code: def _get_populate_from(self, model_instance):
          
  
          if hasattr(self.populate_from, '__call__'):
              tz = self.populate_from(model_instance)
          else:
              from_attr = getattr(model_instance, self.populate_from)
              tz = callable(from_attr) and from_attr() or from_attr
  
          try:
              tz = pytz.timezone(str(tz))
          except pytz.UnknownTimeZoneError:
              # It was a valiant effort. Resistance is futile.
              raise
  
          # If we have a timezone, set the instance's timezone attribute
          self.timezone = tz
  
          return tz
docs: Retrieves the timezone or None from the `populate_from` attribute.