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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def username_validator(self, form, field):
 """Ensure that Usernames contains at least 3 alphanumeric characters. Override this method to customize the username validator. """

        username = field.data
        if len(username) < 3:
            raise ValidationError(
                _('Username must be at least 3 characters long'))
        valid_chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-._'
        chars = list(username)
        for char in chars:
            if char not in valid_chars:
                raise ValidationError(
                    _("Username may only contain letters, numbers, '-', '.' and '_'"))