Being deployment-specific with instance folders

Flask provides yet another method for configuration, where we can efficiently manage deployment-specific parts. Instance folders allow us to segregate deployment-specific files from our version-controlled application. We know that configuration files can be separate for different deployment environments, such as development and production, but there are also many more files, such as database files, session files, cache files, and other runtime files. In this recipe, we will create an instance folder, which will act like a holder bin for such kinds of files.

How to do it...

By default, the instance folder is picked up from the application automatically if we have a folder named instance in our application at the application level, as follows:

my_app/ 
    - app.py 
    - instance/ 
        - config.cfg 

We can also explicitly define the absolute path of the instance folder using the instance_path parameter on our application object, as follows:

app = Flask( 
    __name__, instance_path='/absolute/path/to/instance/folder' 
) 

To load the configuration file from the instance folder, we will use the instance_relative_config parameter on the application object, as follows:

app = Flask(__name__, instance_relative_config=True) 

This tells the application to load the configuration file from the instance folder. The following example shows how this will work:

app = Flask( 
    __name__, instance_path='path/to/instance/folder', 
    instance_relative_config=True 
) 
app.config.from_pyfile('config.cfg', silent=True) 

How it works...

In the preceding code, first, the instance folder is loaded from the given path, and then, the configuration file is loaded from the file named config.cfg in the given instance folder. Here, silent=True is optional and is used to suppress the error if config.cfg is not found in the instance folder. If silent=True is not given and the file is not found, then the application will fail, giving the following error:

   IOError: [Errno 2] Unable to load configuration file (No such file 
   or 
   directory): '/absolute/path/to/config/file'
  

It might seem that loading the configuration from the instance folder using instance_relative_config is redundant work and could be moved to one of the configuration methods itself. However, the beauty of this process lies in the fact that the instance folder concept is completely independent of configuration, and instance_relative_config just complements the configuration object.