code: def input_classes(self):
          
          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)
docs: Full list of classes for the class attribute of the input, returned as a string with
          spaces separating each class.