add multiple columns to pandas dataframe

Question

The following Python code works fine

import pandas as pd

df = pd.DataFrame(data = {'a': [1, 2, 3], 'b': [4, 5, 6]})

def myfun(a, b):
  return [a + b, a - b]

df[['x', 'y']] = df.apply(
    lambda row: myfun(row.a, row.b), axis=1)

The resulting pandas dataframe looks like:

print(df)

   a  b  x  y
0  1  4  5 -3
1  2  5  7 -3
2  3  6  9 -3

However, if I try to add two more columns,

df[['xx','yy']] = df.apply(lambda row: myfun(row.a, row.b), axis=1)

I get the error message,

KeyError: "['xx' 'yy'] not in index"

How come? And what is the correct way to do this?

Many thanks!

//A

jezrael · Accepted Answer · 2018-05-18 11:46:28Z

2

Need convert return output to Series:

def myfun(a, b):
  return pd.Series([a + b, a - b])

df[['x', 'y']] = df.apply(
    lambda row: myfun(row.a, row.b), axis=1)
print (df)
   a  b  x  y
0  1  4  5 -3
1  2  5  7 -3
2  3  6  9 -3

answered May 18, 2018 at 11:46

jezrael

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Comments

jpp · Accepted Answer · 2018-05-18 11:50:02Z

0

You can assign to a tuple of series:

df['xx'], df['yy'] = df.apply(lambda row: myfun(row.a, row.b), axis=1)

But this is inefficient versus direct assignment: don't use pd.DataFrame.apply unless you absolutely must, it's just a fancy loop.

df['xx'] = df['a'] + df['b']
df['yy'] = df['a'] - df['b']

answered May 18, 2018 at 11:50

jpp

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