code: def load_values(self):
          
  
          for config_name, evar in self.evar_defs.items():
              if evar.is_required and evar.name not in os.environ:
                  raise RuntimeError((
                      "Missing required environment variable: {evar_name}\n"
                      "{help_txt}"
                  ).format(evar_name=evar.name, help_txt=evar.help_txt))
              # Env var is present. Transfer its value over.
              if evar.name in os.environ:
                  self[config_name] = os.environ.get(evar.name)
              else:
                  self[config_name] = evar.default_val
              # Perform any validations or transformations.
              for filter in evar.filters:
                  current_val = self.get(config_name)
                  new_val = filter(current_val, evar)
                  self[config_name] = new_val
          # This is the top-level filter that is often useful for checking
          # the values of related env vars (instead of individual validation).
          self._filter_all()
docs: Go through the env var map, transferring the values to this object
          as attributes.
  
          :raises: RuntimeError if a required env var isn't defined.