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Working with CSV files in Python

December 1, 2021


CSV (Comma Separated Values) is a basic file format for tabular data. Most programs create CSV files. They allow you to handle data from spreadsheets and databases. For example, tabular data can be exported to a CSV file and imported into a spreadsheet to analyze, graph, or publish data mining results.

These files have extensive support from several programming languages. CSV files may be directly accessed and manipulated by text file input and string manipulation language.


  • Have Python IDE installed.
  • Have some background information in the python programming language.

Table of contents

Write a CSV file

Python provides a built-in CSV module. This module contains two CSV writing classes:

  • Using CSV.writer class
  • Using CSV.DictWriter class

Using csv.writer class

csv.writer writes data to a CSV file. By default, user data is transformed into a delimited string. The CSV file object will not be recognized if quoted fields do not include \n.

To write to a CSV file, use the writer class. The writer class is a subclass of the csv.Dialect class. The csv.Dialect class provides a set of parameters that can be used to customize the CSV file.

  • writerow(): This technique writes one row. This technique may create a field row.
  • writerows(): This technique writes numerous rows at once. This is for row lists.

To illustrate the use of the writerow() class, let’s create a sample CSV file, student_file.csv, as shown below.

import csv
with open('student_file.csv', 'w', newline='') as file:
    writer = csv.writer(file)
    writer.writerow(["2021", "Student details"])
    writer.writerow([1, "carteblanche kin", "computer science"])
    writer.writerow([2, "Marion koech", "data science"])

The code above will output the following file-

2021, Student details
1, carteblanche kin, computer science
2, Marion koech, data science

To illustrate the use of the writerows() class, let’s create a sample CSV file, student_file.csv as shown below:

import csv
csv_rowlist = [["2021", "Student details"], [1, "carteblanche kin", "computer science"],
               [2, "Marion koech", "data science"]]
with open('protagonist.csv', 'w') as file:
    writer = csv.writer(file)

The program’s output is the same as in the writerow() example above.

Using csv.DictWriter class

This class builds a column-to-dictionary writer object. This class supports two CSV writing methods. They are:

  • writeheader(): A simple CSV file with field names you choose publishes the first row.

  • writerows(): writerows function writes all rows with just values.

Read a CSV file

The CSV module or pandas library can read CSV files. To be able to read CSV files, one can use one of the below methods:

Using csv.reader():

The CSV file is first opened using the open() function in r mode (specify read mode when opening a file) and then read using the reader() method of the CSV module.

The with keyword simplifies exception handling and immediately ends the CSV file.

Using CSV.DictReader() class:

The CSV file is opened using open(), then read using the CSV module’s DictReader class, which works like a reader but converts CSV data to a dictionary. The file’s first line contains dictionary keys.

Using pandas.read_csv() method:

Using pandas library methods to read a CSV file is straightforward. Consider the fonteds.csv CSV file. This is the file used to illustrate one of the methods.

Giant file

import pandas
csvFile = pandas.read_csv('fonteds.csv')

The output

SCHOOL            CEO         YEAR

0 KU    JACOB MUDAVAI         2010

1 MUST   GARETH JASON         2015

2 MMU   DICKSON NJOGU         2013

From the code above, import pandas is used to import the pandas module, csvFile = pandas.read_csv('fonteds.csv') is used to read the fonteds.csv file and print(csvFile) is used to output the read csv file.

Reading from a specific row

Here we will create a CSV file with multiple rows and columns to illustrate how to read from a specific row.

Create a CSV file by entering the below data in your notepad and saving it as student-data.csv. The file will be used to show how to manipulate the CSV files.

RegNo   Name        Course  year-of-study   Department
001     James       BCS     2.1             Computing
002     John        BFF     1.2             IT
003     Christine   BSS     4.2             SPAS
004     Lilian      BCOM    3.1             Business
005     Beth        BIT     2.2             IT

To read a specific row in the CSV file, we use the read_cv function from Panda’s library. The example below illustrates how to read from a specific row.

