Delayed App Config

[bormanjo]

Adds the ability to set/update Dash.config attributes via **kwargs passed to Dash.init_app().

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Make AttributeDict additive
  • Update Dash.__init__() and Dash.init_app()
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

Alternative Multi-Page App Example

The delayed app config allows app routes to be decoupled from Dash.__init__() when server=False

from dash import Dash, html
from flask import Flask

server = Flask(__name__)

app1 = Dash(name='app1', server=False)
app1.layout = html.Div('This is App1')

app2 = Dash(name='app2', server=False)
app2.layout = html.Div('This is App2')

routes = {
    '/dash-app1/': app1,
    '/dash-app2/': app2,
}

for route, app in routes.items():
    app.init_app(server, routes_pathname_prefix=route, requests_pathname_prefix=route)


@server.route('/')
def index():
    return "<p>Hello, World!</p>"

server.run()

[jborman-exos]

@alexcjohnson Thoughts on this? The two percy render tests that failed appear to be issues over the change in month/year (e.g expected December 2021, got January 2022)

[jborman-exos]

@alexcjohnson Does this look good to approve + merge?

[alexcjohnson]

Looks great, thanks very much @jborman-exos! 💃

[t89]

This set of changes seems to cause an incompatibility with jupyter-dash 0.4.0: plotly/jupyter-dash#75 (comment)