By default, a #Python #Pandas series has a RangeIndex, like a string or list:

s = pd.Series([10, 20, 30])
s.index # RangeIndex(start=0, stop=3, step=1)

Set an index like this:

s = pd.Series([10, 20, 30], index=list('abc'))
s.index # Index(['a', 'b', 'c'], dtype='str')