How is the system of creating forms with server-side validation error handling implemented?

In Python, frameworks such as Django or Flask with WTForms are often used to create forms with server-side validation handling.

Main idea:

• A form class is defined with fields and validators.
• When data is received from the user, an instance of the form is created with this data.
• The validation method (e.g., form.validate()) is called to check the correctness of the data.
• If validation fails, errors are stored in the form object and can be displayed to the user.

Example with WTForms:

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

class UserForm(FlaskForm):
    name = StringField('Name', validators=[DataRequired()])
    age = IntegerField('Age', validators=[NumberRange(min=18, max=100)])

# In request handler
form = UserForm(request.form)
if form.validate():
    # Process valid data
    pass
else:
    # form.errors contains validation errors
    pass

Thus, server-side validation is centralized, errors are accumulated, and can be conveniently returned to the client for display.