python - Accessing every 1st element of Pandas DataFrame column containing lists

Question

I have a Pandas DataFrame with a column containing lists objects

      A
0   [1,2]
1   [3,4]
2   [8,9] 
3   [2,6]

How can I access the first element of each list and save it into a new column of the DataFrame? To get a result like this:

      A     new_col
0   [1,2]      1
1   [3,4]      3
2   [8,9]      8
3   [2,6]      2

I know this could be done via iterating over each row, but is there any "pythonic" way?

Answer 1

As always, remember that storing non-scalar objects in frames is generally disfavoured, and should really only be used as a temporary intermediate step.

That said, you can use the .str accessor even though it's not a column of strings:

>>> df = pd.DataFrame({"A": [[1,2],[3,4],[8,9],[2,6]]})
>>> df["new_col"] = df["A"].str[0]
>>> df
        A  new_col
0  [1, 2]        1
1  [3, 4]        3
2  [8, 9]        8
3  [2, 6]        2
>>> df["new_col"]
0    1
1    3
2    8
3    2
Name: new_col, dtype: int64