# Import pandas 
import pandas as pd
# Specify the file location of our CSV file
data = pd.read_csv('File-location/student-data.csv')
# Extract top four data of the specified rows
print (data[0:4]['year-of-study'])

The above code will output the following information:

0   2.1
1   1.2
2   4.2
3   3.1
Name: Name, dtype: float64

Read a precise column

The pandas library’s read_csv method may additionally read specified columns. This is done using the .loc() multi-axes indexing function. First, let’s look into an example program. This example will show the Name and Course columns for all rows.

We will use the student-data.csv file from the previous example.

# Import the pandas module
import pandas as pd
# Specify the file location of our CSV file
data = pd.read_csv('File-location/student-data.csv')
print (data.loc[:,['Name','Course']])

The output

    Name        Course  
0   James       BCS                  
1   John        BFF                
2   Christine   BSS                
3   Lilian      BCOM               
4   Beth        BIT    

Manipulate CSV files

Since you can’t edit a CSV file while reading from it, you must create a new one and write to it.

We will use the student-data.csv file from the previous example.

From the student-data.csv file above, the data is written using uppercase. To demonstrate how to edit and save CSV files, we will change the uppercase letters in our file to lower case letters.

with open('student-data.csv','r') as f:
    with open('lowwer-case.csv','w') as ff:

The above code creates a new CSV file with all letters in it changed to lower case.

Working with large CSV files in Python

When dealing with CSV data, usually read it in using pandas before munging and analyzing it. However, reading huge files straight into pandas may be difficult (or impossible) on a consumer machine due to memory constraints.

While it is simple to load data from CSV files into a database, there may be situations where you don’t have access to or don’t want to set up a database server. However, if you need to look at data in these big files for a short time, here is one method to accomplish it using Python and pandas.

Here is a method for handling large.csv files. The dataset we will be using is gender_voice_dataset.

Using pandas.read_csv

Large files may be handled by reading them in manageable size pieces, processing them before reading the next part. The chunk size option determines the number of lines. This method returns an iterator. For processing, a section of the file is read at a time.

To read a dataset without chunks, use the code below:

import pandas as pd # import pandas module
import numpy as np  # Import numpy module
import time         # import time module

s_time = time.time() # This initilizes time module
df = pd.read_csv("voice.csv") # This captures the time taken to read data from our file 
e_time = time.time()

print("Read without chunks: ", (e_time-s_time), "seconds") # print command is used to output the line specified while e_time-s_time outputs the time in seconds

df.sample(10) # This specifies the time taken, 10 seconds


Convert multiple JSON files to CSV files

A JSON file contains basic data structures and objects in JavaScript Object Notation (JSON). The most common use case is sending data between an internet app and a server.

A CSV file is created by concatenating, merging or joining several JSON files (at least one column must be the same in each file) and saving the result as a flattened data frame. The following sample will help you understand the task’s whole procedure:

Example program:

We will input two JSON files and output a CSV. The used JSON files are:

first.json file

    "NO":{ // Declaring the regestration number of the student
    "Name":{// Declaring the name of the student
    "Marks":{// Declaring the marks of the student
    "Grade":{// Declaring the grade of the student

second.json file

     "NO":{ // Declaring the regestration number of the student
    "Name":{ // Declaring the name of the student
    "Marks":{ // Declaring the marks of the student
    "Grade":{ //  Declaring the grade of the student

Follow the steps below to be able to convert

  • Load JSON files with a pandas data frame.
  • Merge the data frames.
  • Create a CSV file from the concatenated data.

The result is shown in the code.

import pandas as pd
df1 = pd.read_json('first.json')#
df2 = pd.read_json('second.json')
df = pd.concat([df1,df2])
result = pd.read_csv("CSV.csv")

The output:


Creating a data frame using CSV files

As with an excel file, CSV files include comma-separated values. Pandas is the core Python data science module. When evaluating data, we often deal with large datasets in CSV format. Creating a pandas data frame from CSV files is easy.

Download the example CSV file here.

A dataframe can be created by:

  • read_csv() method
  • read_table() method
  • csv module

Let’s look at an example using the read_table() method.

import pandas as pd
df = pd.read_table("dataframe.csv", delimiter =", ")



Thank you for reading till the end. This tutorial taught us how to work with CSV files in Python. We learned how to write and read CSV files, work with large CSV files, convert multiple JSON files to CSV files, and finally, create a data frame using CSV files.

Happy coding!

Peer Review Contributions by: Odhiambo Paul