You won't want to make connecting to the db happen at import time. Go ahead and configure your app at import time because you can always tweak the configuration in your tests before attempting to test or run your app. In the example below you'll have your db connection behind some functions that use the application config so in a unittest you can actually change the db connection to point to a different file and then go ahead and connect explicitly in your setup.

Say you have a myapp package containing myapp.py which looks like:

# myapp/myapp.py
from __future__ import with_statement
from sqlite3 import dbapi2 as sqlite3
from contextlib import closing
from flask import Flask, request, session, g, redirect, url_for, abort, \
     render_template, flash

# configuration
DATABASE = '/tmp/flaskr.db'
DEBUG = True
SECRET_KEY = 'development key'
USERNAME = 'admin'
PASSWORD = 'default'

# create our little application :)
app = Flask(__name__)
app.config.from_object(__name__)
app.config.from_envvar('MYAPP_SETTINGS', silent=True)

def connect_db():
    """Returns a new connection to the database."""
    return sqlite3.connect(app.config['DATABASE'])


def init_db():
    """Creates the database tables."""
    with closing(connect_db()) as db:
        with app.open_resource('schema.sql') as f:
            db.cursor().executescript(f.read())
        db.commit()


@app.before_request
def before_request():
    """Make sure we are connected to the database each request."""
    g.db = connect_db()


@app.after_request
def after_request(response):
    """Closes the database again at the end of the request."""
    g.db.close()
    return response

@app.route('/')
def show_entries():
    cur = g.db.execute('select title, text from entries order by id desc')
    entries = [dict(title=row[0], text=row[1]) for row in cur.fetchall()]
    return render_template('show_entries.html', entries=entries)

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

Your test file myapp/test_myapp.py will look like this:

import os
import myapp
import unittest
import tempfile

class MyappTestCase(unittest.TestCase):

    def setUp(self):
        self.db_fd, myapp.app.config['DATABASE'] = tempfile.mkstemp()
        self.app = myapp.app.test_client()
        myapp.init_db()

    def tearDown(self):
        os.close(self.db_fd)
        os.unlink(myapp.app.config['DATABASE'])

    def test_empty_db(self):
        rv = self.app.get('/')
        assert 'No entries here so far' in rv.data

Of course if you'd like to use SQLAlchemy you'll have to update the connect_db and init_db functions appropriately but hopefully you get the idea.

it’s just a config option. Here’s ours:

CopySQLALCHEMY_ENGINE_OPTIONS = {
    "pool_pre_ping": True,
    "pool_recycle": 300,
}