code: def clean_subject_location(self):
          
          cleaned_data = super(ImageAdminForm, self).clean()
          subject_location = cleaned_data['subject_location']
          if not subject_location:
              # if supplied subject location is empty, do not check it
              return subject_location
  
          # use thumbnail's helper function to check the format
          coordinates = normalize_subject_location(subject_location)
  
          if not coordinates:
              err_msg = ugettext_lazy('Invalid subject location format. ')
              err_code = 'invalid_subject_format'
  
          elif (
              coordinates[0] > self.instance.width
              or coordinates[1] > self.instance.height
          ):
              err_msg = ugettext_lazy(
                  'Subject location is outside of the image. ')
              err_code = 'subject_out_of_bounds'
          else:
              return subject_location
  
          self._set_previous_subject_location(cleaned_data)
          raise forms.ValidationError(
              string_concat(
                  err_msg,
                  ugettext_lazy('Your input: "{subject_location}". '.format(
                      subject_location=subject_location)),
                  'Previous value is restored.'),
              code=err_code)
docs: Validate subject_location preserving last saved value.
  
          Last valid value of the subject_location field is shown to the user
          for subject location widget to receive valid coordinates on field
          validation errors.