Automation and Scripting / Phase 1: OS & File System Automation

The os Module - Navigating Directories

Use os to inspect directories, change the working directory, and work with environment variables.

1 min readupdated 6 Oct 2026

diagram os.walk hands you a list you are meant to edit mermaid

  flowchart TD
  A["os.walk('.')"] --> B["yield (root, dirs, files)"]
  B --> C["your loop body runs"]
  C --> D{"did you modify dirs IN PLACE?"}
  D -->|"dirs.remove('b')"| E["that subtree is never visited"]
  D -->|"dirs = [...] -- rebinding"| F["no effect; the walker still has the old list"]
  D -->|no| G["descend into every subdirectory"]
  E --> H["next root"]
  G --> H
  H --> B
  I["topdown=False"] --> J["children first -- pruning no longer works"]

Each step yields the current directory, its subdirectories and its files. The subdirectory list is not just information -- with the default top-down walk, os.walk reads it again after your loop body to decide where to go next. Removing an entry from it in place is how you skip a whole subtree, which is the difference between a script that finishes and one that walks into node_modules.

Pruning works, but only in place and only top-down

Verified on a tree of top.txt, a/1.txt, a/b/2.txt, a/b/c/3.txt:

Python

for root, dirs, files in os.walk("."):
    if "b" in dirs:
        dirs.remove("b")     # in place

The walk visited . and ./a, then stopped — a/b and a/b/c were never entered.

Two conditions, and both matter:

It has to be in place. dirs.remove(...), dirs[:] = [...] or del dirs[i] all work. Writing dirs = [d for d in dirs if ...] rebinds a local name, the walker keeps its own list, and the filter silently does nothing.

It only works with topdown=True, the default. With topdown=False the children have already been walked by the time you see the list, so editing it changes nothing.

This is the standard way to skip .git, node_modules or __pycache__, and it is much faster than walking them and discarding the results.

What `os` is for

The os module provides:

current working directory
directory listing
environment variables
basic path manipulation (but prefer pathlib for paths)

Current working directory

Python

import os
 
print(os.getcwd())

List files

Python

import os
 
for name in os.listdir("."):
    print(name)

Change directory

Python

import os
 
os.chdir("/tmp")
print(os.getcwd())

Environment variables

Python

import os
 
print(os.environ.get("HOME"))

Notes

For paths, use pathlib.Path whenever possible.

🧪 Try It Yourself

Exercise 1 – List Files with os.listdir

exerciseYour turn

script.py

Output

Press Run to see what your code prints.

Exercise 2 – Join Paths with os.path.join

exerciseYour turn

script.py

Output

Press Run to see what your code prints.

Exercise 3 – Write and Read a File

exerciseYour turn

script.py

Output

Press Run to see what your code prints.

The os Module - Navigating Directories

What os is for

Current working directory

List files

Change directory

Environment variables

Notes

🧪 Try It Yourself

Exercise 1 – List Files with os.listdir

Exercise 2 – Join Paths with os.path.join

Exercise 3 – Write and Read a File

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What `os` is for