Working with Files in Python

file_object = open(file, mode, encoding=None, errors=None, buffering=-1)

with open("example.txt", "r") as f:
    content = f.read()
# File is automatically closed here

f = open("example.txt", "r")
content = f.read()
f.close()  # Must be called explicitly

with open("example.txt", "r", encoding="utf-8") as f:
    content = f.read()
    print(content)

with open("example.txt", "r") as f:
    line = f.readline()
    while line:
        print(line, end="")
        line = f.readline()

with open("example.txt", "r") as f:
    lines = f.readlines()
    for line in lines:
        print(line.strip())

with open("example.txt", "r") as f:
    for line in f:
        print(line.strip())

with open("example.txt", "r") as f:
    chunk = f.read(100)  # Read first 100 characters
    print(chunk)

with open("output.txt", "w", encoding="utf-8") as f:
    f.write("Hello, World!\n")
    f.write("Second line\n")

lines = ["line one\n", "line two\n", "line three\n"]
with open("output.txt", "w") as f:
    f.writelines(lines)

with open("log.txt", "a", encoding="utf-8") as f:
    f.write("New log entry\n")

try:
    with open("new_file.txt", "x") as f:
        f.write("Created for the first time\n")
except FileExistsError:
    print("File already exists!")

with open("image.png", "rb") as f:
    data = f.read()
    print(type(data))   # <class 'bytes'>
    print(data[:8])     # First 8 bytes

with open("source.png", "rb") as src:
    with open("copy.png", "wb") as dst:
        dst.write(src.read())

with open("example.txt", "r") as f:
    print(f.tell())          # 0 — at the beginning
    f.read(5)                # Read 5 characters
    print(f.tell())          # 5 — cursor moved
    f.seek(0)                # Go back to start
    print(f.read())          # Read everything again

with open("example.txt", "r+") as f:
    content = f.read()
    f.seek(0)
    f.write("OVERWRITTEN start\n")

with open("example.txt", "w+") as f:
    f.write("Hello\n")
    f.seek(0)
    print(f.read())   # Hello

try:
    with open("missing.txt", "r") as f:
        content = f.read()
except FileNotFoundError:
    print("File does not exist.")
except PermissionError:
    print("No permission to access the file.")
except IsADirectoryError:
    print("Expected a file but got a directory.")
except OSError as e:
    print(f"OS error: {e}")

import os

import os

cwd = os.getcwd()
print(cwd)   # e.g. /Users/username/projects

os.chdir("/tmp")         # Change working directory
print(os.getcwd())       # /tmp

os.mkdir("new_folder")              # Creates one directory
os.makedirs("parent/child/sub")     # Creates nested directories

os.makedirs("logs/2024", exist_ok=True)

os.rmdir("empty_folder")            # Remove empty directory only

entries = os.listdir(".")           # Returns list of names (files + dirs)
for entry in entries:
    print(entry)

for dirpath, dirnames, filenames in os.walk("."):
    print(f"Directory: {dirpath}")
    for f in filenames:
        print(f"  File: {f}")

os.rename("old_name.txt", "new_name.txt")

os.remove("unwanted.txt")

print(os.path.exists("example.txt"))   # True or False
print(os.path.isfile("example.txt"))   # True if it's a file
print(os.path.isdir("myfolder"))       # True if it's a directory

size = os.path.getsize("example.txt")
print(f"Size: {size} bytes")

import os

path = "/Users/username/projects/app/main.py"

print(os.path.basename(path))      # main.py
print(os.path.dirname(path))       # /Users/username/projects/app
print(os.path.split(path))         # ('/Users/username/projects/app', 'main.py')
print(os.path.splitext(path))      # ('/Users/username/projects/app/main', '.py')

# Join paths safely (handles separators automatically)
full_path = os.path.join("folder", "subfolder", "file.txt")
print(full_path)                   # folder/subfolder/file.txt

# Absolute path
print(os.path.abspath("file.txt")) # /current/working/dir/file.txt

import os

# Get an environment variable (returns None if not set)
home = os.environ.get("HOME")
print(home)   # /Users/username

# Set an environment variable (only for current process)
os.environ["MY_VAR"] = "hello"
print(os.environ["MY_VAR"])   # hello

# List all environment variables
for key, value in os.environ.items():
    print(f"{key} = {value}")

import os

os.system("ls -la")            # Runs a shell command (UNIX)
os.system("dir")               # Windows equivalent

exit_code = os.system("echo Hello")
print(exit_code)               # 0 = success

import os

info = os.stat("example.txt")
print(info.st_size)            # Size in bytes
print(info.st_mtime)           # Last modification time (Unix timestamp)
print(info.st_ctime)           # Creation or metadata change time
print(info.st_mode)            # File permissions (octal)

import os
import datetime

info = os.stat("example.txt")
modified = datetime.datetime.fromtimestamp(info.st_mtime)
print(modified)   # e.g. 2024-04-28 09:15:42

def count_words(filename):
    try:
        with open(filename, "r", encoding="utf-8") as f:
            text = f.read()
            words = text.split()
            return len(words)
    except FileNotFoundError:
        return 0

print(count_words("essay.txt"))

def copy_file(src, dst):
    with open(src, "r", encoding="utf-8") as source:
        with open(dst, "w", encoding="utf-8") as destination:
            for line in source:
                destination.write(line)

copy_file("original.txt", "backup.txt")

import datetime

def log(message, filepath="app.log"):
    timestamp = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    with open(filepath, "a", encoding="utf-8") as f:
        f.write(f"[{timestamp}] {message}\n")

log("Application started")
log("User logged in")

import os

def list_python_files(directory):
    files = []
    for name in os.listdir(directory):
        if name.endswith(".py") and os.path.isfile(os.path.join(directory, name)):
            files.append(name)
    return files

print(list_python_files("."))

import os

def safe_write(filepath, content):
    directory = os.path.dirname(filepath)
    if directory:
        os.makedirs(directory, exist_ok=True)
    with open(filepath, "w", encoding="utf-8") as f:
        f.write(content)

safe_write("output/reports/summary.txt", "Report data here")

def read_config(filepath):
    config = {}
    with open(filepath, "r") as f:
        for line in f:
            line = line.strip()
            if "=" in line and not line.startswith("#"):
                key, value = line.split("=", 1)
                config[key.strip()] = value.strip()
    return config

# config.txt content:
# host = localhost
# port = 8080
cfg = read_config("config.txt")
print(cfg["host"])   # localhost