如何获取唯一 pandas dataframe 列元素的列表？

Question

I am trying to get a list for each unique string in a pandas dataframe column:

import pandas as pd

catalog = {'code': ['A001', 'A001', 'A001', 'A002', 'A002'], 'title': ['director', 'president', 'vice president', 'sales director', 'sales vice president']}

catalog=pd.DataFrame(catalog)

## unique column values ##
codes = catalog['code'].unique()

for code in codes:
     titles = catalog[catalog == code]['title'].tolist()
     print(titles)

Which gives the next output:

[nan, nan, nan, nan, nan]
[nan, nan, nan, nan, nan]

Expected output could look like this:

['director', 'president', 'vice president']
['sales director', 'sales vice president']

What am I missing? Is there any other way to accomplish this task?

Answer 1

Try with

catalog.groupby('code')['title'].unique()
code
A001     [director, president, vice president]
A002    [sales director, sales vice president]
Name: title, dtype: object

Answer 2

Instead of iterating through the unique codes, it's easier to use a groupby:

catalog.groupby("code").title.apply(list)

code
A001    [director, president, vice president]
A002    [sales director, sales vice president]
Name: title, dtype: object

Answer 3

Your code has an issue where you compare the full dataframe when assigning the title variable, instead of comparing with a column:

for code in codes:
    titles = catalog[catalog['code'] == code]['title'].tolist()
    print(titles)

Or:

for code in codes:
    titles = catalog.loc[catalog['code'] == code,'title'].tolist()
    print(titles)

---

['director', 'president', 'vice president']
['sales director', 'sales vice president']