code: def get_widget_css_class(self, field_name, field):
          
          # If we render CheckboxInputs, Bootstrap requires a different
          # widget css class for checkboxes.
          if isinstance(field.widget, forms.CheckboxInput):
              return 'form-check-input'
  
          # Idem for fileinput.
          if isinstance(field.widget, forms.FileInput):
              return 'form-control-file'
  
          return super().get_widget_css_class(field_name, field)
docs: Returns 'form-check-input' if widget is CheckboxInput or 'form-control-file'
          if widget is FileInput. For all other fields return the default value
          from the form property ("form-control").