Python OS, Sys & Pathlib Guide

The os module provides a portable, platform-independent interface to interact with the underlying operating system (Windows, macOS, Linux):

• Environment Variables: os.environ (a dictionary mapping OS environment keys like API_KEY, PATH, USER). Always use os.getenv("KEY", "fallback") for safe access.
• Working Directory: os.getcwd() (get current directory), os.chdir(path) (change directory).
• Directory Management: os.mkdir(path) (create single folder), os.makedirs(path, exist_ok=True) (create nested directory trees safely).
• Listing & Deleting: os.listdir(path), os.remove(file), os.rmdir(empty_dir).

# Inspecting OS environment and directory state:
import os

print("Operating System Name:", os.name) # 'nt' for Windows, 'posix' for Linux/macOS
print("Current Working Directory:", os.getcwd())

# Safe environment variable lookup with getenv:
db_user = os.getenv("DB_USER", "default_admin")
print("Database User:", db_user)

# Inspecting directory contents:
dir_files = [f for f in os.listdir(".") if not f.startswith(".")]
print(f"Top 5 files in current directory: {dir_files[:5]}")

The sys module provides access to variables and functions that interact directly with the CPython interpreter itself:

• sys.argv: A list of command-line argument strings passed to the script during terminal invocation (where sys.argv[0] is the script name).
• sys.exit([code]): Immediately terminates the Python process (exit code 0 signifies clean success, non-zero signifies an error).
• sys.version / sys.version_info: The exact Python version string and tuple.
• sys.platform: Identifies the OS platform ("win32", "darwin" for macOS, "linux").
• sys.getsizeof(object): Returns the exact memory footprint of an object in bytes.

import sys

print(f"🐍 Python Runtime Version: {sys.version.split()[0]}")
print(f"💻 OS Platform Identifier: {sys.platform}")
print(f"📥 Command-line Arguments (sys.argv): {sys.argv}")

# Inspecting memory size of different Python objects in bytes:
int_size = sys.getsizeof(100)
str_size = sys.getsizeof("Hello, World!")
list_size = sys.getsizeof([1, 2, 3, 4, 5])

print(f"\nMemory Footprint:")
print(f"• Integer (100):      {int_size} bytes")
print(f"• String ('Hello..'): {str_size} bytes")
print(f"• List ([1..5]):      {list_size} bytes")

Historically, Python developers manipulated file paths as raw strings using os.path.join(). In modern Python 3.4+ (PEP 428), the standard approach is pathlib.Path, which treats file paths as rich, first-class Python objects with cross-platform slash (/) joining:

Path Object Methods:

• path.exists(): Returns True if file or directory exists.
• path.is_file() / path.is_dir(): Type checks.
• path.name: Full filename ("report.csv").
• path.stem: Filename without extension ("report").
• path.suffix: File extension (".csv").
• path.read_text(encoding="utf-8"): Reads entire text file in one line!
• path.write_text(content, encoding="utf-8"): Writes string to file in one line!

from pathlib import Path

# 1. Building paths with the / operator:
data_folder = Path("my_project") / "data"
config_file = data_folder / "app_config.json"

print("Path Object:", config_file)
print("Filename (name):", config_file.name)
print("Filename stem:  ", config_file.stem)
print("Extension:      ", config_file.suffix)
print("Parent Folder:  ", config_file.parent)

# 2. Writing and reading text files in ONE line with pathlib:
test_file = Path("demo_note.txt")
test_file.write_text("Hello from Python pathlib! 🚀", encoding="utf-8")

file_content = test_file.read_text(encoding="utf-8")
print(f"\nRead file content: '{file_content}'")

# Clean up test file:
if test_file.exists():
    test_file.unlink() # Delete file
    print("Cleaned up demo_note.txt ✅")

Search and iterate through files matching pattern expressions using Globbing:

• path.glob("*.py"): Finds matching files in the current folder.
• path.rglob("*.py"): Recursive Glob — searches the current folder AND all nested sub-folders at any depth!

from pathlib import Path

current_dir = Path(".")

# Find all HTML files in current folder:
html_files = list(current_dir.glob("*.html"))
print(f"Found {len(html_files)} HTML files in current root.")

# Inspect first 3 files:
for f in html_files[:3]:
    print(f"• File: {f.name:25} | Size: {f.stat().st_size} bytes")