Custom validators in WTForms using Flask

Question

I need to make a custom validator in WTForms where the input is to be: number:number - e.g. 2:1

match1 = StringField('Russia-Saudi Arabia', validators=[DataRequired()])

So, my question is - how to create such validator?

I've looked upon the documentation at http://wtforms.readthedocs.io/en/latest/validators.html, but was not very helpful (for me) in this case.

Thanks in advance

Solution

You can write a custom validator within a form by writing a validate_{field_name} method. If it raises a ValidationError, the form will not be valid and will display the error.

For your specific case, here's a solution using regex. It finds the match for the string, and then uses a bit of splitting to get back the scores. After validating the form you can access the scores by form.score1, form.score2.

import re
from flask_wtf import FlaskForm

class MatchForm(FlaskForm):
    match1 = StringField("Russia-Saudi Arabia", validators=[DataRequired()])

    def validate_match1(form, field):
        if not re.search(r"^[0-9]+:[0-9]+$", field.data):
            raise ValidationError("Invalid input syntax")

        s1, s2 = form.data.split(":")
        form.score1 = int(s1)
        form.score2 = int(s2)