新列，每个元素作为列表熊猫

Question

I have some data frames where I want to add new columns, and in this new column each element should be a string for example of two rows,

df
           index colA colB 
           0     a    a1
           1     b    b1

Now I can add new column as

df['colC']=5
           index colA colB colC
           0     a    a1   5
           1     b    b1   5

now I want to add a third column with each element as list

           index colA colB colC
           0     a    a1   ['m','n','p']
           1     b    b1   ['m','n','p']

but,

df['colC']=['m','n','p'] is giving error
ValueError: Length of values does not match length of index

which is obvious.

I know in our example I can do

df['colC']=[['m','n','p'],['m','n','p']]

But I want to set each element to same list of strings, when I do not know number of rows. Can anyone suggest something easy to achieve this.

Answer 1

Adding object ( list ) to cell is tricky

df['colC']=[['m','n','p']]*len(df)

Or

df['colC'] = [list('mnp') for _ in range(len(df))]

df returns:

   index colA colB       colC
0      0    a   a1  [m, n, p]
1      1    b   b1  [m, n, p]