Python Chapter 7: File Handling

Master file operations, data persistence, and exception handling

File Handling Basics

Opening Files - The open() Function

The open() function is used to open files in Python. It returns a file object.

# Basic syntax
file = open('filename.txt', 'mode')
                

Important: Always close files after use with file.close() or use the with statement for automatic closing.

Reading Methods

read()

Reads entire file content as a single string.

with open('file.txt', 'r') as f:
    content = f.read()
    print(content)
                        

readline()

Reads a single line from the file.

with open('file.txt', 'r') as f:
    line = f.readline()
    print(line)
                        

readlines()

Reads all lines and returns them as a list.

with open('file.txt', 'r') as f:
    lines = f.readlines()
    for line in lines:
        print(line)
                        

Iterating Lines

Loop through file line by line (memory efficient).

with open('file.txt', 'r') as f:
    for line in f:
        print(line.strip())
                        

Writing Methods

write()

Writes a string to the file.

with open('file.txt', 'w') as f:
    f.write('Hello World\n')
                        

writelines()

Writes a list of strings to the file.

lines = ['Line 1\n', 'Line 2\n']
with open('file.txt', 'w') as f:
    f.writelines(lines)
                        

The with Statement

Best Practice: The with statement automatically closes the file, even if an exception occurs. It's the recommended way to work with files.

# Without 'with' - must manually close
file = open('file.txt', 'r')
content = file.read()
file.close()  # Must remember to close!

# With 'with' - automatically closes
with open('file.txt', 'r') as file:
    content = file.read()
    # File automatically closed here
                

File Modes Comparison

Mode	Description	Creates File	Position	Truncates
`'r'`	Read only (default)	No (error if missing)	Beginning	No
`'w'`	Write only	Yes	Beginning	Yes
`'a'`	Append only	Yes	End	No
`'r+'`	Read and Write	No (error if missing)	Beginning	No
`'w+'`	Read and Write	Yes	Beginning	Yes
`'a+'`	Read and Append	Yes	End	No
`'b'`	Binary mode (add to any mode)	Depends on base mode	Depends on base mode	Depends on base mode

Interactive Mode Demonstrator

Detailed Mode Explanations

'r' - Read Mode

Opens file for reading. File must exist.

with open('data.txt', 'r') as f:
    content = f.read()
    # FileNotFoundError if file doesn't exist
                        

'w' - Write Mode

Creates new file or overwrites existing file.

with open('data.txt', 'w') as f:
    f.write('New content')
    # WARNING: Erases all existing content!
                        

'a' - Append Mode

Adds content to end of file. Creates if doesn't exist.

with open('log.txt', 'a') as f:
    f.write('New log entry\n')
    # Preserves existing content
                        

'rb' - Binary Read

Reads binary files (images, videos, etc.).

with open('image.png', 'rb') as f:
    binary_data = f.read()
    # Returns bytes object
                        

Interactive Code Simulator

Watch how file operations execute step by step

Python Code

File Contents & Output

Virtual File System

Practice file operations in your browser

Create/Write File

Read File

Your Files

Operation Log

File Paths & OS Module

Path Types

Absolute Path

Complete path from root directory

# Windows
'C:\\Users\\John\\Documents\\file.txt'

# Linux/Mac
'/home/john/documents/file.txt'
                        

Relative Path

Path relative to current directory

# Same directory
'file.txt'

# Subdirectory
'data/file.txt'

# Parent directory
'../file.txt'
                        

Interactive Path Builder

home

user

documents

data

file.txt

Common OS Module Operations

import os

# Get current working directory
current_dir = os.getcwd()

# Check if file exists
if os.path.exists('file.txt'):
    print('File exists')

# Join path components
path = os.path.join('folder', 'subfolder', 'file.txt')

# Get file size
size = os.path.getsize('file.txt')

# List directory contents
files = os.listdir('.')
                

Working with CSV Files

CSV Module Basics

import csv

# Writing CSV
with open('data.csv', 'w', newline='') as file:
    writer = csv.writer(file)
    writer.writerow(['Name', 'Age', 'City'])
    writer.writerow(['Alice', '25', 'New York'])

# Reading CSV
with open('data.csv', 'r') as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)
                

Interactive CSV Demo

Name	Age	City	Occupation

CSV DictWriter & DictReader

import csv

# Writing with DictWriter
data = [
    {'name': 'Alice', 'age': 25, 'city': 'NYC'},
    {'name': 'Bob', 'age': 30, 'city': 'LA'}
]

with open('data.csv', 'w', newline='') as file:
    fieldnames = ['name', 'age', 'city']
    writer = csv.DictWriter(file, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerows(data)

# Reading with DictReader
with open('data.csv', 'r') as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(row['name'], row['age'])
                

Working with JSON Files

JSON Module Basics

import json

# Writing JSON
data = {
    'name': 'Alice',
    'age': 25,
    'skills': ['Python', 'JavaScript']
}

with open('data.json', 'w') as file:
    json.dump(data, file, indent=4)

# Reading JSON
with open('data.json', 'r') as file:
    data = json.load(file)
    print(data['name'])
                

Interactive JSON Demo

Python Dictionary

JSON Output

JSON vs CSV Comparison

Feature	CSV	JSON
Structure	Tabular (rows & columns)	Hierarchical (nested objects)
Data Types	All strings (must convert)	Multiple types (string, number, boolean, null)
Best For	Simple tabular data, spreadsheets	Complex nested data, APIs
Human Readable	Very readable	Readable with formatting
File Size	Smaller for simple data	Larger due to formatting

Exception Handling in File Operations

Common File Exceptions

FileNotFoundError

Raised when trying to open a non-existent file in read mode.

PermissionError

Raised when lacking permission to access a file.

IOError

General input/output operation failure.

IsADirectoryError

Raised when trying to open a directory as a file.

Try-Except-Finally Pattern

try:
    # Try to open and read file
    with open('data.txt', 'r') as file:
        content = file.read()
        print(content)

except FileNotFoundError:
    # Handle missing file
    print('Error: File not found!')

except PermissionError:
    # Handle permission issues
    print('Error: No permission to read file!')

except Exception as e:
    # Handle any other exceptions
    print(f'Unexpected error: {e}')

finally:
    # Always executed (cleanup code)
    print('File operation completed')
                

Interactive Exception Handler

Test different exception scenarios:

Best Practices

1. Use Specific Exceptions: Catch specific exceptions before general ones.

2. Use 'with' Statement: Ensures file is closed even if exception occurs.

3. Provide Helpful Messages: Tell users what went wrong and how to fix it.

4. Log Errors: In production, log errors for debugging.

Complete Exception Handling Example

import os

def safe_file_read(filename):
    """Safely read a file with proper exception handling"""

    # Check if file exists first
    if not os.path.exists(filename):
        return None, "File does not exist"

    try:
        with open(filename, 'r', encoding='utf-8') as file:
            content = file.read()
            return content, None

    except PermissionError:
        return None, "Permission denied"

    except UnicodeDecodeError:
        return None, "File encoding error"

    except Exception as e:
        return None, f"Unexpected error: {str(e)}"

# Usage
content, error = safe_file_read('data.txt')
if error:
    print(f"Error: {error}")
else:
    print(content)
                

File Handling Quiz

Test your knowledge of Python file handling concepts