import pandas as pd
foo = pd.DataFrame([['USA', 1, 2], ['Canada', 3, 4], ['Canada', 5, 6]],
    columns=('Country', 'x', 'y'))
z = foo['x'].where(foo['Country'] == 'USA', foo['y'])
pd.concat([foo['Country'], z], axis=1)
Pandas - conditionally select column based on row value