Python - Working with CSV Files

Working with CSV Files in Python

CSV (Comma-Separated Values) is one of the popular and accessible file formats for storing and exchanging tabular data.

CSV (Comma Separated Values) is a simple file format for storing tabular data, such as a spreadsheet or database. A CSV file stores tabular data (numbers and text) in plain text. Each line in the file is a data record. Each record has one or more fields, separated by commas. The comma used as the field separator is the source of the name of this file format. Python has a built-in module called CSV that works with CSV files.

Reading a CSV file

Reading from a CSV file is done using the reader function (i.e. csv.readers()) which returns each row as a list. The CSV file is opened as a text file with the help of Python’s built-in open() function, which returns a file object.

Example

In this example, we first open the CSV file in READ mode, the file object is converted to csv.reader object, and further operation takes place.

import csv
# Open the CSV file and read it
with open('data.csv', 'r') as file:
    csv_reader = csv.reader(file)
    # Iterate over each row in the CSV file
    for row in csv_reader:
        print(row) # Each row is a list of values

CSV file with Header

If the CSV file contains headers, you can use csv.DictReader(), which reads each row as a dictionary where keys are the column headers.

Suppose we have a details.csv file that contains the following headers name, age, and city.

name, age, city
John, 30, Delhi
Aman, 26, Pune
Bob, 35, Hyderabad

Example

In this example, csv.DictReader() reads each row of the CSV file as a dictionary where the keys are the column headers, and the values are the corresponding values in each row.

import csv
# Reading a CSV file with headers
with open('data_with_headers.csv', 'r') as file:
    csv_reader = csv.DictReader(file)
    # Iterate over each row, now as a dictionary
    for row in csv_reader:
        print(row) # Each row is a dictionary with column names as keys

Writing to CSV File

To write data to a CSV file first, we open the CSV file in the write mode. We then use csv.writer() or csv.DictWriter() function for writing dictionaries. The DictWriter() function is used to write CSV with headers.

Example

Following is a code example of writing a CSV file.

import csv
# Data to write (list of lists)
data = [
    ["Name", "Age", "City"], ["John", 30, "Delhi"], ["Aman", 26, "Pune"], ["Bob", 35, "Hyderabad"]
]

# Write to a CSV file
with open('output.csv', 'w', newline='') as file:
    csv_writer = csv.writer(file)
    # Writing each row of data
    csv_writer.writerows(data)

Example

Let’s write a CSV file with headers using the csv.DictWriter() function:

import csv

# Data to write (list of lists)
data = [
    ["Name", "Age", "City"],["John", 30, "Delhi"], ["Aman", 26, "Pune"], ["Bob", 35, "Hyderabad"]
]
# Write to a CSV file with headers
with open('output_with_headers.csv', 'w', newline='') as file:
    # Specify the fieldnames (column names)
    fieldnames = ['Name', 'Age', 'City']
    csv_writer = csv.DictWriter(file, fieldnames=fieldnames)
    # Write the header (column names)
    csv_writer.writeheader()
    # Write the rows (dictionary values)
    csv_writer.writerows(data)

Modify CSV Data.

Modifying CSV data in Python specifies that we can read the data, modify it in Python, and then write it back to the CSV file.

Example

This example reads the CSV file, modifies Bob's age, and writes the updated data back to the file.

import csv
# Read data from the existing CSV file
with open('data.csv', 'r') as file:
    csv_reader = csv.reader(file)
    data = list(csv_reader)
# Modify the data (e.g., update age)
for row in data:
    # If the name is 'Bob'
    if row[0] == "Bob":
        # Update age to 25
        row[1] = 25

# Write the modified data back to the CSV file
with open('data.csv', 'w', newline='') as file:
    csv_writer = csv.writer(file)
    csv_writer.writerows(data)

In summary, The csv.reader() function reads a CSV file line by line, treating each line as a list of values, while csv.DictReader() reads each line as a dictionary, using column headers as keys On the other hand, csv .writer () allows writing to a CSV file where each line is written as a list, and csv.DictWriter() writes the data into a dictionary with the specified column title, ensuring that each field is formatted properly.