Internal helper to get item value from an environment variable
        if item is controlled by one, and if the variable is set.

        Returns not_set otherwise.

def _get_envvar_value(self):
        """
        Internal helper to get item value from an environment variable
        if item is controlled by one, and if the variable is set.

        Returns not_set otherwise.
        """
        envvar_name = None

        if self.envvar is True:
            envvar_name = self.envvar_name
            if envvar_name is None:
                envvar_name = '_'.join(self.get_path()).upper()
        elif self.envvar:
            envvar_name = self.envvar

        if envvar_name and envvar_name in os.environ:
            return self.type.deserialize(os.environ[envvar_name])
        else:
            return not_set