Change the Index of a pandas DataFrame

import pandas as pd

df = pd.DataFrame(
    {"x": [1, 2, 3, 4, 5], "y": [5, 4, 3, 2, 1]}, index=["r1", "r2", "r3", "r4", "r5"]
)

df.head()

df.index = df.y
df

import pandas as pd

df = pd.DataFrame(
    {"month": [1, 4, 7, 10], "year": [2012, 2014, 2013, 2014], "sale": [55, 40, 84, 31]}
)
df

df2 = df.set_index("month")
df2

df2.columns

df.year.iat[0]

df.year.at[0]

df2.year.iat[0]

df2.year.at[1]

df3 = df.set_index(["year", "month"])
df3

import numpy as np

np.average([20000, 30000], weights=[9, 15])