Pandas：在多索引数据框中设置值

Question

I would like to set a value in a multi-indexed DataFrame. Here is an example:

import pandas as pd
df = pd.DataFrame([[1, 'a' , 1], [1, 'b', 2], [2, 'a', 3]], columns=['x', 'y', 'z'])
df.set_index(['x','y'], inplace=True)
df

gives:

        z
x   y   
1   a   1
    b   2
2   a   3

and then

df.loc[(2,'b')] = 4
df

gives

        z     b
x   y       
1   a   1   NaN
    b   2   NaN
2   a   3   4.0

instead of

        z
x   y       
1   a   1
    b   2
2   a   3
    b   4

So apparently, Pandas does not understand that the dataframe is multi-indexed and it interprets [(2,'b')] as row 2 and column 'b', as if I had written df.loc[2,'b'] .

How can I achieve what I want ?

Additionally, why is the integer 4 converted into a float ?

Thanks.

Answer 1

Also is necessary specify column z :

df.loc[(2,'b'), 'z'] = 4
print (df)
       z
x y     
1 a  1.0
  b  2.0
2 a  3.0
  b  4.0