os - Operating System Interfaces

The os module provides a portable way to interact with the operating system, including file operations, process management, and environment variables.

Module Import

import os

File and Directory Operations

Current Directory

import os

# Get current working directory
cwd = os.getcwd()
print(cwd)  # '/home/user/project'

# Change directory
os.chdir('/tmp')

# Create directory
os.mkdir('new_folder')
os.makedirs('path/to/nested/folder')  # Create nested directories

# Remove directory
os.rmdir('empty_folder')  # Only removes empty directories
os.removedirs('path/to/folder')  # Remove nested empty directories

Listing Directory Contents

import os

# List all files and directories
contents = os.listdir('.')
print(contents)  # ['file1.txt', 'folder1', 'file2.py']

# List with full paths
for item in os.listdir('/path/to/dir'):
    full_path = os.path.join('/path/to/dir', item)
    print(full_path)

# Filter for specific files
python_files = [f for f in os.listdir('.') if f.endswith('.py')]

os.scandir() - Efficient Directory Iteration

import os

# More efficient than listdir for detailed info
with os.scandir('.') as entries:
    for entry in entries:
        if entry.is_file():
            print(f"File: {entry.name} ({entry.stat().st_size} bytes)")
        elif entry.is_dir():
            print(f"Directory: {entry.name}")

os.walk() - Recursive Directory Traversal

import os

# Walk through directory tree
for dirpath, dirnames, filenames in os.walk('.'):
    print(f"Current directory: {dirpath}")
    print(f"Subdirectories: {dirnames}")
    print(f"Files: {filenames}")
    print()

# Find all Python files recursively
for dirpath, _, filenames in os.walk('.'):
    for filename in filenames:
        if filename.endswith('.py'):
            full_path = os.path.join(dirpath, filename)
            print(full_path)

File Operations

Creating and Removing Files

import os

# Remove file
os.remove('file.txt')
os.unlink('file.txt')  # Alias for remove

# Rename/move file
os.rename('old_name.txt', 'new_name.txt')
os.replace('source.txt', 'dest.txt')  # Atomic replacement

File Information

import os
from datetime import datetime

# Get file stats
stats = os.stat('file.txt')
print(f"Size: {stats.st_size} bytes")
print(f"Modified: {datetime.fromtimestamp(stats.st_mtime)}")
print(f"Created: {datetime.fromtimestamp(stats.st_ctime)}")
print(f"Accessed: {datetime.fromtimestamp(stats.st_atime)}")

# Check if path exists
if os.path.exists('file.txt'):
    print("File exists")

# Check if it's a file or directory
if os.path.isfile('file.txt'):
    print("It's a file")

if os.path.isdir('folder'):
    print("It's a directory")

if os.path.islink('symlink'):
    print("It's a symbolic link")

Path Manipulation (os.path)

Joining Paths

import os

# Join path components (cross-platform)
path = os.path.join('folder', 'subfolder', 'file.txt')
print(path)  # 'folder/subfolder/file.txt' on Unix
             # 'folder\\subfolder\\file.txt' on Windows

# Build absolute path
abs_path = os.path.join(os.getcwd(), 'data', 'file.txt')

Path Components

import os

path = '/home/user/documents/file.txt'

# Split into directory and filename
dir_name, file_name = os.path.split(path)
print(dir_name)  # '/home/user/documents'
print(file_name)  # 'file.txt'

# Get directory name
dir_name = os.path.dirname(path)
print(dir_name)  # '/home/user/documents'

# Get filename
file_name = os.path.basename(path)
print(file_name)  # 'file.txt'

# Split extension
name, ext = os.path.splitext('file.txt')
print(name)  # 'file'
print(ext)  # '.txt'

Path Properties

import os

path = '/home/user/file.txt'

# Get absolute path
abs_path = os.path.abspath(path)

# Resolve relative path
real_path = os.path.realpath(path)  # Follows symlinks

# Expand user home directory
expanded = os.path.expanduser('~/documents')
print(expanded)  # '/home/user/documents'

# Check if path is absolute
if os.path.isabs(path):
    print("Absolute path")

Environment Variables

os.environ

Dictionary of environment variables.

import os

# Get environment variable
path = os.environ.get('PATH')
home = os.environ.get('HOME')
user = os.environ.get('USER', 'unknown')  # Default value

# Set environment variable
os.environ['MY_VAR'] = 'value'

# Delete environment variable
del os.environ['MY_VAR']

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

os.getenv() and os.putenv()

import os

# Get environment variable
value = os.getenv('PATH')
value = os.getenv('CUSTOM_VAR', 'default')  # With default

# Set environment variable (prefer os.environ)
os.putenv('MY_VAR', 'value')

Process Management

Current Process

import os

# Get process ID
pid = os.getpid()
print(f"Current PID: {pid}")

# Get parent process ID
ppid = os.getppid()
print(f"Parent PID: {ppid}")

Running Commands

import os

# Execute command (deprecated, use subprocess instead)
os.system('ls -la')

# Get command output (use subprocess.run instead)
# output = os.popen('ls').read()

# Better approach:
import subprocess
result = subprocess.run(['ls', '-la'], capture_output=True, text=True)
print(result.stdout)

File Descriptors

Low-Level File Operations

import os

# Open file (returns file descriptor)
fd = os.open('file.txt', os.O_RDWR | os.O_CREAT)

# Read from file descriptor
data = os.read(fd, 1024)  # Read up to 1024 bytes

# Write to file descriptor
os.write(fd, b'Hello, World!')

# Close file descriptor
os.close(fd)

File Descriptor Operations

import os

# Duplicate file descriptor
fd2 = os.dup(fd)

# Change file position
os.lseek(fd, 0, os.SEEK_SET)  # Beginning
os.lseek(fd, 0, os.SEEK_END)  # End
os.lseek(fd, 10, os.SEEK_CUR)  # Current + 10

