Master File Handling in Python - Read, Write, Manage Files

File Modes Reference

Python's open() function uses modes to specify read/write behavior and text/binary format.

Mode	Action	Creates?	Error if Missing
`'r'`	Read only	No	Yes
`'w'`	Write (overwrite)	Yes	No
`'a'`	Append	Yes	No
`'r+'`	Read + Write	No	Yes
`'b'`	Binary mode	-	-

Basic Read & Write

Writing Files

# Always use 'with' (auto-closes file)
with open('notes.txt', 'w') as file:
    file.write("Hello, Python!\n")
    file.write("Line 2\n")
    file.writelines(["Line 3\n", "Line 4\n"])

print("File created!")
                

Reading Files

# Read entire file
with open('notes.txt', 'r') as file:
    content = file.read()
    print(content)

# Read line by line
with open('notes.txt', 'r') as file:
    for line_num, line in enumerate(file, 1):
        print(f"Line {line_num}: {line.strip()}")

# Read all lines as list
with open('notes.txt', 'r') as file:
    lines = file.readlines()
                

Context Managers (with)

✅ Automatic Cleanup

with open('data.txt', 'w') as f:
    f.write("Safe!")
# File AUTOMATICALLY closed ✓
                        

✅ Exception Safe

try:
    with open('data.txt', 'r') as f:
        data = f.read()
except FileNotFoundError:
    print("File missing")
# File closed even on error ✓
                        

Manual vs Context Manager

# ❌ BAD - Manual close (forget = crash!)
file = open('test.txt', 'w')
file.write("data")
file.close()  # Easy to forget!

# ✅ GOOD - Context manager
with open('test.txt', 'w') as file:
    file.write("data")
# Always closes ✓
                

CSV & JSON Processing

CSV - Tabular Data

import csv

# Write CSV
with open('users.csv', 'w', newline='') as file:
    writer = csv.writer(file)
    writer.writerow(['name', 'age', 'city'])
    writer.writerows([
        ['Alice', 25, 'NYC'],
        ['Bob', 30, 'LA']
    ])

# Read CSV
with open('users.csv', 'r') as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(f"{row['name']} is {row['age']}")
                        

JSON - Structured Data

import json

user_data = {
    "name": "Alice",
    "skills": ["Python", "SQL", "Docker"],
    "active": True
}

# Write JSON
with open('user.json', 'w') as file:
    json.dump(user_data, file, indent=2)

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

Advanced File Operations

File Paths & Directories

import os
from pathlib import Path

# Current directory
print(os.getcwd())

# List files
print(os.listdir('.'))

# Pathlib (modern)
folder = Path('data')
folder.mkdir(exist_ok=True)

# Check existence
if Path('notes.txt').exists():
    print("File found!")

# File size
print(f"Size: {Path('notes.txt').stat().st_size} bytes")
                

Copy, Move, Delete

import shutil

# Copy file
shutil.copy('source.txt', 'backup.txt')

# Move/rename
shutil.move('old.txt', 'new.txt')

# Delete file
# os.remove('temp.txt')  # Single file
# shutil.rmtree('temp_dir')  # Directory
                

Log File Analysis

def analyze_log(filename):
    errors = 0
    warnings = 0
    
    with open(filename, 'r') as file:
        for line in file:
            if 'ERROR' in line:
                errors += 1
            elif 'WARNING' in line:
                warnings += 1
    
    return {'errors': errors, 'warnings': warnings}

stats = analyze_log('app.log')
print(f"Errors: {stats['errors']}, Warnings: {stats['warnings']}")
                

Configuration Files

# config.ini style
config = {
    'database': 'localhost:5432',
    'debug': True,
    'timeout': 30
}

with open('config.txt', 'w') as f:
    for key, value in config.items():
        f.write(f"{key}={value}\n")
                

File Handling Projects

1. Todo List Saver

Save/load todo list to JSON. Add, complete, delete tasks.

2. CSV Gradebook

Read student grades CSV → calculate averages → write report.

3. Log Analyzer

Parse server logs → count errors by type → generate summary.

4. Backup Utility

Copy folder recursively → compress → timestamp naming.

Robust Error Handling

import os

def safe_read(filename):
    try:
        if not os.path.exists(filename):
            raise FileNotFoundError(f"{filename} not found")
        
        with open(filename, 'r') as file:
            return file.read()
    except PermissionError:
        return "Permission denied"
    except UnicodeDecodeError:
        return "Invalid file encoding"
    except Exception as e:
        return f"Error: {e}"

content = safe_read('missing.txt')
print(content)
                

File Handling Pro!

💾 Production Applications:

📊 Data Processing
CSV/JSON pipelines

📝 Log Analysis
Error tracking systems

💾 Backup Tools
Automated file sync

📈 Config Management
App settings loader

🔄 Data Migration
Database import/export

Production-ready data persistence unlocked!
Next: Modules → Error Handling → Deployment

File Handling

File Handling Mastery