<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def positions(self, account: str = '') -> List[Position]: """ List of positions for the given account, or of all accounts if account is left blank. Args: account: If specified, filter for this account name. """

        if account:
            return list(self.wrapper.positions[account].values())
        else:
            return [v for d in self.wrapper.positions.values()
                    for v in d.values()]