def order_target(self,
                     asset,
                     target,
                     limit_price=None,
                     stop_price=None,
                     style=None):
        """Place an order to adjust a position to a target number of shares. If
        the position doesn't already exist, this is equivalent to placing a new
        order. If the position does exist, this is equivalent to placing an
        order for the difference between the target number of shares and the
        current number of shares.

        Parameters
        ----------
        asset : Asset
            The asset that this order is for.
        target : int
            The desired number of shares of ``asset``.
        limit_price : float, optional
            The limit price for the order.
        stop_price : float, optional
            The stop price for the order.
        style : ExecutionStyle
            The execution style for the order.

        Returns
        -------
        order_id : str
            The unique identifier for this order.


        Notes
        -----
        ``order_target`` does not take into account any open orders. For
        example:

        .. code-block:: python

           order_target(sid(0), 10)
           order_target(sid(0), 10)

        This code will result in 20 shares of ``sid(0)`` because the first
        call to ``order_target`` will not have been filled when the second
        ``order_target`` call is made.

        See :func:`zipline.api.order` for more information about
        ``limit_price``, ``stop_price``, and ``style``

        See Also
        --------
        :class:`zipline.finance.execution.ExecutionStyle`
        :func:`zipline.api.order`
        :func:`zipline.api.order_target_percent`
        :func:`zipline.api.order_target_value`
        """
        if not self._can_order_asset(asset):
            return None

        amount = self._calculate_order_target_amount(asset, target)
        return self.order(asset, amount,
                          limit_price=limit_price,
                          stop_price=stop_price,
                          style=style)