add column of length longer than dataframe pandas

June 21, 2024

I have:

df=pd.DataFrame({'col1':[1,2,3,4]})

I want to add a column that is based on a list, but the list is longer than the dataframe

new_col=range(9)
df['new_col']=new_col

I want the original columns to be filled with '', and I get:

pd.DataFrame({'col1': {0: 1, 1: 2, 2: 3, 3: 4, 4: '', 5: '', 6: '', 7: '', 8: ''}, 'new_col': {0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8}})

>Solution :

Assuming a range index, you first need to reindex your DataFrame, using fill_value='' if you want empty strings as filler:

# if longer, reindex to add new rows
df = df.reindex(range(max(len(df), len(new_col))), fill_value='')

# add the new column
df['new_col'] = new_col

Output:

  col1  new_col
0    1        0
1    2        1
2    3        2
3    4        3
4             4
5             5
6             6
7             7
8             8

Alternatively, if the new column is not pre-existing, concat:

df = pd.DataFrame({'col1':[1,2,3,4]})

new_col=range(9)
df = (pd.concat([df.astype(str), pd.DataFrame({'new_col': new_col})], axis=1)
        .fillna('')
      )