environment-variables.md

Python by Example: Environment Variables

Environment variables are key-value pairs set by the OS or shell. They're used for configuration (API keys, paths, feature flags) without hardcoding. Access them with os.environ. Use .get() with a default when the variable might be missing; use os.environ["VAR"] for required variables (it raises if missing).

What you'll learn:

• Reading with os.environ.get()
• Setting variables in the current process
• Platform differences (e.g., HOME vs USERPROFILE)

import os

# Safe read with default
home = os.environ.get("HOME", os.environ.get("USERPROFILE", "/unknown"))
print("HOME:", home)

# Get PATH
path = os.environ.get("PATH", "")
print("PATH has", len(path.split(os.pathsep)), "entries")

# Set for current process
os.environ["MY_VAR"] = "hello"
print("MY_VAR:", os.environ.get("MY_VAR"))

On Windows, HOME may be unset; USERPROFILE is the equivalent. os.pathsep is : on Unix and ; on Windows.

To run this program:

$ python source/environment-variables.py
HOME: /home/user          # varies by OS; on Windows may use USERPROFILE
PATH has 42 entries       # varies by system
MY_VAR: hello

Output varies by OS and environment.

Tip: Never commit secrets (API keys, passwords) to code. Use environment variables or a secrets manager.

Try it: Set an env var in your shell (export MY_VAR=test), then read it in a script.

Source: environment-variables.py

Next: Testing