python - Replace NaN with empty list in a pandas dataframe -

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i'm trying replace nan values in data empty list []. list represented str , doesn't allow me apply len() function. there anyway replace nan value actual empty list in pandas?

in [28]: d = pd.dataframe({'x' : [[1,2,3], [1,2], np.nan, np.nan], 'y' : [1,2,3,4]})  in [29]: d out[29]:            x  y 0  [1, 2, 3]  1 1     [1, 2]  2 2        nan  3 3        nan  4  in [32]: d.x.replace(np.nan, '[]', inplace=true)  in [33]: d out[33]:            x  y 0  [1, 2, 3]  1 1     [1, 2]  2 2         []  3 3         []  4  in [34]: d.x.apply(len) out[34]: 0    3 1    2 2    2 3    2 name: x, dtype: int64

this works using isnull , loc mask series:

in [90]: d.loc[d.isnull()] = d.loc[d.isnull()].apply(lambda x: []) d  out[90]: 0    [1, 2, 3] 1       [1, 2] 2           [] 3           [] dtype: object  in [91]: d.apply(len)  out[91]: 0    3 1    2 2    0 3    0 dtype: int64

you have using apply in order list object not interpreted array assign df try align shape original series

edit

using updated sample following works:

in [100]: d.loc[d['x'].isnull(),['x']] = d.loc[d['x'].isnull(),'x'].apply(lambda x: []) d  out[100]:            x  y 0  [1, 2, 3]  1 1     [1, 2]  2 2         []  3 3         []  4  in [102]:     d['x'].apply(len)  out[102]: 0    3 1    2 2    0 3    0 name: x, dtype: int64

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