nr.seed(2021)
def ran_walk(start = '1-1990', end = '1-2021', freq = 'M', sd = 1.0, mean = 0):
    dates = pd.date_range(start = start, end = end, freq = freq)
    walk = pd.Series(nr.normal(loc = mean, scale = sd, size = len(dates)),
                    index = dates)
    return(walk.cumsum())
walk = ran_walk()   
plot_ts(walk, 'random walk')