os.environ / os.getenv()

import os

# Returns all environment variables as a dictionary-like object
os.environ

# Returns the value of an environment variable (raises KeyError if not found)
os.environ['VARIABLE_NAME']

# Returns the value of an environment variable (returns a default value if not found)
os.getenv('VARIABLE_NAME', default_value)

# Sets an environment variable (affects the current process only)
os.environ['VARIABLE_NAME'] = 'value'

os_environ_basic.py

import os

path = os.environ['PATH']
print(f'PATH: {path}')

db_host = os.getenv('DB_HOST', 'localhost')
db_port = os.getenv('DB_PORT', '5432')
print(f'DB connection: {db_host}:{db_port}')

if 'API_KEY' in os.environ:
    api_key = os.environ['API_KEY']
    print(f'API key is configured.')
else:
    print('API_KEY is not set.')

Running the code produces the following output:

python3 os_environ_basic.py
PATH: /usr/local/bin:/usr/bin:/bin
DB connection: localhost:5432
API_KEY is not set.

os_environ_required.py

import os

def get_required_env(name):
    value = os.getenv(name)
    if value is None:
        raise ValueError(f'Environment variable {name} is not set.')
    return value

try:
    secret = get_required_env('SECRET_KEY')
    print(f'SECRET_KEY: {secret}')
except ValueError as e:
    print(e)

os.environ['MY_APP_DEBUG'] = 'true'
print(os.getenv('MY_APP_DEBUG'))

Running the code produces the following output:

python3 os_environ_required.py
Environment variable SECRET_KEY is not set.
true

os_environ_list.py

import os

for key, value in os.environ.items():
    print(f'{key}={value}')

Running the code produces the following output:

python3 os_environ_list.py
TERM=xterm-256color
SHELL=/bin/zsh
HOME=/Users/user
PATH=/usr/local/bin:/usr/bin:/bin
...

Common Mistake 1: Accessing a missing variable with os.environ[]

os.environ['VARIABLE_NAME'] raises a KeyError when the variable does not exist. For variables that may not be set, using os.getenv() avoids the KeyError.

import os

api_key = os.environ['API_KEY']
print(api_key)

The same logic can also be written as:

import os

api_key = os.getenv('API_KEY', 'default_key')
print(api_key)

Common Mistake 2: Committing a .env file to Git

Including a .env file containing sensitive information such as API keys or passwords in a Git repository can lead to a data breach. Adding it to .gitignore keeps it out of version control.

requirements.txt

The same logic can also be written as:

.env
requirements.txt

Environment variables are a mechanism for controlling program behavior from outside the code. They let you manage database connection details, API keys, and environment flags (such as switching between production and development) without hardcoding them.

For managing environment variables in production, the python-dotenv library is a popular choice. A common pattern is to define variables in a .env file at the project root and load them with load_dotenv().

Always add any .env file containing sensitive information — such as API keys or passwords — to .gitignore to keep it out of version control. Accidentally committing it to a repository can lead to a data breach.

For reading command-line arguments and system information, see 'sys.argv / sys.exit() / sys.path'.