Date slider with Plotly Dash does not work

Question

I am trying to build a dash app with a datetime slider, but the following code does not work - the url displays "Error loading layout". Am I doing something stupid, or are the any tricks needed to build a slider on datetime objects? Reproducible code follows:

import dash
import dash_core_components as dcc
import dash_html_components as html
import datetime

base = datetime.datetime.today()
date_list = [base - datetime.timedelta(days=x) for x in range(0, 10)]

app = dash.Dash()

app.layout = html.Div([
    html.Label('Slider'),
    dcc.RangeSlider(min=min(date_list), 
                    max=max(date_list), 
                    value=min(date_list))
    ])

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

Answer 1

Plotly-Dash does currently not support datetime objects.

A solution for you problem is to described in my answer to an other question Plotly datetime

---

One possible solution is to use two different arguments at the initialization:

• value
• marks

Example by using pandas for timestamp generation:

Needed imports:

import pandas as pd
import time

Some needed functions:

daterange = pd.date_range(start='2017',end='2018',freq='W')

def unixTimeMillis(dt):
    ''' Convert datetime to unix timestamp '''
    return int(time.mktime(dt.timetuple()))

def unixToDatetime(unix):
    ''' Convert unix timestamp to datetime. '''
    return pd.to_datetime(unix,unit='s')

def getMarks(start, end, Nth=100):
    ''' Returns the marks for labeling. 
        Every Nth value will be used.
    '''

    result = {}
    for i, date in enumerate(daterange):
        if(i%Nth == 1):
            # Append value to dict
            result[unixTimeMillis(date)] = str(date.strftime('%Y-%m-%d'))

    return result

For the RangeSlider object you can initialize it like this:

dcc.RangeSlider(
                id='year_slider',
                min = unixTimeMillis(daterange.min()),
                max = unixTimeMillis(daterange.max()),
                value = [unixTimeMillis(daterange.min()),
                         unixTimeMillis(daterange.max())],
                marks=getMarks(daterange.min(),
                            daterange.max())
            )

Answer 2

This should definitely work and is much easier than any other solution

marks = {int(i):str(j) for i,j in zip(range(len(df.years)), df[“years”])}

And then you can mimic the same in the functional callbacks to update the values by dictionary keys.