def input_classes(self):
        """
        Full list of classes for the class attribute of the input, returned as a string with
        spaces separating each class.
        """
        classes = [self.base_input_css_class]
        if self.css_class:
            classes.append(self.css_class)

        if self.style == styles.BOOTSTRAP_4 and self.error:
            classes.append('is-invalid')

        return ' '.join(classes)
Full list of classes for the class attribute of the input, returned as a string with
        spaces separating each class.