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Content:

Section 1. File Operations Basics

Section 2. with Statement

Section 3. File System Operations

Section 4. Encoding in Python

Section 5. Practical Applications

 

Section 1. File Operations Basics

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1. What is a File?

• Data container stored on disk
• Like a water bottle stores liquid
• Two main types:
  • Text files: Human-readable (.txt, .csv)
  • Binary files: Special formats (.jpg, .docx)

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2. File Paths Explained

# Absolute path (full address)
"/home/student/documents/diary.txt"

# Relative path (from current location)
"../pictures/vacation.jpg"

• Think of paths like home addresses for files

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3. Opening Files: The open() Function

# Modes
f = open("list.txt", "r")   # Read
f = open("new.txt", "w")    # Write (overwrite)
f = open("log.txt", "a")    # Append

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4. Reading Files - The read() Family

file = open("classlist.txt", "r")
print(file.read())       # Read entire file
file.seek(0)            # Reset to beginning

print(file.readline())   # Read first line
file.seek(0)   

print(file.readlines())  # Read all lines as list
file.close()

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5. Writing Files - write() vs writelines()

# Write individual strings
file = open("groceries.txt", "w")
file.write("Milk\n")
file.write("Eggs\nBread\n")
file.close()

# Write multiple lines from list
items = ["Apples\n", "Coffee\n"]
file = open("shopping.txt", "w")
file.writelines(items)
file.close()

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6. Case 1: Read Class List

# File: classlist.txt
# Alice
# Bob
# Charlie

file = open("classlist.txt", "r")
students = file.readlines()
file.close()

for student in students:
    print("Hello " + student.strip() + "!")

Output:

Hello Alice!
Hello Bob!
Hello Charlie!

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7. Case 2: Create Shopping List

file = open("groceries.txt", "w")
file.write("Shopping List:\n")
file.write("- Milk\n- Eggs\n- Bread")
file.close()

File contents:

Shopping List:
- Milk
- Eggs
- Bread

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8. Case 3: Append to Diary

# Existing content:
# 2024-03-01: Learned Python basics

file = open("diary.txt", "a")
file.write("\n2024-06-01: Learned file operations!")
file.close()

Updated file:

2024-03-01: Learned Python basics
2024-06-01: Learned file operations!

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9. Important Reminders

1. Always close files!
2. Double-check file modes
3. Use relative paths for portability
4. Test with small files first

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Section 2. with Statement

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What is with?

• Special syntax for resource management
• Makes code cleaner and safer
• Automatically handles cleanup

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Why Use with? (1/2)

Problem without with:

f = open('diary.txt', 'r')
content = f.read()
# What if we forget to close?
# f.close()

File remains open if exception occurs!

1. Data may not be fully written to the file (most dangerous!)
2. File handle leakage (exhaustion of file descriptors)
3. The file may be locked and inaccessible to other programs/processes
4. Automatic closing by garbage collection is unreliable

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Why Use with? (2/2)

Solution with with:

with open('diary.txt', 'r') as f:
    content = f.read()
# File automatically closes here!

Guaranteed cleanup
Safer code

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with Basic Syntax

with expression as variable:
    # Your code block

Real example:

with open('secrets.txt', 'r') as secret_file:
    print(secret_file.read())

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Handling Multiple Files

Copy file contents safely:

with open('original.txt', 'r') as src: 
    with open('backup.txt', 'w') as dst:
        dst.write(src.read())

Two files managed at once!

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Case 1: Read Config File

Read server configuration:

with open('config.cfg', 'r') as config_file:
    settings = config_file.readlines()

print("Server timeout:", settings[0])
print("Max users:", settings[1])

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Case 2: Safe File Copy

Copy cat pictures safely:

with open('cat.jpg', 'rb') as source_cat:
    with open('cat_backup.jpg', 'wb') as backup_cat:
        backup_cat.write(source_cat.read())

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Case 3: Program Logging

Record program activity:

with open('app.log', 'a') as log_file:
    log_file.write('Program started\n')
    # Your code here
    log_file.write('Operation completed\n')

📝 Logs always get saved!

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with Superpowers

1. Automatic cleanup
2. Error-safe operations
3. Cleaner code
4. Multiple resource handling

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When to Use with?

