def axify(expr, axis):
    """ inject axis argument into expression

    Helper function for compute_up(Summary, np.ndarray)

    >>> from blaze import symbol
    >>> s = symbol('s', '10 * 10 * int')
    >>> expr = s.sum()
    >>> axify(expr, axis=0)
    sum(s, axis=(0,))
    """
    return type(expr)(expr._child, axis=axis)