How to Use Environment Variables in Flask for Configuration

In Flask, you can access environment variables using the os.environ dictionary from Python's os module. This lets you keep sensitive data like API keys or configuration settings outside your code by setting them in your system or .env files and reading them in your Flask app.

Syntax

To use environment variables in Flask, first import the os module. Then access variables with os.environ.get('VARIABLE_NAME'). This returns the value of the environment variable or None if it is not set.

You can also provide a default value like os.environ.get('VARIABLE_NAME', 'default_value') to avoid errors if the variable is missing.

python

import os

secret_key = os.environ.get('SECRET_KEY', 'defaultsecret')
print(secret_key)

Output

defaultsecret

Example

This example shows a simple Flask app that reads a secret key from an environment variable to configure the app securely. If the variable is not set, it uses a fallback value.

python

import os
from flask import Flask

app = Flask(__name__)

# Get secret key from environment variable
app.config['SECRET_KEY'] = os.environ.get('SECRET_KEY', 'fallbacksecret')

@app.route('/')
def home():
    return f"Secret key is: {app.config['SECRET_KEY']}"

if __name__ == '__main__':
    app.run(debug=True)

Output

Secret key is: fallbacksecret

Common Pitfalls

• Not setting environment variables: If you forget to set them, your app may use insecure defaults or crash.
• Using os.environ['VAR'] without get: This raises a KeyError if the variable is missing. Use get with a default instead.
• Hardcoding secrets: Avoid putting sensitive info directly in your code; use environment variables instead.
• Forgetting to load .env files: Flask does not load .env files automatically; use packages like python-dotenv to load them in development.

python

import os

# Wrong: raises KeyError if SECRET_KEY not set
# secret_key = os.environ['SECRET_KEY']

# Right: safe access with default
secret_key = os.environ.get('SECRET_KEY', 'defaultsecret')

Quick Reference

Remember these tips when using environment variables in Flask:

• Use os.environ.get('VAR_NAME') to safely access variables.
• Set environment variables outside your code (e.g., in your OS or .env files).
• Use python-dotenv to load .env files during development.
• Never commit secrets or keys to version control.

Key Takeaways

Use os.environ.get('VAR_NAME') to access environment variables safely in Flask.

Set sensitive data like secret keys outside your code to keep them secure.

Use python-dotenv to load .env files automatically during development.

Avoid KeyError by providing default values when accessing environment variables.

Never hardcode secrets or commit them to version control.