Python Pandas – Propagate non-null values backward

Use the “method” parameter of the fillna() method. For backward fill, use the value ‘bfill’ as shown below −

fillna(method='bfill')

Let’s say the following is our CSV file opened in Microsoft Excel with some NaN values −

At first, import the required library −

import pandas as pd

Load data from a CSV file into a Pandas DataFrame −

dataFrame = pd.read_csv("C:\\Users\\amit_\\Desktop\\SalesData.csv")

Example

Following is the code −

import pandas as pd

# Load data from a CSV file into a Pandas DataFrame
dataFrame = pd.read_csv("C:\\Users\\amit_\\Desktop\\SalesData.csv")
print("DataFrame...\n",dataFrame)

# propagate non null values backward
res = dataFrame.fillna(method='bfill')
print("\nDataFrame after backward fill...\n",res)

Output

This will produce the following output −

DataFrame...
       Car   Reg_Price   Units
0      BMW        2500   100.0
1    Lexus        3500     NaN
2     Audi        2500   120.0
3   Jaguar        2000     NaN
4  Mustang        2500   110.0

DataFrame after backward fill...
       Car   Reg_Price   Units
0      BMW        2500   100.0
1    Lexus        3500   120.0
2     Audi        2500   120.0
3   Jaguar        2000   110.0
4  Mustang        2500   110.0