Returns all transactions whose given ``field`` matches ``key``.

        Returns:
            A ``Transactions`` object.

def group_by(self, key, field=lambda x: x.xfer):
        """Returns all transactions whose given ``field`` matches ``key``.

        Returns:
            A ``Transactions`` object.
        """
        return Transactions([t for t in self.trans if field(t) == key])