Slice multi-index pandas dataframe by date

Question

Say I have the following multi-index dataframe:

arrays = [np.array(['bar', 'bar', 'bar', 'bar', 'foo', 'foo', 'foo', 'foo']),
          pd.to_datetime(['2020-01-01', '2020-01-02', '2020-01-03', '2020-01-04', '2020-01-01', '2020-01-02', '2020-01-03', '2020-01-04'])]
df = pd.DataFrame(np.zeros((8, 4)), index=arrays)

                 0    1    2    3
bar 2020-01-01  0.0  0.0  0.0  0.0
    2020-01-02  0.0  0.0  0.0  0.0
    2020-01-03  0.0  0.0  0.0  0.0
    2020-01-04  0.0  0.0  0.0  0.0
foo 2020-01-01  0.0  0.0  0.0  0.0
    2020-01-02  0.0  0.0  0.0  0.0
    2020-01-03  0.0  0.0  0.0  0.0
    2020-01-04  0.0  0.0  0.0  0.0

How do I select only the part of this dataframe where the first index level = 'bar' , and date > 2020.01.02 , such that I can add 1 to this part?

To be clearer, the expected output would be:

                 0    1    2    3
bar 2020-01-01  0.0  0.0  0.0  0.0
    2020-01-02  0.0  0.0  0.0  0.0
    2020-01-03  1.0  1.0  1.0  1.0
    2020-01-04  1.0  1.0  1.0  1.0
foo 2020-01-01  0.0  0.0  0.0  0.0
    2020-01-02  0.0  0.0  0.0  0.0
    2020-01-03  0.0  0.0  0.0  0.0
    2020-01-04  0.0  0.0  0.0  0.0

I managed slicing it according to the first index:

df.loc['bar']

But then I am not able to apply the condition on the date.

Answer 1

Here is possible compare each level and then set 1 , there is : for all columns in DataFrame.loc :

m1 = df.index.get_level_values(0) =='bar' 
m2 = df.index.get_level_values(1) > '2020-01-02'

df.loc[m1 & m2, :] = 1
print (df)

                  0    1    2    3
bar 2020-01-01  0.0  0.0  0.0  0.0
    2020-01-02  0.0  0.0  0.0  0.0
    2020-01-03  1.0  1.0  1.0  1.0
    2020-01-04  1.0  1.0  1.0  1.0
foo 2020-01-01  0.0  0.0  0.0  0.0
    2020-01-02  0.0  0.0  0.0  0.0
    2020-01-03  0.0  0.0  0.0  0.0
    2020-01-04  0.0  0.0  0.0  0.0

Answer 2

#give ur index names :
df.index = df.index.set_names(["names","dates"])

#get the indices that match ur condition
index = df.query('names=="bar" and dates>"2020-01-02"').index

#assign 1 to the relevant points
#IndexSlice makes slicing multiindexes easier ... here though, it might be seen as overkill
idx = pd.IndexSlice
df.loc[idx[index],:] = 1


                 0  1   2   3
names   dates               
bar 2020-01-01  0.0 0.0 0.0 0.0
    2020-01-02  0.0 0.0 0.0 0.0
    2020-01-03  1.0 1.0 1.0 1.0
    2020-01-04  1.0 1.0 1.0 1.0
foo 2020-01-01  0.0 0.0 0.0 0.0
    2020-01-02  0.0 0.0 0.0 0.0
    2020-01-03  0.0 0.0 0.0 0.0
    2020-01-04  0.0 0.0 0.0 0.0