Validation rules in Flask - Mini Project: Build & Apply

Flask Form Validation Rules

📖 Scenario: You are building a simple web form using Flask where users can submit their name and age. To ensure the data is correct, you need to add validation rules to the form fields.

🎯 Goal: Create a Flask app with a form that validates the user's name to be non-empty and the age to be a number between 1 and 120.

📋 What You'll Learn

Create a Flask app with a route for the form

Create a form class with fields name and age

Add validation rules: name must not be empty, age must be an integer between 1 and 120

Render the form in a template and show validation errors if any

💡 Why This Matters

🌍 Real World

Forms with validation are common in websites for user registration, surveys, and data collection.

💼 Career

Understanding form validation in Flask is essential for backend web development roles that handle user input securely and correctly.

1

Set up Flask app and form class

Create a Flask app instance called app and define a form class called UserForm that inherits from FlaskForm. Add two fields: name as a StringField and age as an IntegerField.

Flask

# Your code here

Need a hint?

Import Flask, FlaskForm, StringField, and IntegerField. Create app and UserForm with the two fields.

2

Add validation rules to form fields

Add validators to the name and age fields in UserForm. Use DataRequired() for name to ensure it is not empty. Use NumberRange(min=1, max=120) for age to ensure it is between 1 and 120.

Flask

from flask import Flask
from flask_wtf import FlaskForm
from wtforms import StringField, IntegerField

app = Flask(__name__)
app.config['SECRET_KEY'] = 'secret-key'

class UserForm(FlaskForm):
    # Add validators to name and age fields below
    name = StringField('Name')
    age = IntegerField('Age')

Need a hint?

Import DataRequired and NumberRange from wtforms.validators. Add them inside the validators list for each field.

3

Create route to display and process the form

Create a route / with a function index that creates an instance of UserForm. If the form is submitted and valid, just return the string 'Form submitted'. Otherwise, render a template called form.html passing the form instance as form.

Flask

from flask import Flask, render_template
from flask_wtf import FlaskForm
from wtforms import StringField, IntegerField
from wtforms.validators import DataRequired, NumberRange

app = Flask(__name__)
app.config['SECRET_KEY'] = 'secret-key'

class UserForm(FlaskForm):
    name = StringField('Name', validators=[DataRequired()])
    age = IntegerField('Age', validators=[NumberRange(min=1, max=120)])

# Create route '/' and function index below
# Your code here

Need a hint?

Use @app.route with methods=['GET', 'POST']. Inside the function, create form = UserForm(). Use form.validate_on_submit() to check if valid. Return the string or render the template accordingly.

4

Create the HTML template to show the form and errors

Create a template file form.html that displays the form fields name and age with their labels and input boxes. Show validation error messages below each field if present. Add a submit button labeled Submit. Use form.hidden_tag() for CSRF protection.

Flask

from flask import Flask, render_template
from flask_wtf import FlaskForm
from wtforms import StringField, IntegerField
from wtforms.validators import DataRequired, NumberRange

app = Flask(__name__)
app.config['SECRET_KEY'] = 'secret-key'

class UserForm(FlaskForm):
    name = StringField('Name', validators=[DataRequired()])
    age = IntegerField('Age', validators=[NumberRange(min=1, max=120)])

@app.route('/', methods=['GET', 'POST'])
def index():
    form = UserForm()
    if form.validate_on_submit():
        return 'Form submitted'
    return render_template('form.html', form=form)

# Create form.html template with form fields, errors, and submit button
# Your code here

Need a hint?

Use {{ form.hidden_tag() }} for CSRF. Show each field label and input. Loop over form.field.errors to show errors in red. Add a submit button.