Reading and Writing CSV Files with csv Module in Python

CSV (Comma Separated Values) files are widely used for storing tabular data. Python provides the csv module, which makes it easy to read from and write to CSV files. In this article, we will explore how to read and write CSV files using the csv module with examples.

1. Importing the csv Module

Before working with CSV files, we need to import Python's built-in csv module. The csv module provides functionality for reading and writing CSV files, and it works seamlessly with Python's file handling system.

    import csv
        

2. Reading CSV Files

The csv.reader() function allows you to read CSV files. It returns an iterator, which can be used to iterate over each row in the file. Each row is returned as a list of values.

Example: Reading a CSV File

    import csv

    # Open the CSV file in read mode
    with open('data.csv', 'r') as file:
        csv_reader = csv.reader(file)
        
        # Iterate over each row and print it
        for row in csv_reader:
            print(row)
        

In this example, the CSV file data.csv is opened, and the rows are read using csv.reader(). Each row is printed as a list of values.

3. Reading CSV Files with a Header

If your CSV file contains a header (i.e., the first row contains column names), you can use the csv.DictReader() function. This will allow you to access the data by column name instead of by index.

Example: Using csv.DictReader() to Read a CSV File

    import csv

    # Open the CSV file in read mode
    with open('data.csv', 'r') as file:
        csv_reader = csv.DictReader(file)
        
        # Iterate over each row and print the values using column names
        for row in csv_reader:
            print(row['Name'], row['Age'], row['City'])
        

In this example, csv.DictReader() is used to read the CSV file with headers. The values are accessed by column name (e.g., row['Name'], row['Age'], and row['City']).

4. Writing to CSV Files

To write data to a CSV file, you can use the csv.writer() function. This function allows you to write rows to a CSV file in the form of lists or tuples.

Example: Writing to a CSV File

    import csv

    # Open the CSV file in write mode
    with open('output.csv', 'w', newline='') as file:
        csv_writer = csv.writer(file)
        
        # Write the header (optional)
        csv_writer.writerow(['Name', 'Age', 'City'])
        
        # Write some rows of data
        csv_writer.writerow(['John Doe', 30, 'New York'])
        csv_writer.writerow(['Jane Smith', 25, 'Los Angeles'])
        

In this example, the file output.csv is created and opened in write mode. The header is written first using csv_writer.writerow(), followed by two rows of data. The newline='' argument is used to prevent blank lines between rows when writing to a CSV file.

5. Writing to CSV Files with a Dictionary

If you have data stored as dictionaries, you can use the csv.DictWriter() function to write rows to a CSV file. This allows you to write data with field names directly from a dictionary.

Example: Using csv.DictWriter() to Write to a CSV File

    import csv

    # Open the CSV file in write mode
    with open('output.csv', 'w', newline='') as file:
        fieldnames = ['Name', 'Age', 'City']
        csv_writer = csv.DictWriter(file, fieldnames=fieldnames)
        
        # Write the header
        csv_writer.writeheader()
        
        # Write some rows of data
        csv_writer.writerow({'Name': 'John Doe', 'Age': 30, 'City': 'New York'})
        csv_writer.writerow({'Name': 'Jane Smith', 'Age': 25, 'City': 'Los Angeles'})
        

In this example, csv.DictWriter() is used to write a dictionary to the CSV file. The writeheader() method writes the field names (header), and writerow() writes the rows of data using dictionaries.

6. Conclusion

Python's csv module provides a powerful and easy way to read from and write to CSV files. The csv.reader() and csv.DictReader() functions allow you to read CSV files either as lists or dictionaries. The csv.writer() and csv.DictWriter() functions allow you to write data to CSV files. By mastering these functions, you can easily handle CSV files in your Python programs.