Validating fields on the server side

We have forms and fields, but we need to validate them in order to make sure that only the correct data goes through to the database and errors are handled beforehand rather than corrupting the database. These validations can also prevent the application against cross-site scripting (XSS) and CSRF attacks. WTForms provides a whole lot of field types that themselves have validations written for them by default. Apart from these, there are a bunch of validators that can be used on the basis of choice and need. We will use a few of them to understand this concept further.

How to do it…

It is pretty easy to add validations to our WTForm fields. We just need to pass a validators parameter, which accepts a list of validators to be implemented. Each of the validators can have their own arguments, which enable us to control the validations to a great extent.

Let's modify our ProductForm class to have validations:

from decimal import Decimal
from wtforms.validators...