Python 'KeyError' when reading an environment variable with os.environ['NAME'], or getenv returning None because a .env file was not loaded: how do I fix it?

Cause: os.environ is a mapping, so indexing a variable that is not set raises KeyError: 'NAME'. os.getenv('NAME') returns None instead, which then fails later (for example in int(None) or a string concatenation). Common reasons the variable is missing: it is set in a .env file but nothing loaded it; the code reads the variable before load_dotenv() runs (for example at import time of another module); the process was started by a service manager, IDE or container with a different environment than your shell; or the shell exported it after the process started. os.environ is captured when the os module is first imported, and changes made outside Python afterwards are not reflected. Fix (best first): 1. Decide whether the variable is required. For required settings, fail clearly: value = os.environ.get('NAME'); if not value: raise RuntimeError('NAME is not set'). For optional ones, use os.getenv('NAME', 'default'). Remember environment values are always strings, so convert numbers and booleans yourself. 2. If you use a .env file with python-dotenv, call load_dotenv() once at the very start of the program, before modules that read configuration are imported. By default it does not override variables already present in the real environment; pass override=True only if you want the file to win. dotenv_values() returns a dict without touching os.environ. 3. Check the actual environment of the process: print(sorted(os.environ)) (never log secret values), and check how it is launched (systemd Environment=/EnvironmentFile=, docker run -e or --env-file, CI secrets configuration). 4. Set variables in Python through os.environ['NAME'] = 'value', not os.putenv directly, since putenv does not update os.environ. 5. Keep .env files out of version control and never commit real secrets. Verify: python -c "import os; print('NAME' in os.environ)" in the same launch context prints True, and the application starts without the KeyError. Sources: https://docs.python.org/3/library/os.html https://saurabh-kumar.com/python-dotenv/