Python 3 — File I/O Operations

# File modes:
# 'r'  — Read (default) — error if file doesn't exist
# 'w'  — Write — creates new or overwrites existing
# 'a'  — Append — adds to end of file
# 'x'  — Exclusive create — error if file exists
# 'b'  — Binary mode (add to others: 'rb', 'wb')

# Reading entire file (with statement auto-closes file)
# Create a test file first:
with open("example.txt", "w") as f:
    f.write("Hello, Python!\n")
    f.write("File handling is easy.\n")
    f.write("Line 3 here.\n")

# Now read it back
with open("example.txt", "r") as f:
    content = f.read()    # Read entire file as string
    print(content)

# Read line by line
with open("example.txt", "r") as f:
    for line in f:
        print(line.strip())  # strip() removes newline chars

# Create a sample file
lines_data = ["Alice,92,A\n", "Bob,78,B\n", "Charlie,85,B+\n"]
with open("students.txt", "w") as f:
    f.writelines(lines_data)

# .read() — entire file as one string
with open("students.txt") as f:
    data = f.read()
    print(repr(data))

# .readline() — one line at a time
with open("students.txt") as f:
    first_line = f.readline()
    second_line = f.readline()
    print(first_line.strip())
    print(second_line.strip())

# .readlines() — all lines as a list
with open("students.txt") as f:
    all_lines = f.readlines()
    print(all_lines)
    print(f"Total lines: {len(all_lines)}")

# Write mode ('w') — creates or OVERWRITES
with open("log.txt", "w") as f:
    f.write("=== Application Log ===\n")
    f.write("Session started\n")
    for i in range(1, 4):
        f.write(f"Event {i}: User action\n")

# Append mode ('a') — adds to existing file
with open("log.txt", "a") as f:
    f.write("New event added\n")
    f.write("Session ended\n")

# writelines() — write a list of strings
data = [f"Student {i}: Score {i*10}\n" for i in range(1, 6)]
with open("scores.txt", "w") as f:
    f.writelines(data)

# Read back to verify
with open("log.txt") as f:
    print(f.read())

import csv

# Writing CSV
students = [
    ["Name", "Age", "Score", "Grade"],
    ["Alice", 20, 92, "A"],
    ["Bob", 22, 78, "B"],
    ["Charlie", 21, 85, "B+"]
]

with open("students.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(students)

# Reading CSV
with open("students.csv", "r") as f:
    reader = csv.reader(f)
    header = next(reader)   # Skip header row
    print(f"Columns: {header}")
    for row in reader:
        name, age, score, grade = row
        print(f"{name}: Score {score} ({grade})")

# DictWriter — write with headers as keys
with open("data.csv", "w", newline="") as f:
    fieldnames = ["product", "price", "quantity"]
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerow({"product": "Laptop", "price": 999.99, "quantity": 5})

import json

# Python dict to JSON file
config = {
    "app_name": "Our Compiler",
    "version": "2.0",
    "languages": ["Python", "Java", "C++"],
    "settings": {
        "theme": "dark",
        "font_size": 14
    }
}

# Write JSON
with open("config.json", "w") as f:
    json.dump(config, f, indent=2)  # indent for pretty print

# Read JSON back into Python dict
with open("config.json", "r") as f:
    loaded = json.load(f)
    print(loaded["app_name"])         # Our Compiler
    print(loaded["languages"])        # ['Python', 'Java', 'C++']
    print(loaded["settings"]["theme"]) # dark

# Convert dict to JSON string
json_str = json.dumps(config, indent=2)
print(json_str[:100] + "...")

# Parse JSON string to dict
data = json.loads('{"name": "Balaji", "age": 25}')
print(data["name"])  # Balaji

import os

# Check if file exists before reading
filename = "example.txt"
if os.path.exists(filename):
    with open(filename) as f:
        print(f.read())
else:
    print(f"File '{filename}' not found!")

# File info
if os.path.exists(filename):
    size = os.path.getsize(filename)
    print(f"File size: {size} bytes")

# Create directory
os.makedirs("output/data", exist_ok=True)  # Creates nested dirs

# List files with specific extension
for f in os.listdir("."):
    if f.endswith(".txt"):
        print(f"Found text file: {f}")

# Rename and delete
# os.rename("old.txt", "new.txt")
# os.remove("temp.txt")

def read_file_safely(filename):
    """Read a file safely, returning None if it fails."""
    try:
        with open(filename, 'r') as f:
            return f.read()
    except FileNotFoundError:
        print(f"❌ Error: '{filename}' does not exist.")
        return None
    except PermissionError:
        print(f"❌ Error: No permission to read '{filename}'.")
        return None
    except Exception as e:
        print(f"❌ Unexpected error: {e}")
        return None

content = read_file_safely("example.txt")
if content:
    print(f"File loaded ({len(content)} chars)")

missing = read_file_safely("nonexistent.txt")
print(f"Result: {missing}")  # None