define col as index pandas code example

Example 1: set index to column pandas

df = df.set_index('col')
df['col'] = df.index

Example 2: add an index column pandas

import numpy as np
import pandas as pd
df = pd.DataFrame({'month': [2, 5, 8, 10],
                   'year': [2017, 2019, 2018, 2019],
                   'sale': [60, 45, 90, 36]})
df.set_index('month')