Inserting Lists into Pandas Cells

Problem

In Python, attempting to insert a list into a cell of a Pandas DataFrame can result in errors or unexpected results. For example, when trying to insert a list into cell 1B of a DataFrame df:

df = pd.DataFrame({'A': [12, 23], 'B': [np.nan, np.nan]})
abc = ['foo', 'bar']

The following attempts to insert the abc list into 1B, but they produce errors or incorrect insertion:

1. df.ix[1,'B'] = abc - Error: Must have equal len keys and value when setting with an iterable
2. df.ix[1,'B'] = [abc] - Inserts a list with one element: [['foo', 'bar']]
3. df.ix[1,'B'] = ', '.join(abc) - Inserts a string: "foo, bar"
4. df.ix[1,'B'] = [', '.join(abc)] - Inserts a list with one element: ['foo, bar']

Solution

To insert lists into cells of a DataFrame without errors, use the at method, which always refers to a single value:

df.at[1, 'B'] = ['foo', 'bar']

This will insert the abc list into 1B as expected:

    A  B
0  12  NaN
1  23  ['foo', 'bar']

Note that the DataFrame column must have dtype=object to allow list insertion. For example:

df['B'] = df['B'].astype('object')