"""
 db.py  

 database interface
 ... in this example, just stored in a json file

 Notes :

    This is *not* a great approach for several reasons, 
    but it's simple and illustrates several basic notions.

    "g" (think "globals") is a flask special object/dict 
    that stores request data.

    g['db'] will store our session and form data
    as a python dictionary.

    The database itself is stored in an "instance" directory,
    which is where anything for one deployed instantiation
    of this app should be put.

"""
from flask import current_app, g
import simplejson

def open_db():
    """ modify the global "g" object by adding a 'db' (database) key 
        containing the data found by reading the json database 
    """
    if 'db' not in g:
        try:
            with open(current_app.config['JSONDATA']) as jsonfile:
                g.db = simplejson.loads(jsonfile.read())
        except:
            g.db = {}

def close_db(e=None):
    """ write the database to the json file, and
        remove the 'db' key from the global "g" object.
    """
    # (This is called by the flask framework,
    #  which may passes an exception argument which we'll ignore)
    if 'db' in g:
        with open(current_app.config['JSONDATA'], 'w') as jsonfile:
            jsonfile.write(simplejson.dumps(g.db))
        g.pop('db', None)

def init_app(app):
    """ initialize any database aspects of this app """
    # add "close_db" to the tasks to be done when a request finishes.
    app.teardown_appcontext(close_db)