Dash URL State example

app.py

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

app = dash.Dash()

app.config.suppress_callback_exceptions = True


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


def apply_default_value(params):
    def wrapper(func):
        def apply_value(*args, **kwargs):
            if 'id' in kwargs and kwargs['id'] in params:
                kwargs['value'] = params[kwargs['id']]
            return func(*args, **kwargs)
        return apply_value
    return wrapper


def build_layout(params):
    layout = [
        html.H2('URL State demo', id='state'),
        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 parse_state(url):
    parse_result = urlparse(url)
    params = parse_qsl(parse_result.query)
    state = dict(params)
    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 build_layout(state)


component_ids = [
    'dropdown',
    'input',
    'slider'
]


@app.callback(Output('url', 'search'),
              inputs=[Input(i, 'value') for i in component_ids])
def update_url_state(*values):
    state = urlencode(dict(zip(component_ids, values)))
    return f'?{state}'


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

requirements.txt

dash==0.20.0
dash-renderer==0.11.2
dash-html-components==0.8.0
dash-core-components==0.18.0

Hi, I changed the code a bit to work with arbitrary attributes (not just 'value'), allow for saving lists of items as well (e.g. for multi dropdowns) and save the whole thing in a base64 encoded json:

(Also nicely saves the particular tab that you're on, when working with tabs)

def apply_default_value(params):
    def wrapper(func):
        def apply_value(*args, **kwargs):
            if 'id' in kwargs and kwargs['id'] in params:
                kwargs[params[kwargs['id']][0]] = params[kwargs['id']][1]
                #kwargs['value'] = params[kwargs['id']][1]
            return func(*args, **kwargs)
        return apply_value
    return wrapper

def parse_state(url):
    parse_result = urlparse(url)
    query_string = parse_qsl(parse_result.query)
    if query_string:
        encoded_state = query_string[0][1]
        state = dict(json.loads(urlsafe_b64decode(encoded_state)))
    else:
        state = dict()
    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 build_layout(state)

component_ids = {
    'tabs' : 'value',
    'tab1_multidropdown' : 'value',
    'tab2_datepicker' : 'start_date'
}

@app.callback(Output('url', 'search'),
              inputs=[Input(id, param) for id, param in component_ids.items()])
def update_url_state(*values):
    state = dict(zip(list(component_ids.keys()), list(zip(component_ids.values(), values))))
    encoded = urlsafe_b64encode(json.dumps(state).encode())
    params = urlencode(dict(params=encoded))
    return f'?{params}'

The original gist does not work anymore with the stricter type enforcement of recent Dash:

The low-footprint fix I tried is to try to coerce the vale into its existing type (won't work if it's init with None):

def apply_default_value(params):
    def wrapper(func):
        def apply_value(*args, **kwargs):
            if 'id' in kwargs and kwargs['id'] in params:
                hval = kwargs['value']  # hardcoded value
                sval = params[kwargs['id']]  # saved value
                ctor = None if hval is None or isinstance(hval, str) else type(hval)
                kwargs['value'] = ctor(sval) if ctor else sval
            return func(*args, **kwargs)
        return apply_value
    return wrapper

It works but it's brittle (so I moved to another approach: plotly/dash#188 (comment) )