8.3 Writing to CSV Files in Python

CSV (Comma-Separated Values) files are a ubiquitous format for storing structured tabular data, widely used for data exchange and in spreadsheet applications. Python's built-in csv module offers a straightforward and efficient way to write data to CSV files.

This guide will walk you through the essential steps and common techniques for writing data to CSV files using Python.

1. Importing the csv Module

Before you can interact with CSV files, you need to import the csv module:

import csv

2. Using csv.writer() to Write Data

To write data to a CSV file, you first create a writer object using the csv.writer() function. This object provides methods to write rows to the file.

csv.writer() Syntax and Parameters

csv.writer(file_object, delimiter=',', quotechar='"', quoting=csv.QUOTE_MINIMAL)

• file_object: The file object that has been opened in write mode ('w').
• delimiter: The character used to separate values in each row. The default is a comma (,).
• quotechar: The character used to quote fields that contain special characters (like the delimiter itself). The default is a double quote (").
• quoting: Controls when to quote fields. Common options include:
  • csv.QUOTE_MINIMAL (default): Only quote fields containing special characters (delimiter, quotechar, or newline).
  • csv.QUOTE_ALL: Quote all fields.
  • csv.QUOTE_NONNUMERIC: Quote all non-numeric fields.
  • csv.QUOTE_NONE: Never quote fields, but raise an error if a special character is encountered.

Key Methods of the Writer Object:

• writerow(row_list): Writes a single row of data to the CSV file. row_list should be an iterable (e.g., a list or tuple) where each element is a field in the row.
• writerows(list_of_rows): Writes multiple rows of data to the CSV file. list_of_rows should be an iterable of iterables (e.g., a list of lists), where each inner iterable represents a row.

3. Writing Multiple Rows at Once (writerows())

The writerows() method is efficient for writing a collection of rows simultaneously.

Example: Writing Multiple Rows

import csv

# Data to be written
data = [
    ['Name', 'Department', 'Joining Year'],
    ['Rahul', 'IT', 2022],
    ['Anjali', 'HR', 2021],
    ['Suman', 'Finance', 2023]
]

# Writing to a CSV file
with open('employees.csv', 'w', newline='') as file:
    writer = csv.writer(file)
    writer.writerows(data)

print("Data written to employees.csv")

Explanation:

1. The file employees.csv is opened in write mode ('w').
2. The newline='' argument is crucial. It prevents Python from automatically adding extra blank lines between rows on some operating systems, ensuring clean CSV output.
3. writer.writerows(data) writes all the rows from the data list into the employees.csv file.

4. Writing Rows Individually (writerow())

If you need more control or are processing data row by row, you can use the writerow() method.

Example: Writing Rows One by One

import csv

header = ['ID', 'Course', 'Duration']
row1 = [101, 'Python', '3 months']
row2 = [102, 'Java', '2 months']

with open('courses.csv', 'w', newline='') as file:
    writer = csv.writer(file)
    writer.writerow(header) # Write the header row
    writer.writerow(row1)   # Write the first data row
    writer.writerow(row2)   # Write the second data row

print("Data written to courses.csv")

Explanation:

This example demonstrates writing each row—the header and the subsequent data rows—using separate calls to writerow().

5. Customizing Delimiter and Quoting Behavior

The csv.writer() allows for customization of delimiters and quoting for advanced scenarios.

Example: Custom Delimiter and Quoting

import csv

data = [
    ['Student', 'Score'],
    ['Nina', 88],
    ['Ravi', 92],
    ['Alice Smith', 95] # Example with a space in the name
]

# Writing with semicolon delimiter and quoting all fields
with open('results.csv', 'w', newline='') as file:
    writer = csv.writer(file, delimiter=';', quotechar='"', quoting=csv.QUOTE_ALL)
    writer.writerows(data)

print("Data written to results.csv with custom settings")

Explanation:

• The delimiter=';' changes the separator from a comma to a semicolon.
• quoting=csv.QUOTE_ALL ensures that every field is enclosed in double quotes, which is useful for consistency or when fields might contain the delimiter or other special characters. Notice how "Alice Smith" is quoted even without a special character, due to QUOTE_ALL.

Summary of Key csv.writer Components

Interview Questions

• How do you write data to a CSV file using Python’s csv module?
• What is the difference between writerow() and writerows() in the csv module?
• Why is newline='' used when opening a CSV file for writing in Python?
• How can you change the delimiter while writing CSV files in Python?
• What are quotechar and quoting parameters used for in csv.writer()?
• How would you write rows one by one to a CSV file?
• Can you explain the different quoting options in the Python csv module?
• How do you handle writing CSV files with special characters in fields?
• What precautions should be taken when writing CSV files to ensure cross-platform compatibility?
• How do you write CSV files with non-default delimiters and quoting styles?