Codectionary / Developer documentation / Python

os Module

The os module provides functions for interacting with the operating system - working with file paths, listing directory contents, creating and removing folders, and reading environment variables. Its os.path submodule (or the more modern pathlib alternative) handles cross-platform file path manipulation, so your code works correctly on Windows, macOS, and Linux without hardcoding path separators.

Syntax

import os

Examples

Working with Paths

Joining and inspecting file paths in a cross-platform way.

import os

path = os.path.join("data", "reports", "summary.csv")
print(path)  # data/reports/summary.csv (or data\\reports\\summary.csv on Windows)

print(os.path.exists(path))   # True or False
print(os.path.basename(path)) # summary.csv
print(os.path.dirname(path))  # data/reports

Listing Directory Contents

Getting a list of files and folders within a directory.

import os

files = os.listdir(".")  # current directory
print(files)

for name in os.listdir("."):
    if os.path.isfile(name):
        print(f"File: {name}")
    elif os.path.isdir(name):
        print(f"Directory: {name}")

Environment Variables

Reading environment variables, commonly used for configuration like API keys.

import os

api_key = os.environ.get("API_KEY", "default-key")
print(api_key)

# Setting an environment variable for the current process
os.environ["DEBUG"] = "true"

Creating and Removing Directories

Managing folders on the filesystem.

import os

os.makedirs("output/reports", exist_ok=True)  # creates nested dirs, no error if they exist
print(os.path.exists("output/reports"))  # True

os.rmdir("output/reports")  # removes an empty directory

Best practices

  • Use os.path.join() (or pathlib) instead of manually concatenating strings with "/" or "\\" for cross-platform compatibility
  • Use os.environ.get(key, default) rather than os.environ[key] when a variable might not be set, to avoid a KeyError
  • Use os.makedirs(path, exist_ok=True) instead of os.mkdir() when a path might have missing parent directories
  • Consider pathlib.Path for new code - it offers a more modern, object-oriented API that many developers find more readable than os.path

At a glance

Purpose
Scripting and general-purpose applications
File extension
.py
Runs in
Python interpreter
Usually used with
Python standard library and packages

Specifications & further reading

Related Python documentation

import
The import statement brings code from another module or package into the current file, letting you reuse functions, classes, and variables defined elsewhere - whether from Python's standard library, a third-party package, or your own project files. Python offers several import styles: importing an entire module, importing specific names directly, and renaming imports with 'as' to avoid naming conflicts or shorten long module names.if __name__ == '__main__'
Every Python module has a built-in __name__ variable. When a file is run directly, __name__ is set to '__main__'; when the same file is imported into another module, __name__ is set to the module's actual name instead. The if __name__ == '__main__': guard lets you write code that only runs when the file is executed directly, not when it's imported elsewhere - essential for writing reusable modules that also work as standalone scripts.File Handling
Python's built-in open() function reads from and writes to files on disk. Using it with the 'with' statement (a context manager) ensures the file is automatically closed when the block ends, even if an error occurs - this is the recommended way to work with files. The mode argument controls behavior: 'r' for reading, 'w' for writing (overwriting), 'a' for appending, among others.datetime Module
The datetime module provides classes for working with dates and times - creating them, formatting them for display, parsing them from text, and performing date arithmetic. The datetime class combines both a date and a time, while timedelta represents a duration, letting you add or subtract time spans from a date. strftime() formats a datetime as text, and strptime() parses text back into a datetime.