app.py

os
from flask_extended import Flask

app = Flask(__name__)
app.config.from_yaml(os.join(app.root_path, 'config.yml'))

config.yml

COMMON: &common
  SECRET_KEY: insecure
    
DEVELOPMENT: &development
  <<: *common
  DEBUG: True
  
STAGING: &staging
  <<: *common
  SECRET_KEY: sortasecure
  
PRODUCTION: &production
  <<: *common
  SECRET_KEY: shouldbereallysecureatsomepoint
    
    
  

flask_extended.py

"""
 You need to install PyYAML library
 using pip:

    pip install PyYAML
"""

import os
import yaml

from flask import Flask as BaseFlask, Config as BaseConfig


class Config(BaseConfig):
    """Flask config enhanced with a `from_yaml` method."""

    def from_yaml(self, config_file):
        """
            To make this work in HEROKU, you must set the configuration variable
            called "FLASK_ENV".

            Type:
                heroku config:set FLASK_ENV=production

            Verified type:
                heroku config

            You should see it listed.

            FLASK_ENV:    production
            GEM_PATH:     vendor/bundle/ruby/1.9.1
            LANG:         en_US.UTF-8
            PATH:         bin:vendor/bundle/ruby/1.9.1/bin:/usr/local/bin:/usr/bin:/bin
            RACK_ENV:     production
            RAILS_ENV:    production
        """

        # Read 'FLASK_ENV' config value, if it does not exists set 'development'
        # as default
        env = os.environ.get('FLASK_ENV', 'development')

        # Add 'ENVIRONMENT' variable as a reference
        self['ENVIRONMENT'] = env.lower()

        # Load everything from the YAML file
        with open(config_file) as yaml_file:
            config_values = yaml.load(yaml_file)

        # Get all the variables from the proper 'section' in the YAML.
        # The section we are getting is defined by 'FLASK_ENV' value.
        # e.g
        #
        # DEVELOPMENT:
        #    DEBUG: True
        #    FB_KEY: ABCDEF
        config_values = config_values.get(env.upper(), config_values)

        # Assign values to FLASK's app.config
        for key in config_values.iterkeys():
            # We look at key with uppercase only
            if key.isupper():
                self[key] = config_values[key]


class Flask(BaseFlask):
    """Extended version of `Flask` that implements custom config class"""

    def make_config(self, instance_relative=False):
        root_path = self.root_path
        if instance_relative:
            root_path = self.instance_path
        return Config(root_path, self.default_config)