apply after set_index dataframe code example

Example 1: pandas create a column from index

df.reset_index(level=0, inplace=True)

Example 2: how to change indeces in pandas dataframe

>>> df.set_index('month')
       year  sale
month
1      2012    55
4      2014    40
7      2013    84
10     2014    31

Example 3: set index in datarame

df = df.set_index('col')