Merge multiple column integer values into one column in python pandas code example

Example 1: add two column values of a datframe into one

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

Example 2: assign multiple columns pandas

import pandas as pd

df = {'col_1': [0, 1, 2, 3],
        'col_2': [4, 5, 6, 7]}
df = pd.DataFrame(df)

df[[ 'column_new_1', 'column_new_2','column_new_3']] = [np.nan, 'dogs',3]  #thought this wo