This is the 2nd chapter of the Dash Tutorial.
The previous chapter covered installation
and the next chapter covers Dash callbacks.

Note:

  1. The layout is composed of a tree of “components” like html.Div
    and dcc.Graph.
  2. The dash_html_components library has a component for every HTML
    tag. The html.H1(children='Hello Dash') component generates
    a <h1>Hello Dash<h1> HTML element in your application.
  3. Not all components are pure HTML. The dash_core_components describe
    higher-level components that are interactive and are generated with
    JavaScript, HTML, and CSS through the React.js library.
  4. Each component is described entirely through keyword attributes.
    Dash is declarative: you will primarily describe your application
    through these attributes.
  5. The children property is special. By convention, it’s always the
    first attribute which means that you can omit it:
    html.H1(children='Hello Dash') is the same as html.H1('Hello Dash').
    Also, it can contain a string, a number, a single component, or a
    list of components.
  6. The fonts in your application will look a little bit different than
    what is displayed here. This application is using a
    custom CSS stylesheet and Dash Enterprise Design Kit
    to modify the default styles of the elements.
    You can learn more about custom CSS in the
    css tutorial.

Making your first change

Dash includes “hot-reloading”, this features is activated by default when
you run your app with app.run_server(debug=True). This means that Dash
will automatically refresh your browser when you make a change in your code.

Give it a try: change the title “Hello Dash” in your application or change the
x or the y data. Your app should auto-refresh with your change.

Don’t like hot-reloading? You can turn this off with app.run_server(dev_tools_hot_reload=False).

Learn more in Dash Dev Tools documentation Questions? See the community forum hot reloading discussion.

More about HTML

dash_html_components (dash.html as of Dash v2.0) contains a component class
for every HTML tag as well as keyword arguments for all of the HTML arguments.

Let’s customize the text in our app by modifying the inline styles of the
components. Create a file named app.py with the following code:

# Run this app with `python app.py` and
# visit <a href="http://127.0.0.1:8050/">http://127.0.0.1:8050/</a> in your web browser.

import dash
import dash_core_components as dcc
import dash_html_components as html
import plotly.express as px
import pandas as pd

app = dash.Dash(__name__)

colors = {
    'background': '#111111',
    'text': '#7FDBFF'
}

# assume you have a "long-form" data frame
# see <a href="https://plotly.com/python/px-arguments/">https://plotly.com/python/px-arguments/</a> for more options
df = pd.DataFrame({
    "Fruit": ["Apples", "Oranges", "Bananas", "Apples", "Oranges", "Bananas"],
    "Amount": [4, 1, 2, 2, 4, 5],
    "City": ["SF", "SF", "SF", "Montreal", "Montreal", "Montreal"]
})

fig = px.bar(df, x="Fruit", y="Amount", color="City", barmode="group")

fig.update_layout(
    plot_bgcolor=colors['background'],
    paper_bgcolor=colors['background'],
    font_color=colors['text']
)

app.layout = html.Div(style={'backgroundColor': colors['background']}, children=[
    html.H1(
        children='Hello Dash',
        style={
            'textAlign': 'center',
            'color': colors['text']
        }
    ),

    html.Div(children='Dash: A web application framework for Python.', style={
        'textAlign': 'center',
        'color': colors['text']
    }),

    dcc.Graph(
        id='example-graph-2',
        figure=fig
    )
])

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

Hello Dash

Dash: A web application framework for Python.

In this example, we modified the inline styles of the html.Div
and html.H1 components with the style property.

html.H1('Hello Dash', style={'textAlign': 'center', 'color': '#7FDBFF'})
is rendered in the Dash application as
&lt;h1&gt;Hello Dash&lt;h1&gt;.

There are a few important differences between the dash_html_components
and the HTML attributes:

  1. The style property in HTML is a semicolon-separated string. In Dash,
    you can just supply a dictionary.
  2. The keys in the style dictionary are camelCased.
    So, instead of text-align, it’s textAlign.
  3. The HTML class attribute is className in Dash.
  4. The children of the HTML tag is specified through the children keyword
    argument. By convention, this is always the first argument and
    so it is often omitted.

Besides that, all of the available HTML attributes and tags are available
to you within your Python context.

Reusable Components

By writing our markup in Python, we can create complex reusable
components like tables without switching contexts or languages.

