Unit 10: File Handling in Python

# Step 1: Open file in write mode ('w')
file_handle = open("sample_demo.txt", "w", encoding="utf-8")

# Step 2: Write text data to the file
file_handle.write("DevAscent Academy - Unit 10 File Handling\n")
file_handle.write("Learning Python persistent storage step by step!\n")

# Step 3: Explicitly close the file to flush buffer to disk
file_handle.close()

print("File written and closed successfully!")
print("Is the file closed?:", file_handle.closed)

# 1. Overwriting with 'w'
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("Line 1: Initial content.\n")

# 2. Appending with 'a' (adds to end without destroying Line 1)
with open("notes.txt", "a", encoding="utf-8") as f:
    f.write("Line 2: Appended message.\n")

# 3. Reading back with 'r'
with open("notes.txt", "r", encoding="utf-8") as f:
    content = f.read()

print("Current contents of notes.txt:")
print(content)

# 4. Exclusive creation with 'x' (catches existing file safely)
try:
    with open("notes.txt", "x", encoding="utf-8") as f:
        f.write("This will fail because notes.txt already exists!")
except FileExistsError:
    print("Safe Catch: notes.txt already exists, 'x' mode prevented accidental overwrite!")

# Setup dummy file for demonstration
with open("students.txt", "w", encoding="utf-8") as f:
    f.write("101,Rahul Sharma,BCA\n102,Pooja Verma,B.Tech\n103,Amit Kumar,BCA\n")

# Method 1: read() -> Entire file as a single string
with open("students.txt", "r", encoding="utf-8") as f:
    full_text = f.read()
    print("--- 1. read() Output (Single String) ---")
    print(full_text)

# Method 2: readlines() -> List of string lines
with open("students.txt", "r", encoding="utf-8") as f:
    lines_list = f.readlines()
    print("--- 2. readlines() Output (List) ---")
    print(lines_list)

# Method 3: BEST PRACTICE for Big Files -> Direct Loop Iteration
print("\n--- 3. Best Practice: Memory-Efficient for-loop ---")
with open("students.txt", "r", encoding="utf-8") as f:
    for line_no, line in enumerate(f, start=1):
        clean_line = line.strip()  # removes trailing newline '\n'
        print(f"Line {line_no}: {clean_line}")

# Demonstrating automatic closure after the with block
with open("autoclose_test.txt", "w", encoding="utf-8") as file:
    file.write("Testing automatic closure!\n")
    print("Inside the 'with' block: Is file closed? ->", file.closed)

# Notice we are now OUTSIDE the with block (unindented)
print("Outside the 'with' block: Is file closed? ->", file.closed)

import csv

# --- 1. Writing to a CSV File using csv.writer ---
student_records = [
    ["Roll No", "Name", "Marks", "City"],
    [101, "Aman Gupta", 88, "Delhi"],
    [102, "Neha Sharma", 92, "Jaipur"],
    [103, "Karan Joshi", 76, "Mumbai"]
]

# newline='' prevents blank lines on Windows systems
with open("students.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerows(student_records)
print("CSV File written successfully!")

# --- 2. Reading a CSV File using csv.DictReader (Header-aware!) ---
print("\nReading CSV using csv.DictReader:")
with open("students.csv", "r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(f"  Student #{row['Roll No']} -> {row['Name']} ({row['City']}) Scored: {row['Marks']}")

import json

# Python Dictionary with nested structure
college_profile = {
    "institution": "DevAscent Academy",
    "established": 2024,
    "active": True,
    "courses": ["Python for Systems", "Data Structures", "Web Dev"],
    "contact": {
        "email": "support@devascentacademy.me",
        "city": "New Delhi"
    }
}

# 1. Serialization: Saving Python dict to 'profile.json' with indent=4
with open("profile.json", "w", encoding="utf-8") as f:
    json.dump(college_profile, f, indent=4)
print("JSON file saved successfully!")

# 2. Deserialization: Reading JSON file back into Python dictionary
with open("profile.json", "r", encoding="utf-8") as f:
    loaded_data = json.load(f)

print("\nDeserialized Python Data Type:", type(loaded_data).__name__)
print(f"Institution : {loaded_data['institution']}")
print(f"City        : {loaded_data['contact']['city']}")
print(f"Courses (3) : {', '.join(loaded_data['courses'])}")

import csv
import json
import datetime

def log_system_event(action_message):
    """Appends a timestamped audit log to a text file."""
    timestamp = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    with open("audit_log.txt", "a", encoding="utf-8") as log_file:
        log_file.write(f"[{timestamp}] {action_message}\n")

def save_roster_csv(filename, students):
    """Exports student records to CSV format."""
    fieldnames = ["id", "name", "course", "gpa"]
    with open(filename, "w", newline="", encoding="utf-8") as f:
        writer = csv.DictWriter(f, fieldnames=fieldnames)
        writer.writeheader()
        writer.writerows(students)
    log_system_event(f"Exported {len(students)} student records to {filename}")

# Execute Workflow
student_roster = [
    {"id": 101, "name": "Ananya Roy", "course": "BCA 1st Year", "gpa": 8.9},
    {"id": 102, "name": "Siddharth Rao", "course": "B.Tech CSE", "gpa": 9.2},
    {"id": 103, "name": "Kavita Sen", "course": "BCA 1st Year", "gpa": 7.8}
]

save_roster_csv("roster_2026.csv", student_roster)

print("=== AUDIT LOG CONTENTS ===")
with open("audit_log.txt", "r", encoding="utf-8") as f:
    print(f.read().strip())

print("\n=== GENERATED CSV PREVIEW ===")
with open("roster_2026.csv", "r", encoding="utf-8") as f:
    print(f.read().strip())