import pandas as pd
df = pd.DataFrame([['A', 1], ['B', 2], ['C', 3]] , columns = ['col1', 'col2'])
.assign(col3 = 2 * col2)

df <- data.frame(col1 = LETTERS[1:3], col2 = 1:3) %>% mutate(col3 = 2 * col2)