posix

Key Features

• Wraps the C library calls standardized by POSIX, such as open(), fork(), the exec family, stat(), and waitpid()
• Exposes the process environment at interpreter startup through the posix.environ dictionary, whose keys and values are bytes on Unix
• Defines the numeric constants used by those system calls, including file-mode flags, exit codes, and signal-related values
• Raises OSError when a system call fails and lets type errors propagate as usual

Frequently Used Classes and Functions

Examples

Reading a variable from the startup environment, which is keyed by bytes on Unix:

>>> import posix

>>> posix.environ[b"HOME"]
b'/home/realpython'

Listing the entries of the current directory using the POSIX name:

>>> import posix

>>> sorted(posix.listdir(b"."))[:3]
[b'README.md', b'pyproject.toml', b'src']

Calling the preferred portable equivalents from the os module, which forwards to posix on Unix:

>>> import os, posix

>>> os.getcwd is posix.getcwd
True
>>> os.getcwd()
'/home/realpython'

Common Use Cases

The most common reasons to care about posix include:

• Understanding how the os module delegates to operating system calls on Unix
• Reading POSIX-specific constants and error codes that appear in tracebacks
• Auditing or teaching how Python exposes low-level system calls
• Inspecting the raw bytes-keyed environ mapping that os.environ wraps on Unix

Real-World Example

Consider an audit script that lists the POSIX system calls an application touches indirectly through the os module:

import os
import posix

os_names = {name for name in dir(os) if not name.startswith("_")}
posix_names = {name for name in dir(posix) if not name.startswith("_")}

shared = sorted(os_names & posix_names)
os_only = sorted(os_names - posix_names)

print(f"Shared with posix: {len(shared)}")
print(f"Added by os only:  {len(os_only)}")
print("First shared:", shared[:3])
print("First os-only:", os_only[:3])

Running the script on a Linux system prints a small report:

$ python audit_posix.py
Shared with posix: 312
Added by os only:  58
First shared: ['CLD_CONTINUED', 'CLD_DUMPED', 'CLD_EXITED']
First os-only: ['GenericAlias', 'Mapping', 'MutableMapping']

The shared set is exactly the surface that os inherits from posix, while the os-only names are the portable extras the standard library layers on top.

Related Resources

Tutorial

Python's pathlib Module: Taming the File System

Handle file and folder paths the modern way with Python pathlib, a built-in module whose semantics work the same on every OS.

intermediate python stdlib

For additional information on related topics, take a look at the following resources:

• The subprocess Module: Wrapping Programs With Python (Tutorial)
• How to Get a List of All Files in a Directory With Python (Tutorial)
• Build a Python Directory Tree Generator for the Command Line (Tutorial)
• Speed Up Your Python Program With Concurrency (Tutorial)
• Reading and Writing Files in Python (Guide) (Tutorial)
• Using Python's pathlib Module (Course)
• Python's pathlib Module: Taming the File System (Quiz)
• Using the Python subprocess Module (Course)
• The subprocess Module: Wrapping Programs With Python (Quiz)
• Listing All Files in a Directory With Python (Course)
• How to Get a List of All Files in a Directory With Python (Quiz)
• Speed Up Python With Concurrency (Course)
• Python Concurrency (Quiz)
• Reading and Writing Files in Python (Course)
• Reading and Writing Files in Python (Quiz)