Using functions on multi-indexed dataframe

I think this is a basic question but I have not been able to find a usable solution yet. I have some data that is multi-index by month and year as in this attached figure

I want to do some transformations on some columns for each year and month. Let’s say I have some function:

def foo(series):
   return series/series.max()

So I would like to apply this function to some column (say, vol) for every month of every year, rather than for all the data at once. Can someone help?

>Solution :

You can use GroupBy.transform, because function return Series:

df['new'] = df.groupby(['year','month'])['vol'].transform(foo)

Alternative here is:

df['new'] = df['vol'].div(df.groupby(['year','month'])['vol'].transform('max'))