Query parameter validation in FastAPI - Mini Project: Build & Apply

Query Parameter Validation with FastAPI

📖 Scenario: You are building a simple web API that returns items based on a query parameter. You want to make sure the query parameter is validated properly to avoid errors and unexpected results.

🎯 Goal: Create a FastAPI app with a GET endpoint /items/ that accepts a query parameter q. Validate that q is a string with a minimum length of 3 and a maximum length of 10 characters.

📋 What You'll Learn

Create a FastAPI app instance named app

Define a GET endpoint /items/ that accepts a query parameter q

Use FastAPI's Query to validate q with min_length=3 and max_length=10

Return a JSON response with the key query and the value of q

💡 Why This Matters

🌍 Real World

Validating query parameters is essential in web APIs to ensure clients send correct and expected data, preventing errors and improving API reliability.

💼 Career

Backend developers often use FastAPI or similar frameworks to build APIs that require input validation to maintain data integrity and security.

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
# Your code here

Need a hint?

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

2

Import Query for parameter validation

Import Query from fastapi to use for query parameter validation.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

# Import Query for validation
# Your code here

Need a hint?

Add Query to the import statement from fastapi.

3

Define GET endpoint with validated query parameter

Define a GET endpoint /items/ using @app.get("/items/"). Create a function read_items that accepts a query parameter q of type str. Use Query to validate q with min_length=3 and max_length=10.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

# Define GET endpoint with query parameter validation
# Your code here

Need a hint?

Use Query(..., min_length=3, max_length=10) to require and validate the query parameter.

4

Complete the FastAPI app with proper async function

Ensure the endpoint function read_items is declared as async and returns a dictionary with the key query and the value of q.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(q: str = Query(..., min_length=3, max_length=10)):
    # Return the query parameter in a dictionary
    # Your code here

Need a hint?

Make sure the function is async and returns the query parameter in a dictionary.