How to convert dataframe column into list of lists json format?

My dataframe looks like below and I want to use sales column and convert it into json files for each month as a list of lists.

for date in date_range:
        df.loc[df['dt'] == date]['sales'].to_json(f"{out_path}/sales_{date.replace('-', '_')}.json", orient='values',indent=2)

Using this, I am getting this format of json files:

[
156,
192
]

However, I want to have:

[
 [156],
 [192]
]

I have created this as a toy example. I want to implement this on a very large dataframe for several months data. Can anyone point me how to achieve this? Thank you.

You can try:

    df = df.groupby('dt',as_index=False).agg({'sales':list})
    df['sales'] = df['sales'].apply(lambda x: [[e] for e in x])
    df.apply(lambda row: pd.Series(row['sales']).to_json(
             f"{out_path}/sales_{row['dt'].replace('-','_')}.json",
             orient='values',indent=2), axis=1)