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What are environment variables?

Environment variables are settings stored outside your code. Think of them as configuration that your program can read.

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The problem they solve

# BAD: Never hardcode sensitive data!
api_key = "sk-1234567890abcdef"  # Everyone can see this!
database = "production_database"  # Can't change without editing code
Problems:
  • Secrets visible in code
  • Can’t share code safely
  • Different settings for different computers

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How environment variables work

Environment variables live in your system, not your code: 
import os

# Read from environment
api_key = os.environ.get('API_KEY')
database = os.environ.get('DATABASE_NAME', 'default.db')

print(f"Using database: {database}")

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Setting environment variables

You can set them temporarily in your terminal: 
# Mac/Linux
export API_KEY=sk-1234567890abcdef
python app.py

# Windows
set API_KEY=sk-1234567890abcdef
python app.py
But they disappear when you close the terminal!

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Reading in Python

import os

# Method 1: Get with default
api_key = os.environ.get('API_KEY', 'demo-key')

# Method 2: Check if exists
if 'API_KEY' in os.environ:
    api_key = os.environ['API_KEY']
else:
    print("No API key found")

# Method 3: Will crash if not found
api_key = os.environ['API_KEY']  # KeyError if missing!

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Common uses

  • API Keys: OPENAI_API_KEY, GITHUB_TOKEN
  • Database URLs: DATABASE_URL, REDIS_URL
  • App Settings: DEBUG=True, PORT=8000
  • File Paths: LOG_DIR, UPLOAD_FOLDER

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Important notes

  • Environment variables are always strings
  • Names are UPPERCASE by convention
  • They’re different on each computer
  • They don’t persist between terminal sessions

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What’s next?

Setting variables manually is tedious. Let’s use .env files for a better workflow.

Using .env files

Easier way to manage variables