import pandas as pd
import numpy.random as nr
nr.seed(2021)
def trend(start = '1-1990', end = '1-2020', freq = 'M', slope = 0.02, sd = 1.0, mean = 0):
    dates = pd.date_range(start = start, end = end, freq = freq)
    trend = pd.Series([slope*x for x in range(len(dates))],
                    index = dates)
    trend = trend + nr.normal(loc = mean, scale = sd, size = len(dates))
    return(trend)

trends = trend()   
plot_ts(trends, 'random walk')