How to add multiple column to a dataframe

Question

I have a function which returns a list of lists and I'd like to add multiple columns to my dataframe based on the return value. Here is how the return value of my function looks like

[[1,2,3],[3,4,3],[1,6,7],[4,7,6]]

I would like to add three columns to my dataframe. I have the following code

 col_names = ['A','B','C']
 df[col_names] = func()

but it gives me an error. How can I add 3 new columns?

What is the error? What should the output columns look like? Which list elements go into which columns? — G. Anderson
– G. Anderson, Commented Feb 12, 2019 at 18:55

anky · Accepted Answer · 2019-02-12 18:58:33Z

2

you can pass the list directly:

pd.DataFrame([[1,2,3],[3,4,3],[1,6,7],[4,7,6]],columns=['A','B','C'])

   A  B  C
0  1  2  3
1  3  4  3
2  1  6  7
3  4  7  6

Or if you have defined the list as l = [[1,2,3],[3,4,3],[1,6,7],[4,7,6]], pass it to the Dataframe:

df = pd.Dataframe(l,columns=['A','B','C'])

answered Feb 12, 2019 at 18:58

anky

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3 Comments

HHH Over a year ago

this is different than what I want. I want to append a bunch of new columns to my existing dataframe

anky Over a year ago

can you show your function , a sample df and what you want to achieve?

G. Anderson Over a year ago

@H.Z. simply do this, then concat it onto your existing dataframe, or merge if you need to do it on key columns

gold_cy · Accepted Answer · 2019-02-12 19:00:57Z

0

Here is one way to do it:

df = pd.DataFrame({'foo': ['bar', 'buzz', 'fizz']})

def buzz():
    return [[1,2,3],[3,4,3],[1,6,7],[4,7,6]]


pd.concat([df, pd.DataFrame.from_records(buzz(), columns=col_names)], axis=1)

    foo  A  B  C
0   bar  1  2  3
1  buzz  3  4  3
2  fizz  1  6  7
3   NaN  4  7  6

answered Feb 12, 2019 at 19:00

gold_cy

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2 Comments

HHH Over a year ago

in my case df has 865 record and the new dataframe also has the same number of records, so I expect the resulting dataframe to have 865 records as well. but it has around 1600 records!

gold_cy Over a year ago

does the function that returns data return 865 lists with 3 items each?

Vaishali · Accepted Answer · 2019-02-12 19:09:42Z

0

Here is an example with a dummy function that returns a list of list. Since the function returns a list, you need to pass it to pd.DataFrame constructor before assigning it to the existing dataframe.

def fn(l1,l2,l3):
    return [l1,l2,l3]
df = pd.DataFrame({'col': ['a', 'b', 'c']})
col_names = ['A','B','C']
df[col_names] = pd.DataFrame(fn([1,2,3], [3,4,3], [4,7,6]))

You get

    col A   B   C
0   a   1   2   3
1   b   3   4   3
2   c   4   7   6

answered Feb 12, 2019 at 19:09

Vaishali

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2 Comments

HHH Over a year ago

for some reason, when I do that all the new added columns (A,B, C) would have only Null

Vaishali Over a year ago

What does your function look like? I created a dummy function that returns list of lists. You need to replace fn(...) with your function

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