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Working with CSV Files

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Introduction

CSV (Comma-Separated Values) files are a common format for storing tabular data. They are simple text files where each line represents a row, and columns are separated by commas. Python provides several ways to work with CSV files, making it easy to read from and write to these files.

Key Concepts

  1. CSV File Structure:

    • Each line in a CSV file corresponds to a row in the table.
    • Columns are separated by commas.
    • The first row often contains headers (column names).

  2. Python CSV Module:

    • The csv module in Python provides functionality to both read from and write to CSV files.
    • Key classes and functions include csv.reader, csv.writer, csv.DictReader, and csv.DictWriter.

Reading CSV Files

Using csv.reader

The csv.reader class allows you to read CSV files row by row.

import csv

with open('example.csv', mode='r') as file:
    csv_reader = csv.reader(file)
    for row in csv_reader:
        print(row)

Explanation:

  • open('example.csv', mode='r'): Opens the file in read mode.
  • csv.reader(file): Creates a CSV reader object.
  • for row in csv_reader: Iterates over each row in the CSV file.
  • print(row): Prints each row.

Using csv.DictReader

The csv.DictReader class reads each row into a dictionary, using the first row as the keys.

import csv

with open('example.csv', mode='r') as file:
    csv_reader = csv.DictReader(file)
    for row in csv_reader:
        print(row)

Explanation:

  • csv.DictReader(file): Creates a CSV reader object that maps the information in each row to a dictionary using the headers as keys.

Writing CSV Files

Using csv.writer

The csv.writer class allows you to write data to a CSV file row by row.

import csv

data = [
    ['Name', 'Age', 'City'],
    ['Alice', 30, 'New York'],
    ['Bob', 25, 'Los Angeles'],
    ['Charlie', 35, 'Chicago']
]

with open('output.csv', mode='w', newline='') as file:
    csv_writer = csv.writer(file)
    csv_writer.writerows(data)

Explanation:

  • open('output.csv', mode='w', newline=''): Opens the file in write mode.
  • csv.writer(file): Creates a CSV writer object.
  • csv_writer.writerows(data): Writes multiple rows to the CSV file.

Using csv.DictWriter

The csv.DictWriter class writes dictionaries to a CSV file, using the dictionary keys as column headers.

import csv

data = [
    {'Name': 'Alice', 'Age': 30, 'City': 'New York'},
    {'Name': 'Bob', 'Age': 25, 'City': 'Los Angeles'},
    {'Name': 'Charlie', 'Age': 35, 'City': 'Chicago'}
]

with open('output.csv', mode='w', newline='') as file:
    fieldnames = ['Name', 'Age', 'City']
    csv_writer = csv.DictWriter(file, fieldnames=fieldnames)
    csv_writer.writeheader()
    csv_writer.writerows(data)

Explanation:

  • csv.DictWriter(file, fieldnames=fieldnames): Creates a CSV writer object that uses the specified fieldnames as headers.
  • csv_writer.writeheader(): Writes the header row to the CSV file.
  • csv_writer.writerows(data): Writes multiple rows to the CSV file.

Practical Exercises

Exercise 1: Reading a CSV File

Task: Write a Python script to read a CSV file named students.csv and print each row.

Solution:

import csv

with open('students.csv', mode='r') as file:
    csv_reader = csv.reader(file)
    for row in csv_reader:
        print(row)

Exercise 2: Writing to a CSV File

Task: Write a Python script to write the following data to a CSV file named employees.csv:

Name Age Department
John 28 HR
Jane 32 IT
Mike 40 Finance

Solution:

import csv

data = [
    ['Name', 'Age', 'Department'],
    ['John', 28, 'HR'],
    ['Jane', 32, 'IT'],
    ['Mike', 40, 'Finance']
]

with open('employees.csv', mode='w', newline='') as file:
    csv_writer = csv.writer(file)
    csv_writer.writerows(data)

Exercise 3: Using csv.DictReader and csv.DictWriter

Task: Write a Python script to read a CSV file named products.csv and write its content to another CSV file named products_copy.csv using csv.DictReader and csv.DictWriter.

Solution:

import csv

with open('products.csv', mode='r') as infile, open('products_copy.csv', mode='w', newline='') as outfile:
    csv_reader = csv.DictReader(infile)
    fieldnames = csv_reader.fieldnames
    csv_writer = csv.DictWriter(outfile, fieldnames=fieldnames)
    
    csv_writer.writeheader()
    for row in csv_reader:
        csv_writer.writerow(row)

Common Mistakes and Tips

  • File Modes: Ensure you open the file in the correct mode ('r' for reading, 'w' for writing).
  • Newline Parameter: When writing CSV files, use newline='' to prevent extra blank lines.
  • Header Row: When using csv.DictReader or csv.DictWriter, ensure the header row is correctly handled.

Conclusion

In this section, you learned how to work with CSV files in Python using the csv module. You can now read from and write to CSV files using both csv.reader/csv.writer and csv.DictReader/csv.DictWriter. These skills are essential for handling tabular data in many real-world applications. Next, we will explore JSON data handling, which is another common format for data interchange.

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Python Programming Course

Module 1: Introduction to Python

Module 2: Control Structures

Module 3: Functions and Modules

Module 4: Data Structures

Module 5: Object-Oriented Programming

Module 6: File Handling

Module 7: Error Handling and Exceptions

Module 8: Advanced Topics

Module 9: Testing and Debugging

Module 10: Web Development with Python

Module 11: Data Science with Python

Module 12: Final Project

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