Context in https://github.com/plotly/dash/issues/188

dash_state_demo_datepicker.py

# Copyright (C) 2022 by Edward O.
# Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without l> imitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
# The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE

import ast
import re

import flask

import dash
import dash_core_components as dcc
import dash_html_components as html
from urllib.parse import urlparse, parse_qsl, urlencode, quote
from dash.dependencies import Input, Output

app = dash.Dash()

url_bar_and_content_div = html.Div([
    dcc.Location(id='url', refresh=False),
    html.Div(id='page-layout')
])



def apply_default_value(params):
    """
    :param params: the state decoded from url
    """
    def wrapper(func):
        def apply_value(*args, **kwargs):
            if 'id' in kwargs and kwargs['id'] in params:
                param_key_dict = params[kwargs['id']]
                kwargs.update(param_key_dict)
            return func(*args, **kwargs)
        return apply_value
    return wrapper


def page_layout(params=None):

    params = params or {}
    layout = [
        html.H2('URL State demo', id='state'),
        apply_default_value(params)(dcc.DatePickerRange)(
            id='picker',
        ),
        apply_default_value(params)(dcc.Dropdown)(
            id='dropdown',
            options=[{'label': i, 'value': i} for i in ['LA', 'NYC', 'MTL']],
            value='LA'
        ),
        apply_default_value(params)(dcc.Input)(
            id='input',
            placeholder='Enter a value...',
            value=''
        ),
        apply_default_value(params)(dcc.Slider)(
            id='slider',
            min=0,
            max=9,
            marks={i: 'Label {}'.format(i) for i in range(10)},
            value=5,
        ),
        html.Br(),
    ]

    return layout


def app_layout():
    if flask.has_request_context():  # for real
        return url_bar_and_content_div
    # validation only
    return html.Div([
        url_bar_and_content_div,
        *page_layout()
    ])


app.layout = app_layout


ID_PARAM_SEP = '::'


def parse_state(url):
    parse_result = urlparse(url)
    query_string = parse_qsl(parse_result.query)

    state = {}
    for key, value in query_string:
        if ID_PARAM_SEP in key:
            id, param = key.split(ID_PARAM_SEP)
        else:
            id, param = key, 'value'
        state.setdefault(id, {})[param] = ast.literal_eval(value)

    return state


@app.callback(Output('page-layout', 'children'),
              inputs=[Input('url', 'href')])
def page_load(href):
    if not href:
        return []
    state = parse_state(href)
    return page_layout(state)


component_ids = {
    'picker': ['start_date', 'end_date'],
    'dropdown': ['value'],
    'input': ['value'],
    'slider': ['value'],
}


def param_string(id, p):
    return id if p == 'value' else id + ID_PARAM_SEP + p


RE_SINGLE_QUOTED = re.compile("^'|'$")
def myrepr(o):
    """Optional but chrome URL bar hates "'" """
    return RE_SINGLE_QUOTED.sub('"', repr(o))


@app.callback(Output('url', 'search'),
              inputs=[Input(id, p) for id, param in component_ids.items() for p in param])
def update_url_state(*values):
    """Updates URL from component values."""

    keys = [param_string(id, p) for id, param in component_ids.items() for p in param]
    state = dict(zip(keys, map(myrepr, values)))
    params = urlencode(state, safe="%/:?~#+!$,;'@()*[]\"", quote_via=quote)
    return f'?{params}'


if __name__ == '__main__':
    app.run_server(debug=True)