Python pandas pivot_table margins keyError

Question

Consider the following dataframe:

test = pd.DataFrame({'A': [datetime.datetime.now(), datetime.datetime.now()], 'B': [1, 2]})

If I use pivot_table like below then everything is fine:

test.pivot_table(index = 'A', aggfunc = {'B': 'mean'}, margins = True)

However, if I do the following, I can't set margins = True (throws the error KeyError: 'A' ):

test.pivot_table(index = test['A'], aggfunc = {'B': 'mean'}, margins = True)

I am really confused. Let's say I need do something like below AND need to set margin = True . Is that impossible?

test.pivot_table(index = test['A'].dt.year, aggfunc = {'B': 'mean'}, margins = True)

Answer 1

Try:

test['Ax']=test['A'].dt.year

test.pivot_table(index = 'Ax' , aggfunc = 'mean', values='B', margins = True)

Outputs:

        B
Ax
2020  1.5
All   1.5

Explanation : in case if you don't pass values it will default to df.columns (all columns of dataframe, that you're pivoting over). https://github.com/pandas-dev/pandas/blob/v0.25.3/pandas/core/reshape/pivot.py#L87

So technically by passing no values you were passing ALL columns into values , yet at the same time providing function for just one, so this is where this KeyError was coming from.

Source here is oddly off with the documentation: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.pivot_table.html