您應該如何使用環境變量配置Python應用程序？

使用環境變量配置Python應用，首先在開發時通過python-dotenv加載.env文件中的變量，1. 使用os.getenv()並提供默認值避免錯誤，2. 按環境（如開發、生產）結構化配置，3. 生產環境中通過部署平台設置環境變量，4. 始終將環境變量作為配置的唯一來源，確保敏感信息不進入代碼或版本控制，應用行為僅由環境決定。

You should use environment variables to configure a Python application by loading them at runtime to keep configuration separate from code, especially for settings that vary between environments (like development, staging, and production). This approach follows the twelve-factor app methodology and helps keep sensitive data like API keys and database URLs out of version control.

Use a .env file during development

During development, it's common to use a .env file to store environment variables locally. This file is listed in .gitignore so it's not committed to version control. You can use the python-dotenv package to load these variables:

 pip install python-dotenv

Create a .env file in your project root:

 DATABASE_URL=postgresql://user:pass@localhost:5432/mydb
SECRET_KEY=your-secret-key
DEBUG=True

Then load it in your Python code:

 from dotenv import load_dotenv
import os

load_dotenv() # Load environment variables from .env

database_url = os.getenv("DATABASE_URL")
secret_key = os.getenv("SECRET_KEY")
debug = os.getenv("DEBUG", "False").lower() == "true"

Note : Never commit .env files with secrets to Git. Use a .env.example file to show required variables (without values) for onboarding new developers.

Access variables using os.getenv() with defaults

Always use os.getenv() instead of os.environ[] to avoid KeyError exceptions. Provide sensible defaults when possible:

 import os

port = int(os.getenv("PORT", 8000))
debug = os.getenv("DEBUG", "False").lower() == "true"
allowed_hosts = os.getenv("ALLOWED_HOSTS", "localhost,127.0.0.1").split(",")

This makes your app more resilient and easier to run in different environments without requiring every variable to be set.

Use environment-specific configuration

Structure your configuration based on the current environment. For example:

 import os

env = os.getenv("ENV", "development")

if env == "production":
    database_url = os.getenv("DATABASE_URL")
    debug = False
elif env == "staging":
    database_url = os.getenv("DATABASE_URL")
    debug = True
else:
    database_url = "sqlite:///dev.db"
    debug = True

Alternatively, use a config class pattern:

 class Config:
    SECRET_KEY = os.getenv("SECRET_KEY", "dev-secret")
    DATABASE_URL = os.getenv("DATABASE_URL")

class DevelopmentConfig(Config):
    DEBUG = True

class ProductionConfig(Config):
    DEBUG = False

config = {
    "development": DevelopmentConfig,
    "production": ProductionConfig
}[os.getenv("ENV", "development")]

Set environment variables in production

In production, don't rely on .env files. Instead, set environment variables through your deployment platform:

• Docker : Use environment in docker-compose.yml or ENV in Dockerfile
• Kubernetes : Use env in pod spec or ConfigMaps/Secrets
• Cloud platforms (eg, Heroku, AWS, GCP) : Use platform-specific config commands or UI
• System level : Set in shell (eg, export DATABASE_URL=... ) or systemd service files

For example, on Heroku:

 heroku config:set DATABASE_URL=your-prod-db-url

Or in Docker:

 # docker-compose.yml
environment:
  - DATABASE_URL=postgresql://...
  - DEBUG=False

Summary

• Use python-dotenv and .env files only for development
• Always use os.getenv() with fallbacks to avoid crashes
• Keep secrets out of code and version control
• Structure config by environment
• Set real environment variables in production via deployment tools

Basically, treat environment variables as the single source of truth for configuration—your app should behave differently based solely on its environment, not on hardcoded values.

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