Working with Files

# First create a file to read, so the examples below have something to open
# (writing is covered in detail further down)
with open("data.txt", "w") as file:
    file.write("First line\nSecond line\nThird line\n")

# Open a file for reading
file = open("data.txt", "r")
content = file.read()
file.close()  # Always close files!
print(content)

# Open a file for writing
file = open("output.txt", "w")
file.write("Hello, World!")
file.close()

# Using 'with' statement (recommended)
with open("data.txt", "r") as file:
    content = file.read()
    # File is automatically closed when block exits
    # Even if an error occurs!
print("Closed automatically:", file.closed)

# Equivalent to:
file = open("data.txt", "r")
try:
    content = file.read()
finally:
    file.close()  # Always executed
print("Closed manually:", file.closed)

# Read entire file as string
with open("data.txt", "r") as file:
    content = file.read()
    print(content)

# Read entire file as list of lines
with open("data.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line.strip())  # strip() removes newline characters

# Method 1: Iterate directly over file object
with open("data.txt", "r") as file:
    for line in file:
        print(line.strip())  # Process each line

# Method 2: Using readline() in a loop
with open("data.txt", "r") as file:
    line = file.readline()
    while line:
        print(line.strip())
        line = file.readline()

# Method 3: Read specific number of characters
with open("data.txt", "r") as file:
    chunk = file.read(100)  # Read first 100 characters
    print(chunk)

def read_file_safely(filename):
    """Read file content with proper error handling."""
    try:
        with open(filename, "r") as file:
            content = file.read()
            return content
    except FileNotFoundError:
        print(f"Error: File '{filename}' not found.")
        return None
    except PermissionError:
        print(f"Error: Permission denied to read '{filename}'.")
        return None
    except Exception as e:
        print(f"Unexpected error: {e}")
        return None

# Usage
content = read_file_safely("data.txt")
if content:
    print("File read successfully!")
    print(content)

# Write mode ("w") - overwrites existing file
with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is line 2\n")
    file.write("This is line 3")

# Append mode ("a") - adds to end of file
with open("output.txt", "a") as file:
    file.write("\nThis line is appended")

# Writing multiple lines
lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open("output.txt", "w") as file:
    file.writelines(lines)

# Or use write() with join
with open("output.txt", "w") as file:
    file.write("\n".join(["Line 1", "Line 2", "Line 3"]))

# Each "w" wiped the file, so only the last version survives
with open("output.txt", "r") as file:
    print(file.read())

def write_file_safely(filename, content):
    """Write content to file with error handling."""
    try:
        with open(filename, "w") as file:
            file.write(content)
        print(f"Successfully wrote to {filename}")
        return True
    except PermissionError:
        print(f"Error: Permission denied to write to '{filename}'.")
        return False
    except Exception as e:
        print(f"Unexpected error: {e}")
        return False

# Usage
success = write_file_safely("output.txt", "Hello, World!")
if success:
    print("File written successfully!")

import os

# Relative path (relative to current working directory)
with open("data.txt", "r") as file:
    content = file.read()
print("data.txt:", len(content), "characters")

# Relative path with subdirectory
os.makedirs("data", exist_ok=True)
with open("data/input.txt", "w") as file:
    file.write("nested file\n")

with open("data/input.txt", "r") as file:
    content = file.read()
print("data/input.txt:", content.strip())

# Absolute paths start from the root of the filesystem. These are shown as
# syntax only, since they will not exist on your machine:
#
# with open("/home/user/documents/data.txt", "r") as file:
#     content = file.read()
#
# On Windows a backslash starts an escape, so double them (or use a raw string):
#
# with open("C:\\Users\\User\\Documents\\data.txt", "r") as file:
#     content = file.read()

import os

# Join path components (handles / or \ automatically)
file_path = os.path.join("data", "input", "file.txt")
print(f"Joined path: {file_path}")
# On Linux/Mac: "data/input/file.txt"
# On Windows: "data\input\file.txt"

# Create it so there is something to read
os.makedirs(os.path.join("data", "input"), exist_ok=True)
with open(file_path, "w") as file:
    file.write("nested file\n")

with open(file_path, "r") as file:
    content = file.read()
print(f"Contents: {content.strip()}")

# Check if file exists
if os.path.exists("data.txt"):
    print("File exists!")
else:
    print("File does not exist!")

# The next two depend on where you run the script, so your paths will differ
current_dir = os.getcwd()
print(f"Current directory: {current_dir}")

abs_path = os.path.abspath("data.txt")
print(f"Absolute path: {abs_path}")

from pathlib import Path

# Create path object
file_path = Path("data") / "input" / "file.txt"
print(file_path)  # Works on all platforms!

# Make sure the folder exists, then write through the Path itself
file_path.parent.mkdir(parents=True, exist_ok=True)
file_path.write_text("Hello, World!")

# Read it back with pathlib's read_text method
content = file_path.read_text()
print(f"Contents: {content}")

# Check if exists
if file_path.exists():
    print("File exists!")

# Get parent directory
parent = file_path.parent
print(f"Parent: {parent}")

# Get filename
filename = file_path.name
print(f"Filename: {filename}")

# Create a small CSV to work with
with open("data.csv", "w") as file:
    file.write("Alice,30\nBob,25\nCharlie,35\n")

# Reading and processing a simple CSV file
with open("data.csv", "r") as file:
    for line in file:
        # Split by comma (simple CSV parsing)
        fields = line.strip().split(",")
        if len(fields) >= 2:
            name = fields[0]
            age = fields[1]
            print(f"Name: {name}, Age: {age}")

# Writing CSV-like data
data = [
    ["Alice", "30", "Engineer"],
    ["Bob", "25", "Designer"],
    ["Charlie", "35", "Manager"]
]

with open("output.csv", "w") as file:
    for row in data:
        file.write(",".join(row) + "\n")

# Create the configuration file this example reads
with open("config.txt", "w") as file:
    file.write("# App settings\nhost=localhost\nport=8080\ndebug=True\n")

# Simple key=value configuration file
config = {}
with open("config.txt", "r") as file:
    for line in file:
        line = line.strip()
        if line and not line.startswith("#"):  # Skip empty lines and comments
            key, value = line.split("=", 1)
            config[key.strip()] = value.strip()

print(config)

import datetime

def log_message(message, log_file="app.log"):
    """Append timestamped message to log file."""
    timestamp = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    log_entry = f"[{timestamp}] {message}\n"

    with open(log_file, "a") as file:
        file.write(log_entry)

# Usage
log_message("Application started")
log_message("Processing data...")
log_message("Application finished")

# Read the log back. "a" appends, so running this twice doubles the entries.
with open("app.log", "r") as file:
    print(file.read(), end="")