Numeric validation (gt, lt, ge, le) in FastAPI - Mini Project: Build & Apply

Numeric Validation with FastAPI

📖 Scenario: You are building a simple FastAPI app that accepts a number from users. You want to make sure the number is within a certain range before processing it.

🎯 Goal: Create a FastAPI endpoint that accepts a query parameter value which must be a number greater than 10 and less than or equal to 100. Use FastAPI's numeric validation features to enforce these rules.

📋 What You'll Learn

Create a FastAPI app instance named app

Define a GET endpoint at path /check-number

Add a query parameter value of type int

Use FastAPI's Query with gt=10 and le=100 to validate value

Return a JSON response with the key value and the validated number

💡 Why This Matters

🌍 Real World

Validating numeric inputs is common in web APIs to ensure data integrity and prevent invalid data from causing errors or security issues.

💼 Career

Understanding how to use FastAPI's built-in validation helps backend developers build robust and user-friendly APIs that enforce rules on incoming data.

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 validation

Import Query from fastapi to use for numeric validation of query parameters.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

# Import Query for parameter validation
# Your code here

Need a hint?

Import Query alongside FastAPI in the same line.

3

Define GET endpoint with validated query parameter

Define a GET endpoint at /check-number with a query parameter value of type int. Use Query with gt=10 and le=100 to validate value.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

# Define GET endpoint /check-number with validated query parameter 'value'
# Your code here

Need a hint?

Use @app.get('/check-number') decorator and define value with Query(..., gt=10, le=100).

4

Complete the FastAPI app

Ensure the FastAPI app code includes the import statements, app instance, and the GET endpoint with numeric validation as defined.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get('/check-number')
async def check_number(value: int = Query(..., gt=10, le=100)):
    return {"value": value}

# Finalize the app code
# Your code here

Need a hint?

Make sure all parts are present and correctly indented.