def weighted_sum(df, var, wt="s006"):
    """
    Return the weighted sum of specified variable

    Parameters
    ----------
    df: Pandas DataFrame
        data overwhich to compute weighted sum
    var: str
        variable name from df for which to computer weighted sum
    wt: str
        name of weight variable in df

    Returns
    -------
    float
        weighted sum
    """
    return (df[var] * df[wt]).sum()