def binop_return_dtype(op, left, right):
    """
    Compute the expected return dtype for the given binary operator.

    Parameters
    ----------
    op : str
        Operator symbol, (e.g. '+', '-', ...).
    left : numpy.dtype
        Dtype of left hand side.
    right : numpy.dtype
        Dtype of right hand side.

    Returns
    -------
    outdtype : numpy.dtype
        The dtype of the result of `left <op> right`.
    """
    if is_comparison(op):
        if left != right:
            raise TypeError(
                "Don't know how to compute {left} {op} {right}.\n"
                "Comparisons are only supported between Factors of equal "
                "dtypes.".format(left=left, op=op, right=right)
            )
        return bool_dtype

    elif left != float64_dtype or right != float64_dtype:
        raise TypeError(
            "Don't know how to compute {left} {op} {right}.\n"
            "Arithmetic operators are only supported between Factors of "
            "dtype 'float64'.".format(
                left=left.name,
                op=op,
                right=right.name,
            )
        )
    return float64_dtype