Insert a new column in existing DataFrame

A pandas dataframe is implemented as an ordered dict of columns. In this article, you will get to know how to add a new column to an existing data frame. So first let's create a data frame with values.

 
import pandas as pd
import numpy as np
df = pd.DataFrame()
df['Name'] = ['John', 'Doe', 'Bill','Jim','Harry','Ben']
df['TotalMarks'] = [82, 38, 63,22,55,40]
df['Grade'] = ['A', 'E', 'B','E','C','D']
df['Promoted'] = [True, False,True,False,True,True]
df

 
    Name  TotalMarks Grade  Promoted
0   John          82     A      True
1    Doe          38     E     False
2   Bill          63     B      True
3    Jim          22     E     False
4  Harry          55     C      True
5    Ben          40     D      True

Using [] accessor

 
df['Age'] = [12, 12, 13, 12, 13, 12] #Adding column 'Age'
df

 
    Name  TotalMarks Grade  Promoted  Age
0   John          82     A      True   12
1    Doe          38     E     False   12
2   Bill          63     B      True   13
3    Jim          22     E     False   12
4  Harry          55     C      True   13
5    Ben          40     D      True   12

Here, when you use the [] to pandas DataFrame is quietly performing an outer join or outer merge using the index of the left hand dataframe and the index of the right hand series. df['column'] = series.

Using insert() method

 
df.insert(loc, column, value)

You can inset a column to pandas DataFrmae at a specified index using insert() method.

 
df.insert(1,"Age",[12, 12, 13, 12, 13, 12])
df

 
    Name  Age  TotalMarks Grade  Promoted
0   John   12          82     A      True
1    Doe   12          38     E     False
2   Bill   13          63     B      True
3    Jim   12          22     E     False
4  Harry   13          55     C      True
5    Ben   12          40     D      True

Here you can see the column 'Age' inserted at the index position of 1 using insert() method.

Using assign() method

Pandas DataFrame has an assign() function that allows you to create a new dataframe with new column added to the old dataframe.

 
new_df = df.assign(Age = [12, 12, 13, 12, 13, 12])
new_df

 
    Name  TotalMarks Grade  Promoted  Age
0   John          82     A      True   12
1    Doe          38     E     False   12
2   Bill          63     B      True   13
3    Jim          22     E     False   12
4  Harry          55     C      True   13
5    Ben          40     D      True   12

Next : Change the order of columns in Pandas dataframe