Environment Variables in Python

Reading Environment Variables

Python provides environment variable access through the os module. The os.environmapping contains all environment variables as strings. Use dictionary-style access or the os.getenv()function:

import os

# Raises KeyError if not set
db_url = os.environ['DATABASE_URL']

# Returns None if not set (or a default)
log_level = os.getenv('LOG_LEVEL', 'info')

# Check if a variable exists
if 'API_KEY' in os.environ:
    api_key = os.environ['API_KEY']

The difference between os.environ['KEY'] and os.getenv('KEY') is error handling: the former raises KeyError for missing variables, while the latter returns None(or your default). Use os.environ[] for required variables and os.getenv() for optional ones.

Setting Environment Variables

Set variables in the shell before running your script, or modify os.environ at runtime:

# Shell
export DATABASE_URL=postgres://localhost/mydb
python app.py

# In Python
os.environ['MY_VAR'] = 'value'
del os.environ['MY_VAR']  # Remove a variable

Popular Libraries

python-dotenv loads variables from a .env file into os.environ. Call load_dotenv() at the start of your application:

from dotenv import load_dotenv
load_dotenv()  # loads .env from current directory

db_url = os.getenv('DATABASE_URL')

pydantic-settings (part of the Pydantic ecosystem) provides typed, validated settings classes that read from environment variables automatically:

from pydantic_settings import BaseSettings

class Settings(BaseSettings):
    database_url: str
    port: int = 3000
    debug: bool = False

    class Config:
        env_file = '.env'

settings = Settings()
# settings.database_url is validated and typed

environs is a simpler alternative that provides type casting and validation without needing a full settings class.

Common Gotchas

• All environment variable values are strings. You must explicitly cast to int,float, or bool. Be careful with boolean parsing — bool('false')returns True in Python because it is a non-empty string.
• os.environ is a mutable mapping. Changes persist for the lifetime of the process and are inherited by child processes, but do not affect the parent shell.
• In Django, environment variables are commonly used in settings.py. Make sureload_dotenv() runs before Django imports settings, typically in manage.pyor wsgi.py.