How to Use Config in Flask: Simple Guide with Examples

In Flask, you use app.config to manage configuration settings by assigning key-value pairs or loading from files like .py or .env. You can set configs directly or load them using app.config.from_pyfile() or app.config.from_mapping().

Syntax

Flask configuration is stored in app.config, a dictionary-like object. You can set values directly or load from external files.

• app.config['KEY'] = 'value': Set a single config key.
• app.config.from_pyfile('config.py'): Load config from a Python file.
• app.config.from_mapping({'KEY': 'value'}): Load config from a dictionary.

python

from flask import Flask

app = Flask(__name__)

# Set config directly
app.config['DEBUG'] = True

# Load config from a Python file
app.config.from_pyfile('config.py')

# Load config from a dictionary
app.config.from_mapping(
    SECRET_KEY='mysecret',
    DATABASE_URI='sqlite:///mydb.sqlite'
)

Example

This example shows how to create a Flask app, set configuration values directly, and access them inside a route.

python

from flask import Flask

app = Flask(__name__)

# Set configuration values
app.config['DEBUG'] = True
app.config['SECRET_KEY'] = 'supersecretkey'

@app.route('/')
def home():
    debug_mode = app.config['DEBUG']
    secret = app.config['SECRET_KEY']
    return f"Debug is {debug_mode}, Secret Key is {secret}"

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

Output

Debug is True, Secret Key is supersecretkey

Common Pitfalls

Common mistakes when using Flask config include:

• Trying to access config keys before setting or loading them, causing KeyError.
• Forgetting to reload or restart the app after changing config files.
• Using incorrect file paths or formats when loading config from files.
• Modifying app.config after the app has started without proper context.

python

from flask import Flask

app = Flask(__name__)

# Wrong: Accessing config before setting it
# print(app.config['MY_SETTING'])  # KeyError

# Right: Set config first
app.config['MY_SETTING'] = 'value'
print(app.config['MY_SETTING'])  # Outputs: value

Output

value

Quick Reference

Method	Description
app.config['KEY'] = value	Set a single configuration key directly
app.config.from_pyfile('filename.py')	Load configuration from a Python file
app.config.from_mapping(dict)	Load configuration from a dictionary or mapping
app.config.from_envvar('ENV_VAR')	Load config from a file path stored in an environment variable
app.config.get('KEY', default)	Get a config value with a default fallback

Key Takeaways

Use app.config to store and access Flask configuration settings as key-value pairs.

Load configuration from files using from_pyfile or from_mapping for better organization.

Always set or load config before accessing keys to avoid errors.

Restart your Flask app after changing config files to apply updates.

Use app.config.get() to safely access config values with defaults.