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File Modes and CSV Handling
Lesson 2 of 2

CSV Handling in Python

Learn how to use the Pandas module in Python for data analysis. Explore functions, examples, and best practices for efficient data manipulation.

Handling CSV (Comma-Separated Values) files is a common task in Python, especially for data analysis, automation, and working with spreadsheets. This guide covers reading, writing, and manipulating CSV files with beginner-friendly examples and real-world use cases.

Example Processing CSV Data (Simple Version)

# Read CSV and calculate average
def calculate_average_scores():
    with open('scores.csv', 'r') as file:
        lines = file.readlines()
    
    total = 0
    count = 0
    
    for line in lines:
        name, score = line.strip().split(',')
        total += int(score)
        count += 1
    
    average = total / count
    print(f"Average score: {average:.2f}")
 
# Sample scores.csv:
# Alice,92
# Bob,85
# Charlie,78

2. Reading a CSV File

Using csv.reader() (Basic Method)

import csv
 
with open('employees.csv', 'r') as file:
    csv_reader = csv.reader(file)
    for row in csv_reader:
        print(row)  # Each row is a list

Output Example:

['Name', 'Age', 'Department']
['Alice', '28', 'HR']
['Bob', '32', 'Engineering']
['Charlie', '45', 'Finance']

Real-Life Use Case:

  • Reading an exported list of employees from an HR system.

Using csv.DictReader() (Easier for Column Access)

import csv
 
with open('employees.csv', 'r') as file:
    csv_reader = csv.DictReader(file)
    for row in csv_reader:
        print(row['Name'], row['Department'])  # Access columns by name

Output Example:

Alice HR
Bob Engineering
Charlie Finance

Real-Life Use Case:

  • Extracting specific columns (e.g., only names and emails from a contact list).

2. Writing to a CSV File

Using csv.writer() (Basic Method)

import csv
 
data = [
    ["Product", "Price", "Stock"],
    ["Laptop", "999", "50"],
    ["Phone", "699", "100"],
    ["Tablet", "299", "30"]
]
 
with open('inventory.csv', 'w', newline='') as file:
    csv_writer = csv.writer(file)
    csv_writer.writerows(data)

Real-Life Use Case:

  • Generating an inventory report for an e-commerce store.

Using csv.DictWriter() (Better for Structured Data)

import csv
 
employees = [
    {"Name": "Alice", "Age": 28, "Department": "HR"},
    {"Name": "Bob", "Age": 32, "Department": "Engineering"},
    {"Name": "Charlie", "Age": 45, "Department": "Finance"}
]
 
with open('employees_new.csv', 'w', newline='') as file:
    fieldnames = ["Name", "Age", "Department"]
    writer = csv.DictWriter(file, fieldnames=fieldnames)
    writer.writeheader()  # Writes column names
    writer.writerows(employees)

Real-Life Use Case:

  • Exporting structured data (e.g., employee records) to a CSV file.

3. Modifying CSV Data (Real-Life Example)

Example: Filtering Data (Only Keep Engineering Dept.)

import csv
 
# Read CSV and filter
with open('employees.csv', 'r') as file:
    csv_reader = csv.DictReader(file)
    engineering_employees = [row for row in csv_reader if row['Department'] == 'Engineering']
 
# Write filtered data to a new CSV
with open('engineering_employees.csv', 'w', newline='') as file:
    fieldnames = ["Name", "Age", "Department"]
    writer = csv.DictWriter(file, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerows(engineering_employees)

Real-Life Use Case:

  • Extracting only relevant rows (e.g., employees in a specific department).

4. Working with CSV & Pandas (For Data Analysis)

Pandas is a powerful library for handling CSV files efficiently.

Install Pandas (if not installed)

pip install pandas

Reading CSV with Pandas

import pandas as pd
 
df = pd.read_csv('employees.csv')
print(df.head())  # Shows first 5 rows

Writing CSV with Pandas

df.to_csv('updated_employees.csv', index=False)  # index=False avoids extra column

Real-Life Use Case:

  • Cleaning and analyzing large datasets (e.g., sales records, survey responses).

5. Common CSV Tasks (Cheat Sheet)

Task Code Example
Read CSV csv.reader(file) or pd.read_csv()
Write CSV csv.writer(file) or df.to_csv()
Skip Header next(csv_reader) (before loop)
Read as Dictionary csv.DictReader(file)
Write with Headers csv.DictWriter(file, fieldnames)
Append to CSV Open in 'a' (append) mode

Real-World CSV Use Cases

  1. Importing/Exporting Data (Excel, Google Sheets)
  2. Data Cleaning (Removing duplicates, fixing errors)
  3. Automating Reports (Sales, inventory, logs)
  4. Machine Learning Datasets (Storing training data)
  5. Web Scraping Results (Saving scraped data to CSV)

Summary

  • Use csv.reader() and csv.writer() for basic CSV handling.
  • Use csv.DictReader() and csv.DictWriter() for column-based access.
  • Use Pandas (pd.read_csv(), df.to_csv()) for advanced data manipulation.
  • Real-world applications include HR records, inventory, sales data, and more.
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