python pandas在dataframe列中添加列表作为默认值

Question

I'd like to make a new column in dataframe df that will fill all rows with [np.nan]

    df['new'] = [np.nan]

I get

ValueError: Length of values does not match length of index

If I try

    test['new'] = np.nan
    test['new'] = test['new'].astype('object')
    test['new'] = [np.nan]

I get

ValueError: Length of values does not match length of index

I'd like to be sure that all rows are filled with a list containing nan

Answer 1

You are assign a object

df['New']=[[np.nan]]*len(df)
df
Out[250]: 
        Date  Value    New
0 2017-01-01      1  [nan]
1 2017-02-13      2  [nan]
2 2018-03-01      3  [nan]
3 2018-04-01      4  [nan]