• File operations
• Database connections
• Network resources
• Any operation needing cleanup

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Final Demo: Recipe Reader

with open('recipe.txt', 'r') as recipe:
    print("Today's Recipe:")
    for line in recipe:
        print(line.strip())

File closed automatically!

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Section 3. File System Operations

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01. Path Operations with os.path

• Why check paths?
  • Avoid errors when working with files
• Basic methods:

  import os
    
  # Check if file exists
  print(os.path.exists("diary.txt"))  # Output: True/False
    
  # Check file type
  print(os.path.isfile("photos/"))    # Output: False
  print(os.path.isdir("photos/"))     # Output: True
  

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02. Get File Size

• Measure file content:

  size = os.path.getsize("secrets.txt")
  print("File size: " + str(size) + " bytes") 
  # Output: File size: 8 bytes
  

• Note: 1024 bytes = 1KB

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03. Create Directories

• Make new folders:

  os.mkdir("experiments")  # Simple directory
  os.mkdir("projects/2024") # Error! Parent must exist
  

• Pro tip: Use os.makedirs() for nested folders

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04. List Directory Contents

• See what’s inside:

  files = os.listdir("photos/")
  print(files)
  # Output: []
  

• Remember: Returns names only, not full paths

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05. Rename Files

• Change names safely:

  # Rename single file
  os.rename("old.txt", "new.txt")
    
  # Rename directory
  os.rename("temp/", "backup/")
  

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Case 1: Auto-create Daily Folder

• Problem: Need date-based organization
• Solution:

  from datetime import datetime
    
  today = datetime.now().strftime("%Y%m%d")
  os.mkdir("lab_" + today)
  print("Created: lab_" + today)
    
  

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Case 2: Batch Rename Photos

“replace” function of String

  a="aaabbb"
  b=a.replace('bbb','ccc')
  print(b)

• Before: photo_001.jpg, photo_002.jpg
• After: vacation_001.jpg, vacation_002.jpg

  for file in os.listdir("photos/"):
      if file.startswith("photo"):
          new_name = file.replace("photo", "vacation")
          os.rename("photos/" + file, "photos/" + new_name)
  

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Case 3: Find All Text Files

• Goal: Locate .txt files in directory

  text_files = []
    
  for item in os.listdir("text/"):
      if item[len(item)-4:len(item)] =='.txt':   # or use item.endswith('.txt')
          text_files.append(item)
    
  print("Found text files:", text_files)
  # Output: Found text files: ['notes.txt', 'todo.txt']
  

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Section 4. Encoding in Python

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Why Encoding Matters?

• Computers only understand 0s and 1s
• Encoding = Dictionary for text ↔ numbers conversion
• Common encodings: ASCII, UTF-8, GBK

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ASCII: The Basic Alphabet

• 128 characters (English letters, numbers, basic symbols)
• Example:

  # ASCII character codes
  print(ord('A'))  # Output: 65
  print(chr(65))   # Output: 'A'
  

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UTF-8: The Global Standard

• Supports all languages
• Variable-length encoding (1-4 bytes)
• Python 3’s default encoding

  # Chinese character in UTF-8
  汉字 = "中文"
  print(len(汉字.encode('utf-8')))  # Output: 6 (3 bytes per character)
  

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GBK: Chinese Legacy Encoding

• Simplified Chinese standard
• Fixed 2 bytes per Chinese character
• Common in older systems

  # Same text in GBK
  print(len("中文".encode('gbk')))  # Output: 4 (2 bytes per character)
  

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Setting Encoding Parameters

Always specify encoding when opening files:

# Safe way to open files
file = open('data.txt', 'r', encoding='utf-8')
content = file.read()
file.close()

Common error types:

• UnicodeDecodeError: Can’t decode bytes
• UnicodeEncodeError: Can’t encode characters

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Handling Encoding Errors

Use errors parameter:

# Ignore problematic characters
file = open('data.txt', 'r', encoding='utf-8', errors='ignore')

# Replace errors with ?
file = open('data.txt', 'r', encoding='utf-8', errors='replace')

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Case 1: GBK → UTF-8 Conversion

