KeyError: Missing Environment Variable

Why This Happens

os.environ behaves like a dictionary and raises KeyError for missing keys. Using os.environ.get() with a default value prevents the crash.

The Problem

import os
db_url = os.environ['DATABASE_URL']

The Fix

import os
db_url = os.environ.get('DATABASE_URL', 'sqlite:///local.db')

Step-by-Step Fix

1. 1

   Use os.environ.get()

   Replace os.environ['KEY'] with os.environ.get('KEY', default).

2. 2

   Set the variable

   Set it before running: export DATABASE_URL=... or use python-dotenv.

3. 3

   Provide clear errors

   If required, check at startup and give a helpful error message.