https://app.dataquest.io/m/468/business-metrics/7/date-wrangling
churn = pd.merge(churn, monthly_churn, "left", left_on="yearmonth", right_index=True)
I do not understand why it is needed to set right_index=True
I understand the other parts like join the left dataframe --churn–on its column
left_on=“yearmonth”
it is apparent to me that churn has its index with rows 1,2,3,4…but it is not apparent what the index is in the monthly_churn dataframe