how to insert 0, 1, 2 for all the rows in pandas code example

Example: python push to dataframe pandas

df = pd.DataFrame({
    'a':[1,2,3],
    'b':[5,6,7]
})

df2 = pd.DataFrame({
    'a':[11,12,13],
    'b':[15,16,17]
})

df = df.append(df2, ignore_index = True )

print(df)