Permissions and Ownership

File Permissions

import os
import stat

# Change file permissions
os.chmod('file.txt', 0o644)  # rw-r--r--
os.chmod('script.sh', 0o755)  # rwxr-xr-x

# Using stat constants
os.chmod('file.txt', stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)

# Check permissions
stats = os.stat('file.txt')
mode = stats.st_mode
if mode & stat.S_IXUSR:
    print("File is executable by owner")

Ownership (Unix only)

import os

# Change owner (requires privileges)
os.chown('file.txt', uid, gid)

Platform-Specific

Path Separators

import os

print(os.sep)  # '/' on Unix, '\\' on Windows
print(os.pathsep)  # ':' on Unix, ';' on Windows
print(os.linesep)  # '\n' on Unix, '\r\n' on Windows

# Platform name
print(os.name)  # 'posix', 'nt', 'java'

Special Paths

import os

print(os.devnull)  # '/dev/null' on Unix, 'nul' on Windows
print(os.curdir)  # '.'
print(os.pardir)  # '..'

Practical Examples

Find All Files with Extension

import os

def find_files(directory, extension):
    """Find all files with given extension recursively"""
    matches = []
    for root, dirs, files in os.walk(directory):
        for file in files:
            if file.endswith(extension):
                matches.append(os.path.join(root, file))
    return matches

# Usage
python_files = find_files('.', '.py')
for file in python_files:
    print(file)

Get Directory Size

import os

def get_directory_size(directory):
    """Calculate total size of directory in bytes"""
    total_size = 0
    for dirpath, dirnames, filenames in os.walk(directory):
        for filename in filenames:
            filepath = os.path.join(dirpath, filename)
            if os.path.isfile(filepath):
                total_size += os.path.getsize(filepath)
    return total_size

# Usage
size = get_directory_size('.')
print(f"Directory size: {size / (1024**2):.2f} MB")

Create Backup with Timestamp

import os
from datetime import datetime
import shutil

def backup_file(filepath):
    """Create timestamped backup of file"""
    if not os.path.exists(filepath):
        return None
    
    timestamp = datetime.now().strftime('%Y%m%d_%H%M%S')
    name, ext = os.path.splitext(filepath)
    backup_path = f"{name}_{timestamp}{ext}"
    
    shutil.copy2(filepath, backup_path)
    return backup_path

# Usage
backup = backup_file('important.txt')
print(f"Backup created: {backup}")

Safe File Operations

import os
import tempfile

def safe_write(filepath, content):
    """Write to file atomically using temp file"""
    # Get directory and create temp file in same location
    directory = os.path.dirname(filepath) or '.'
    
    # Write to temporary file
    fd, temp_path = tempfile.mkstemp(dir=directory, text=True)
    try:
        with os.fdopen(fd, 'w') as f:
            f.write(content)
        
        # Atomic replacement
        os.replace(temp_path, filepath)
    except:
        # Clean up temp file on error
        if os.path.exists(temp_path):
            os.remove(temp_path)
        raise

# Usage
safe_write('config.txt', 'new configuration')

Best Practices

Use pathlib for modern path handling: For new code, consider using pathlib.Path instead of os.path:

from pathlib import Path

path = Path('folder') / 'subfolder' / 'file.txt'
if path.exists():
    content = path.read_text()

Be careful with os.remove(): It permanently deletes files without moving them to trash. Always verify the path before removing.

if os.path.exists(filepath) and os.path.isfile(filepath):
    os.remove(filepath)

Use os.path.join() for cross-platform paths: Always use os.path.join() instead of string concatenation with / or \\.

# Good
path = os.path.join('folder', 'file.txt')

# Bad
path = 'folder' + '/' + 'file.txt'

Migration to pathlib

Many os and os.path functions have pathlib equivalents:

import os
from pathlib import Path

# os module → pathlib
os.getcwd()              # Path.cwd()
os.path.exists(path)     # Path(path).exists()
os.path.isfile(path)     # Path(path).is_file()
os.path.isdir(path)      # Path(path).is_dir()
os.listdir(path)         # list(Path(path).iterdir())
os.mkdir(path)           # Path(path).mkdir()
os.rename(old, new)      # Path(old).rename(new)
os.remove(path)          # Path(path).unlink()
os.rmdir(path)           # Path(path).rmdir()

pathlib

Object-oriented filesystem paths

shutil

High-level file operations

glob

Unix-style pathname pattern expansion

tempfile

Temporary files and directories

​Module Import

​File and Directory Operations

​Current Directory

​Listing Directory Contents

​os.scandir() - Efficient Directory Iteration

​os.walk() - Recursive Directory Traversal

​File Operations

​Creating and Removing Files

​File Information

​Path Manipulation (os.path)

​Joining Paths

​Path Components

​Path Properties

​Environment Variables

​os.environ

​os.getenv() and os.putenv()

​Process Management

​Current Process

​Running Commands

​File Descriptors

​Low-Level File Operations

​File Descriptor Operations

​Permissions and Ownership

​File Permissions

​Ownership (Unix only)

​Platform-Specific

​Path Separators

​Special Paths

​Practical Examples

​Find All Files with Extension

​Get Directory Size

​Create Backup with Timestamp

​Safe File Operations

​Best Practices

​Migration to pathlib

​Related Modules