wtforms-usedif.py

class UsedIf(object):
    """
    Make a WTF field "used" if a given condition evaluates to True.
    If the field is not used, validation stops.
    """
    def __init__(self, condition):
        self.condition = condition

    def __call__(self, form, field):
        if self.condition in (True, False):
            if not self.condition:
                field.errors[:] = []
                raise StopValidation()
        elif not self.condition(form, field):
            field.errors[:] = []
            raise StopValidation()