How can I implement advanced validation in Flask?

How can I implement advanced validation in Flask?

You can use libraries like WTForms or Flask-WTF for advanced validation in Flask. They provide a way to define forms and validate input easily.

Can you show me an example of using WTForms for validation?

Sure! Here's a simple example:

from flask import Flask, render_template, request
from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField
from wtforms.validators import DataRequired, Length

app = Flask(__name__)
app.secret_key = 'your_secret_key'

class MyForm(FlaskForm):
    name = StringField('Name', validators=[DataRequired(), Length(min=2, max=20)])
    submit = SubmitField('Submit')

@app.route('/form', methods=['GET', 'POST'])
def form():
    form = MyForm()
    if form.validate_on_submit():
        return f'Hello, {form.name.data}!'
    return render_template('form.html', form=form)

What if I want to add custom validation?

You can create custom validators by defining a function and using it in your form field. Here's an example:

def validate_name(form, field):
    if not field.data.isalpha():
        raise ValidationError('Name must contain only letters.')

class MyForm(FlaskForm):
    name = StringField('Name', validators=[DataRequired(), validate_name])