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)