git_repo/
  venv/
  project/ - All actual Python resides in here
    __init__.py
    ui/
      static/ - Static Flask stuff
      templates/ - HTML templates
      __init__.py
      db.py
      user.py
      views.py
      dev_server.py - Simple entry point for UI dev (i.e. `app.run()`)
    ...

import flask
import project

app = flask.Flask(__name__)

<snip> Some logging setup, Flask config, etc. </snip>

import project.ui.views

import os
import peewee
import project
from project.ui import app

ui_db = os.path.join(project.dir, 'db.db')

class Model(peewee.Model):
    class Meta:
        database = ui_db

import peewee
import project.ui.db

class User(project.ui.db.Model):
    username = peewee.CharField(unique=True)
    password = peewee.CharField()
    name = peewee.CharField()

def get_user(username):
    try:
        return User.select().where(User.username == username)
    except peewee.DoesNotExist:
        return None

def check_user(username, password):
    u = get_user(username)
    if u:
        return u.password == password
    return False

import flask
from project.ui import app
import project.ui.user

@app.route('/login', methods=['GET', 'POST'])
def route_login():
    # Do some stuff using project.ui.user (User, get_user and check_user)
    ...

# Other routes, etc

...
File ".../project/ui/user.py", line 5, in <module>
    class User(project.ui.db.Model):
AttributeError: module 'project' has no attribute 'ui'