Adding the same list to each row in a pandas DataFrame in a new column

Question

In a pandas DataFrame, foo for example:

>>> foo
   col1  col2
0     1     0
1     2     0
2     3     1

If you want to add a new column, all with the same value you can do

>>> foo['col3'] = 1
>>> foo
   col1  col2  col3
0     1     0     1
1     2     0     1
2     3     1     1

If you want to add another new column, all with specific values you can do

>>> foo['col4'] = ['a', 'b', 'c']
>>> foo
   col1  col2  col3  col4
0     1     0     1     a
1     2     0     1     b
2     3     1     1     c

But what I want to do is add the same list to each row as new column. Something like

>>> myList = [True, False, False, True]
>>> foo['col5'] = {...something something something...}
>>> foo
   col1  col2  col3  col4                        col5
0     1     0     1     a  [True, False, False, True]
1     2     0     1     b  [True, False, False, True]
2     3     1     1     c  [True, False, False, True]

Using the previous method results in ValueError('Length of values does not match length of ' 'index'). So at the moment, my {...something something something...} line is foo['col5'] = [myList] * foo.shape[0]. But I'm wondering, is there a better way?

Do you really want references to the same list in each row? Because that's what your current solution does. — miradulo
– miradulo, Commented Apr 9, 2018 at 3:46
@cᴏʟᴅsᴘᴇᴇᴅ Not sure, what did I miss, is it performance or causes error? — niraj
– niraj, Commented Apr 9, 2018 at 4:01

cs95 · Accepted Answer · 2018-04-09 03:58:08Z

8

Use a list comprehension.

v = [True, False, False, True]
df['col5'] = [v for _ in range(len(df))]

df
   col1  col2                        col5
0     1     0  [True, False, False, True]
1     2     0  [True, False, False, True]
2     3     1  [True, False, False, True]

You might be tempted to use

df['col5'] = [True, False, False, True] * len(df)

However, each record actually references the same list. Try this -

df.loc[0, 'col5'][0] = False
df

   col1  col2                         col5
0     1     0  [False, False, False, True]
1     2     0  [False, False, False, True]
2     3     1  [False, False, False, True]

You'll see the change is reflected across all sublists.

answered Apr 9, 2018 at 3:58

cs95

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1 Comment

niraj Over a year ago

Great! I forgot about it :) +1

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