import pandas as pd

df1 = pd.DataFrame({'HPI':[80,85,88,85],
                    'Int_rate':[2, 3, 2, 2],
                    'US_GDP_Thousands':[50, 55, 65, 55]},
                   index = [2001, 2002, 2003, 2004])

df2 = pd.DataFrame({'HPI':[80,85,88,85],
                    'Int_rate':[2, 3, 2, 2],
                    'US_GDP_Thousands':[50, 55, 65, 55]},
                   index = [2005, 2006, 2007, 2008])

df3 = pd.DataFrame({'HPI':[80,85,88,85],
                    'Int_rate':[2, 3, 2, 2],
                    'Low_tier_HPI':[50, 52, 50, 53]},
                   index = [2001, 2002, 2003, 2004])

# df1.set_index('HPI', inplace=True)
concat = pd.concat([df1, df3])

df4 = df1.append(df3)

print(concat)
print(df4)

s = pd.Series([[80, 2, 50],[80, 54, 56], [56, 43, 23]])
print(s)