Pandas column containing lists - iterate through each list problem

Question

Let's say I have a dataframe such as

df = pd.DataFrame({'a': [1,2,3], 'b': [['this', 'is', 'a', 'sentence'],['we', 'like', 'pizza'],['hello', 'world']]})

And I want to iterate through the lists in column b and do something like capitalize every letter. I can do something like

for row in df['b']:
    row = [i.upper() for i in row]
    print(row)
# ['THIS', 'IS', 'A', 'SENTENCE']
# ['WE', 'LIKE', 'PIZZA']
# ['HELLO', 'WORLD']

but it doesn't replace the lists in that column. I thought I was reassigning the row values in the for loop but clearly I am not when I print the dataframe after this:

print(df)
#    a                        b
# 0  1  [this, is, a, sentence]
# 1  2        [we, like, pizza]
# 2  3           [hello, world]

What is the proper way to do this? Thanks!

By assigning back to the iterable you're not updating the dataframe, see [here](https://stackoverflow.com/a/48951427/9698684). Do something like `df['b'] = [[s.upper() for s in l] for l in df.b]` — yatu, Apr 04 '20 at 22:16

score 1 · Accepted Answer · answered Apr 04 '20 at 22:28

1

you can use pandas.DataFrame.apply:

 df['b'] =  df['b'].apply(lambda x: list(map(str.upper, x)))
 df

output:

answered Apr 04 '20 at 22:28

kederrac

16,819
6
32
55

1

I was looking for apply! Thank you! I knew I was missing something. – Azathoth Apr 05 '20 at 21:57

score 1 · Answer 2 · answered Apr 04 '20 at 22:30

1

I don't know if it is a proper way of doing this, but df.b = df.b.apply(lambda x: [i.upper() for i in x]) should work.

answered Apr 04 '20 at 22:30

andrew_the_coder

63
8

Pandas column containing lists - iterate through each list problem

2 Answers2