python pandas add list in dataframe column as default value

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

Why would you want [np.nan] instead of np.nan? That's inefficient and highly inadvisable. Your series will become object type (a sequence of pointers) rather than float. — jpp
– jpp, Commented Aug 6, 2018 at 21:45
Because it will be loaded into a Postgresql table that has a column of array type: bigint[]. It has to match data type — Willem
– Willem, Commented Aug 6, 2018 at 21:47

BENY · Accepted Answer · 2018-08-06 21:46:41Z

5

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]

answered Aug 6, 2018 at 21:46

BENY

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