python - Add column of empty lists to DataFrame

Question

Similar to this question How to add an empty column to a dataframe?, I am interested in knowing the best way to add a column of empty lists to a DataFrame.

What I am trying to do is basically initialize a column and as I iterate over the rows to process some of them, then add a filled list in this new column to replace the initialized value.

For example, if below is my initial DataFrame:

df = pd.DataFrame(d = {'a': [1,2,3], 'b': [5,6,7]}) # Sample DataFrame

>>> df
   a  b
0  1  5
1  2  6
2  3  7

Then I want to ultimately end up with something like this, where each row has been processed separately (sample results shown):

>>> df
   a  b          c
0  1  5     [5, 6]
1  2  6     [9, 0]
2  3  7  [1, 2, 3]

Of course, if I try to initialize like df['e'] = [] as I would with any other constant, it thinks I am trying to add a sequence of items with length 0, and hence fails.

If I try initializing a new column as None or NaN, I run in to the following issues when trying to assign a list to a location.

df['d'] = None

>>> df
   a  b     d
0  1  5  None
1  2  6  None
2  3  7  None

Issue 1 (it would be perfect if I can get this approach to work! Maybe something trivial I am missing):

>>> df.loc[0,'d'] = [1,3]

...
ValueError: Must have equal len keys and value when setting with an iterable

Issue 2 (this one works, but not without a warning because it is not guaranteed to work as intended):

>>> df['d'][0] = [1,3]

C:Python27Scriptsipython:1: SettingWithCopyWarning:
A value is trying to be set on a copy of a slice from a DataFrame

Hence I resort to initializing with empty lists and extending them as needed. There are a couple of methods I can think of to initialize this way, but is there a more straightforward way?

Method 1:

df['empty_lists1'] = [list() for x in range(len(df.index))]

>>> df
   a  b   empty_lists1
0  1  5             []
1  2  6             []
2  3  7             []

Method 2:

 df['empty_lists2'] = df.apply(lambda x: [], axis=1)

>>> df
   a  b   empty_lists1   empty_lists2
0  1  5             []             []
1  2  6             []             []
2  3  7             []             []

Summary of questions:

Is there any minor syntax change that can be addressed in Issue 1 that can allow a list to be assigned to a None/NaN initialized field?

If not, then what is the best way to initialize a new column with empty lists?

Answer 1

One more way is to use np.empty:

df['empty_list'] = np.empty((len(df), 0)).tolist()

---

You could also knock off .index in your "Method 1" when trying to find len of df.

df['empty_list'] = [[] for _ in range(len(df))]

---

Turns out, np.empty is faster...

In [1]: import pandas as pd

In [2]: df = pd.DataFrame(pd.np.random.rand(1000000, 5))

In [3]: timeit df['empty1'] = pd.np.empty((len(df), 0)).tolist()
10 loops, best of 3: 127 ms per loop

In [4]: timeit df['empty2'] = [[] for _ in range(len(df))]
10 loops, best of 3: 193 ms per loop

In [5]: timeit df['empty3'] = df.apply(lambda x: [], axis=1)
1 loops, best of 3: 5.89 s per loop