combine two columns pandas

df["period"] = df["Year"].astype(str) + df["quarter"]

import pandas as pd
T1 = pd.merge(T1, T2, on=T1.index, how='outer')

df["period"] = df["Year"] + df["quarter"]

df = df['Day'].map(str) + '-' + df['Month'].map(str) + '-' + df['Year'].map(str)

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