return {"message": "Good evening, sir."} if formal is True. Otherwise, return {"message": "Hey there!"}.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/greet")
def greet(formal: bool = False):
    # Return formal or casual greeting based on formal
    # Your code here

Need a hint?

Use an if statement to check formal and return the correct message.

4

Add type hints and docstring for clarity

Add a docstring to the greet function explaining that formal controls the greeting style. Ensure the function has a return type hint of dict.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/greet")
def greet(formal: bool = False) -> dict:
    """Return a greeting message. If formal is True, return a formal greeting."""
    if formal:
        return {"message": "Good evening, sir."}
    else:
        return {"message": "Hey there!"}
# Your code here

Need a hint?

Add a docstring inside the function and a return type hint -> dict.