os Module Complexity

The os module provides a way to use operating system-dependent functionality like reading or writing to the file system.

File Operations

Operation	Time	Space	Notes
os.path.exists(path)	O(1)	O(1)	Check if path exists
os.path.isfile(path)	O(1)	O(1)	Check if file
os.path.isdir(path)	O(1)	O(1)	Check if directory
os.listdir(path)	O(n)	O(n)	List directory contents
os.scandir(path)	O(n)	O(1)	Lazy directory iterator
os.walk(path)	O(n)	O(d)	n = total entries, d = max depth (call stack + pending dirs)
os.stat(path)	O(1)	O(1)	Get file statistics
os.lstat(path)	O(1)	O(1)	Stat without following symlink
os.remove(path)	O(1)	O(1)	Delete file
os.mkdir(path)	O(1)	O(1)	Create directory
os.makedirs(path)	O(n)	O(1)	Create directories, n = depth
os.rmdir(path)	O(1)	O(1)	Remove empty directory
os.rename(src, dst)	O(1)	O(1)	Rename/move file
os.chmod(path, mode)	O(1)	O(1)	Change permissions

Path Operations

Operation	Time	Space	Notes
os.path.join(paths)	O(n)	O(n)	Join path components, n = total length
os.path.split(path)	O(n)	O(n)	Split head and tail
os.path.dirname(path)	O(n)	O(n)	Get directory part
os.path.basename(path)	O(n)	O(n)	Get filename part
os.path.splitext(path)	O(n)	O(n)	Split extension
os.path.abspath(path)	O(n)	O(n)	Get absolute path
os.path.realpath(path)	O(n)	O(n)	Resolve symlinks
os.path.normpath(path)	O(n)	O(n)	Normalize path

Common Operations

Checking File Existence

import os

# Fast existence check - O(1)
if os.path.exists("/path/to/file"):
    print("File exists")

# Check type - O(1)
if os.path.isfile("/path/to/file"):
    print("Is a file")

if os.path.isdir("/path/to/dir"):
    print("Is a directory")

Listing Files

import os

# Get all files - O(n) where n = number of items
files = os.listdir("/path/to/dir")

# Better: lazy iteration - O(1) per item
with os.scandir("/path/to/dir") as entries:
    for entry in entries:  # O(1) each
        print(entry.name)
        print(entry.is_file())  # O(1)

Walking Directory Tree

import os

# Walk recursively - O(n) where n = total files
for dirpath, dirnames, filenames in os.walk("/path/to/root"):
    # Process files
    for filename in filenames:
        path = os.path.join(dirpath, filename)
        # Do something - O(1) per file

File Metadata

import os

# Get file stats - O(1)
stats = os.stat("/path/to/file")
print(stats.st_size)      # File size
print(stats.st_mtime)     # Modification time
print(stats.st_mode)      # Permissions

# Check if readable - O(1)
if os.access("/path/to/file", os.R_OK):
    print("Readable")

Path Manipulation

import os

# Join paths - O(n)
path = os.path.join("/home", "user", "documents", "file.txt")

# Split path - O(n)
dirname, filename = os.path.split(path)
# dirname = "/home/user/documents"
# filename = "file.txt"

# Get extension - O(n)
name, ext = os.path.splitext("file.txt")
# name = "file"
# ext = ".txt"

# Get absolute path - O(n)
abs_path = os.path.abspath("./relative/path")

Creating and Removing

import os

# Create directory - O(1)
os.mkdir("/new/dir")

# Create all parent dirs - O(n) where n = depth
os.makedirs("/deep/nested/dir/structure")

# Remove file - O(1)
os.remove("/path/to/file")

# Remove empty dir - O(1)
os.rmdir("/empty/dir")

# Remove tree (careful!) - O(n)
import shutil
shutil.rmtree("/path/to/remove")

Environment Variables

import os

# Get environment - O(1)
home = os.environ['HOME']

# Get with default - O(1)
user = os.environ.get('USER', 'unknown')

# Set environment - O(1)
os.environ['MY_VAR'] = 'value'

# All environment variables - O(m) where m = number of vars
all_vars = os.environ.items()

Process Information

import os

# Current process ID - O(1)
pid = os.getpid()

# Parent process ID - O(1)
ppid = os.getppid()

# Current working directory - O(1)
cwd = os.getcwd()

# Change working directory - O(1)
os.chdir("/new/directory")

Performance Notes

Efficient File Listing

import os

# Less efficient - returns all names, no file type info
files = os.listdir(".")  # Must separate files/dirs separately

# More efficient - lazy, includes file type info
with os.scandir(".") as entries:
    files = [e.name for e in entries if e.is_file()]

Avoiding Unnecessary Stats

import os

path = "/path/to/file"

# Bad: two stat calls
if os.path.exists(path) and os.path.getsize(path) > 0:
    pass

# Better: one stat call
try:
    size = os.path.getsize(path)  # O(1)
    if size > 0:
        pass
except FileNotFoundError:
    pass

Version Notes

• Python 2.x: Basic functionality, limited Unicode support
• Python 3.x: Full Unicode path support
• Python 3.5+: os.scandir() is faster for directory iteration
• Python 3.10+: More type hints, better cross-platform support

Platform Differences

• POSIX (Linux, macOS): Full feature set
• Windows: Some path operations differ (backslashes vs forward slashes)
• Path handling: Use os.path.join() or better: pathlib.Path

Related Modules

• pathlib - Object-oriented path handling (recommended)
• glob - Unix-style pathname expansion
• shutil - High-level file operations
• tempfile - Temporary files

Best Practices

✅ Do:

• Use os.path.join() for cross-platform compatibility
• Use os.scandir() for directory listing (faster)
• Use pathlib.Path for modern code (more readable)
• Check existence before accessing files

❌ Avoid:

• Assuming forward slashes work on all platforms
• Using string concatenation for paths
• Multiple stat calls when one suffices
• Hardcoding paths (use environment variables)