Day 2: Files, Errors & System Interaction

📋 Today's Agenda

📁 File Handling

Reading Files

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

# Read line by line
with open('log.txt', 'r') as file:
    for line in file:
        print(line.strip())

# Read with encoding
with open('data.txt', 'r', encoding='utf-8') as file:
    content = file.read()

# Check if file exists first
import os
if os.path.exists('config.txt'):
    with open('config.txt', 'r') as file:
        print(file.read())

Writing Files

# Write simple text
with open('backup.txt', 'w') as file:
    file.write("Backup created\n")

# Write multiple lines
lines = ["Line 1", "Line 2", "Line 3"]
with open('data.txt', 'w') as file:
    for line in lines:
        file.write(line + '\n')

# Write with timestamp
import datetime
with open('log.txt', 'w') as file:
    timestamp = datetime.datetime.now()
    file.write(f"Created: {timestamp}\n")

Appending to Files

# Append new entry
with open('log.txt', 'a') as file:
    file.write("New log entry\n")

# Append with timestamp
import datetime
with open('log.txt', 'a') as file:
    timestamp = datetime.datetime.now()
    file.write(f"[{timestamp}] User login\n")

# Log function with error handling
def log_event(message):
    try:
        with open('system.log', 'a') as file:
            file.write(f"{message}\n")
    except:
        print("Could not write to log file")

🛡️ Error Handling

Try/Except Basics

# Simple try/except
try:
    with open('file.txt', 'r') as file:
        print(file.read())
except:
    print("File not found")

# Specific error types
try:
    with open('config.txt', 'r') as file:
        content = file.read()
except FileNotFoundError:
    print("File not found")
except PermissionError:
    print("Permission denied")

# Get error details
try:
    result = 10 / 0
except Exception as e:
    print(f"Error: {e}")

# Try/except with else (runs if no error)
try:
    with open('data.txt', 'r') as file:
        content = file.read()
except FileNotFoundError:
    print("File not found")
else:
    print("File read successfully")

# Try/except with finally (always runs)
try:
    file = open('test.txt', 'r')
    content = file.read()
except:
    print("Error")
finally:
    print("This always runs")

📝 Logging

# Basic logging
import logging
logging.basicConfig(level=logging.INFO)
logging.info("Script started")
logging.warning("Warning message")
logging.error("Error message")

# Log to file
import logging
logging.basicConfig(filename='app.log', level=logging.INFO)
logging.info("Application started")

# Different log levels
logging.debug("Debug message")    # Won't show (level is INFO)
logging.info("Info message")      # Shows
logging.warning("Warning message") # Shows
logging.error("Error message")    # Shows

# Log with timestamp
import logging
logging.basicConfig(
    format='%(asctime)s - %(levelname)s - %(message)s',
    level=logging.INFO
)
logging.info("Event occurred")

🖥️ OS Module

import os

# Get current directory
print(os.getcwd())

# List files in directory
files = os.listdir('.')
for file in files:
    print(file)

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

# Get file size
if os.path.exists('log.txt'):
    size = os.path.getsize('log.txt')
    print(f"File size: {size} bytes")

# Create directory
os.makedirs('backup', exist_ok=True)

🔧 SYS Module

import sys

# Get Python version
print(sys.version)

# Get command line arguments
print("Script:", sys.argv[0])
print("Arguments:", sys.argv[1:])

# Get platform
print("Platform:", sys.platform)

# Check Python path
for path in sys.path:
    print(path)

🚀 Subprocess Module

Basic Subprocess

import subprocess

# Run simple command
result = subprocess.run(['echo', 'Hello'], capture_output=True, text=True)
print(result.stdout)

# Run with arguments
result = subprocess.run(['echo', 'test'], capture_output=True, text=True)
print(result.stdout)

# Check return code
result = subprocess.run(['ls'], capture_output=True, text=True)
if result.returncode == 0:
    print("Command successful")
else:
    print("Command failed")

# Get both output and errors
result = subprocess.run(['ls', 'nonexistent'], capture_output=True, text=True)
print("Output:", result.stdout)
print("Errors:", result.stderr)

Safe Command Execution

import subprocess

def run_command(command):
    try:
        result = subprocess.run(command, capture_output=True, text=True, timeout=30)
        return result.stdout
    except:
        return "Command failed"

# Example usage
output = run_command(['echo', 'Hello from subprocess'])
print(output)

🛠️ Hands-On: Log Parser

Simple log reader

def read_log(filename):
    try:
        with open(filename, 'r') as file:
            for line in file:
                print(line.strip())
    except FileNotFoundError:
        print("Log file not found")

# Count lines in log
def count_log_lines(filename):
    try:
        with open(filename, 'r') as file:
            lines = file.readlines()
            print(f"Total lines: {len(lines)}")
    except FileNotFoundError:
        print("Log file not found")

# Find error lines
def find_errors(filename):
    try:
        with open(filename, 'r') as file:
            for line in file:
                if 'ERROR' in line:
                    print(line.strip())
    except FileNotFoundError:
        print("Log file not found")

# Create log summary
def log_summary(filename):
    try:
        with open(filename, 'r') as file:
            lines = file.readlines()
            print(f"Total lines: {len(lines)}")
            error_count = sum(1 for line in lines if 'ERROR' in line)
            print(f"Error lines: {error_count}")
    except FileNotFoundError:
        print("Log file not found")

🛠️ Hands-On: Command Wrapper

import subprocess

def run_safe(command):
    try:
        result = subprocess.run(command, capture_output=True, text=True, timeout=30)
        return result.stdout
    except:
        return "Error running command"

# System info collector
def get_system_info():
    info = {}
    info['user'] = run_safe(['whoami']).strip()
    info['directory'] = run_safe(['pwd']).strip()
    return info

# Check disk space (Linux)
def check_disk():
    try:
        result = subprocess.run(['df', '-h'], capture_output=True, text=True)
        return result.stdout
    except:
        return "Could not check disk space"

# List processes
def list_processes():
    try:
        result = subprocess.run(['ps', 'aux'], capture_output=True, text=True)
        return result.stdout
    except:
        return "Could not list processes"

✅ What We Did Today

• Read and wrote files
• Handled errors safely with try/except
• Used the logging module
• Explored os and sys modules
• Ran system commands with subprocess
• Created log parser and command wrapper scripts

🏠 Homework

• Create a script that reads a log file and counts errors
• Write a function that runs a system command safely
• Practice error handling with file operations