String validation (min, max, regex) in FastAPI - Mini Project: Build & Apply

String validation (min, max, regex) with FastAPI

📖 Scenario: You are building a simple web API using FastAPI. The API will accept a username from users. You want to make sure the username is valid by checking its length and pattern.

🎯 Goal: Create a FastAPI app that accepts a username string with these rules: minimum length 3, maximum length 10, and only letters and numbers allowed. Use Pydantic string validation with min_length, max_length, and regex.

📋 What You'll Learn

Create a FastAPI app instance called app

Create a Pydantic model called User with a username field

Validate username with min_length=3, max_length=10, and a regex that allows only letters and numbers

Create a POST endpoint /users/ that accepts a User model and returns the username

💡 Why This Matters

🌍 Real World

Validating user input is essential in web APIs to prevent bad data and security issues. FastAPI with Pydantic makes this easy and clear.

💼 Career

Backend developers often need to validate incoming data. Knowing how to use FastAPI and Pydantic validation is a valuable skill for building reliable APIs.

1

Create FastAPI app and User model with username field

Write code to import FastAPI and BaseModel from pydantic. Create a FastAPI app instance called app. Create a Pydantic model called User with a single field username of type str.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

# Your code here

Need a hint?

Start by importing FastAPI and BaseModel. Then create app = FastAPI(). Define User class inheriting from BaseModel with a username field of type str.

2

Add validation rules to username field

Modify the username field in the User model to add validation: set min_length=3, max_length=10, and a regex pattern that allows only letters and numbers. Use Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$").

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class User(BaseModel):
    # Add validation to username below
    username: str = Field(...)

Need a hint?

Import Field from pydantic. Use Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$") to add validation to username.

3

Create POST endpoint to accept User model

Add a POST endpoint /users/ to the FastAPI app. The endpoint should accept a User model as input and return a dictionary with the key username and the value from the input model.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class User(BaseModel):
    username: str = Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$")

# Add POST endpoint below

Need a hint?

Use @app.post("/users/") decorator. Define async function create_user with parameter user: User. Return dictionary with username key and user.username value.

4

Add response model to endpoint

Modify the POST endpoint /users/ to include a response_model=User parameter in the decorator. This ensures the response matches the User model.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class User(BaseModel):
    username: str = Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$")

# Add response_model=User to the POST decorator
@app.post("/users/")
async def create_user(user: User):
    return {"username": user.username}

Need a hint?

Add response_model=User inside the @app.post decorator parentheses.