Mastering Flask Web Development - Second Edition

Custom validations

Writing a custom validation function is very simple. All that is required is to write a function that takes the form object and the field object as parameters and raises a WTForm. A ValidationError is raised if the data does not pass the test. Here is an example of a custom email validator:

import re 
import wtforms 
def custom_email(form, field): 
  if not re.match(r"[^@]+@[^@]+.[^@]+", field.data): 
    raise wtforms.ValidationError('Field must be a valid email        address.')

To use this function, just add it to the list of validators for your field.