Одно решение:
data.csv
Age,Population,Seasons
20,100,1
30,340,2
35,45,3
40,90,4
45,9,3
test.py
#!/bin/python
import pandas as pd
df = pd.read_csv('data.csv')
seasons = ['Winter', 'Spring', 'Summer', 'Fall']
df['Seas_Fact'] = [seasons[x-1] for x in df['Seasons']]
print(df)
Вывод:
Age Population Seasons Seas_Fact
0 20 100 1 Winter
1 30 340 2 Spring
2 35 45 3 Summer
3 40 90 4 Fall
4 45 9 3 Summer