get values from a data frame using a series

Question

df = pd.DataFrame({\'a\':[\'x\',\'y\',\'w\',\'x\',\'y\',\'w\',\'x\',\'y\',\'w\',\'x\',\'y\',\'w\'],\'b\':[\'m\',\'m\',\'m\',\'f\',\'f\',\'f\',\'m\',\'m\',\'m\',\'f\'