Convert legacy Chinese files:

# Step 1: Read GBK file
source = open('old_data.txt', 'r', encoding='gbk')
content = source.read()
source.close()

# Step 2: Save as UTF-8
target = open('new_data.txt', 'w', encoding='utf-8')
target.write(content)
target.close()

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Case 2: Reading Chinese CSV

Handle common data formats:

# data.csv contents:
# 姓名,年龄
# 张三,25
# 李四,30

file = open('data.csv', 'r', encoding='gbk')
for line in file:
    name, age = line.strip().split(',')
    print("Name: " + name + ", Age: " + age)
file.close()

file = open('data.csv', 'r', encoding='utf-8')
for line in file:
    name, age = line.strip().split(',')
    print("Name: " + name + ", Age: " + age)
file.close()

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Pro Tips

1. Always check file encoding before processing
2. Use UTF-8 for new projects
3. Test with chardet library if unsure

   # Detect encoding (install chardet first)
   import chardet
   rawdata = open('mystery.txt', 'rb').read()
   result = chardet.detect(rawdata)
   print(result['encoding'])
   

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Section 5. Practical Applications

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1. Log Analyzer: Count 404 Errors

def count_404(log_file):
    counter = 0
    f = open(log_file, 'r')
    for line in f:
        if ' 404 ' in line:
            counter += 1
    f.close()
    return counter

# Usage example
print("Total 404 errors:", count_404('nginx.log'))

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2. Log Analyzer: Time Filter

def filter_by_time(log_file, start_time, end_time):
    results = []
    f = open(log_file, 'r')
    for line in f:
        timestamp = line.split('[')[1].split(']')[0]
        if start_time <= timestamp and timestamp <= end_time:
            results.append(line)
    f.close()
    return results

# Example usage
matches = filter_by_time('nginx.log', '01/Jan/2023:12:00', '01/Jan/2023:12:30')
print("Found " + str(len(matches)) + " entries in time range")

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3. Backup Tool: Basic File Copy

import shutil
from datetime import datetime

def simple_backup(source_dir):
    timestamp = datetime.now().strftime("%Y%m%d_%H%M")
    backup_name = "backups/" + source_dir + "_backup_" + timestamp
    shutil.copytree(source_dir, backup_name)
    print("Backup created: " + backup_name)

# Usage
simple_backup('important_files')

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4. Backup Tool: Validate Backup

import os

def verify_backup(source, backup):
    f1=open(source,'r')
    f2=open(backup,'r')
    f1_content=f1.read()
    f2_content=f2.read()
    f1.close()
    f2.close()
    if f1_content==f2_content:
       print('yes')
    else:
       print('no')

# Example usage
verify_backup('nginx.log','nginx_bac.log')

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5. Text Encryption: Caesar Cipher

def caesar_encrypt(text, shift=3):
    encrypted = []
    for char in text:
        encrypted.append(chr(ord(char) + shift))
    return ''.join(encrypted)

# Example
secret = caesar_encrypt("Hello Students!")
print("Encrypted:", secret)

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6. Text Decryption: Caesar Cipher

def caesar_decrypt(ciphertext, shift=3):
    decrypted = []
    for char in ciphertext:
        decrypted.append(chr(ord(char) - shift))
    return ''.join(decrypted)

# Test decryption
print("Decrypted:", caesar_decrypt(secret))

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7. File Encryption Wrapper

def encrypt_file(input_file, output_file):
    f_in = open(input_file, 'r')
    f_out = open(output_file, 'w')
    
    original_text = f_in.read()
    encrypted = caesar_encrypt(original_text)
    f_out.write(encrypted)
    
    f_in.close()
    f_out.close()

# Usage
encrypt_file('nginx.log', 'nginx.log.enc')

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8. File Decryption Wrapper

def decrypt_file(input_file, output_file):
    f_in = open(input_file, 'r')
    f_out = open(output_file, 'w')
    
    encrypted_text = f_in.read()
    decrypted = caesar_decrypt(encrypted_text)
    f_out.write(decrypted)
    
    f_in.close()
    f_out.close()

# Usage
decrypt_file('nginx.log.enc', 'nginx.log.enc.dec')

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