One of the most frequently posed Flask interview questions for freshers, be ready for it. A validator simply takes an input, checks that it meets some criteria, such as a string length limit, and then returns. If the validation fails, a ValidationError is raised. This method is quite straightforward and adaptable, allowing you to link any number of validators onto fields. In Flask, the validators class of WTForm is listed below: DataRequired: To check whether the input field is empty. Email: To check whether the text in the field follows email ID conventions. IPAddress: To validate IP address in the input field. Length: To verify if the length of the string in the input field is in the given range. NumberRange: To validate a number in input field within the given range. URL: To validate the URL entered in the input field.