Here’s a quick example that generates a Table from a Pandas dataframe.
Create a file named app.py with the following code:

# Run this app with `python app.py` and
# visit <a href="http://127.0.0.1:8050/">http://127.0.0.1:8050/</a> in your web browser.

import dash
import dash_html_components as html
import pandas as pd

df = pd.read_csv('https://gist.githubusercontent.com/chriddyp/c78bf172206ce24f77d6363a2d754b59/raw/c353e8ef842413cae56ae3920b8fd78468aa4cb2/usa-agricultural-exports-2011.csv')


def generate_table(dataframe, max_rows=10):
    return html.Table([
        html.Thead(
            html.Tr([html.Th(col) for col in dataframe.columns])
        ),
        html.Tbody([
            html.Tr([
                html.Td(dataframe.iloc[i][col]) for col in dataframe.columns
            ]) for i in range(min(len(dataframe), max_rows))
        ])
    ])


app = dash.Dash(__name__)

app.layout = html.Div([
    html.H4(children='US Agriculture Exports (2011)'),
    generate_table(df)
])

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

US Agriculture Exports (2011)

Unnamed: 0 state total exports beef pork poultry dairy fruits fresh fruits proc total fruits veggies fresh veggies proc total veggies corn wheat cotton
Alabama 1390.63 34.4 10.6 481.0 4.06 8.0 17.1 25.11 5.5 8.9 14.33 34.9 70.0 317.61
1 Alaska 13.31 0.2 0.1 0.19 0.6 1.0 1.56
2 Arizona 1463.17 71.3 17.9 105.48 19.3 41.0 60.27 147.5 239.4 386.91 7.3 48.7 423.95
3 Arkansas 3586.02 53.2 29.4 562.9 3.53 2.2 4.7 6.88 4.4 7.1 11.45 69.5 114.5 665.44
4 California 16472.88 228.7 11.1 225.4 929.95 2791.8 5944.6 8736.4 803.2 1303.5 2106.79 34.6 249.3 1064.95
5 Colorado 1851.33 261.4 66.0 14.0 71.94 5.7 12.2 17.99 45.1 73.2 118.27 183.2 400.5
6 Connecticut 259.62 1.1 0.1 6.9 9.49 4.2 8.9 13.1 4.3 6.9 11.16
7 Delaware 282.19 0.4 0.6 114.7 2.3 0.5 1.0 1.53 7.6 12.4 20.03 26.9 22.9
8 Florida 3764.09 42.6 0.9 56.9 66.31 438.2 933.1 1371.36 171.9 279.0 450.86 3.5 1.8 78.24
9 Georgia 2860.84 31.0 18.9 630.4 38.38 74.6 158.9 233.51 59.0 95.8 154.77 57.8 65.4 1154.07

More about Visualization

The dash_core_components library includes a component called Graph.

Graph renders interactive data visualizations using the open source
plotly.js JavaScript graphing
library. Plotly.js supports over 35 chart types and renders charts in
both vector-quality SVG and high-performance WebGL.

The figure argument in the dash_core_components.Graph component is
the same figure argument that is used by plotly.py, Plotly’s
open source Python graphing library.
Check out the plotly.py documentation and gallery
to learn more.

Here’s an example that creates a scatter plot from a Pandas dataframe.
Create a file named app.py with the following code:

# Run this app with `python app.py` and
# visit <a href="http://127.0.0.1:8050/">http://127.0.0.1:8050/</a> in your web browser.

import dash
import dash_core_components as dcc
import dash_html_components as html
import plotly.express as px
import pandas as pd


app = dash.Dash(__name__)

df = pd.read_csv('https://gist.githubusercontent.com/chriddyp/5d1ea79569ed194d432e56108a04d188/raw/a9f9e8076b837d541398e999dcbac2b2826a81f8/gdp-life-exp-2007.csv')

fig = px.scatter(df, x="gdp per capita", y="life expectancy",
                 size="population", color="continent", hover_name="country",
                 log_x=True, size_max=60)

app.layout = html.Div([
    dcc.Graph(
        id='life-exp-vs-gdp',
        figure=fig
    )
])

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

These graphs are interactive and responsive.
Hover over points to see their values,
click on legend items to toggle traces,
click and drag to zoom,
hold down shift, and click and drag to pan.

Markdown

While Dash exposes HTML through dash_html_components,
it can be tedious to write your copy in HTML.
For writing blocks of text, you can use the Markdown component in
dash_core_components. Create a file named app.py with the following code:

# Run this app with `python app.py` and
# visit <a href="http://127.0.0.1:8050/">http://127.0.0.1:8050/</a> in your web browser.

import dash
import dash_core_components as dcc
import dash_html_components as html

app = dash.Dash(__name__)

markdown_text = '''
### Dash and Markdown

Dash apps can be written in Markdown.
Dash uses the [CommonMark](http://commonmark.org/)
specification of Markdown.
Check out their [60 Second Markdown Tutorial](http://commonmark.org/help/)
if this is your first introduction to Markdown!
'''

app.layout = html.Div([
    dcc.Markdown(children=markdown_text)
])

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

Dash and Markdown

Dash apps can be written in Markdown.
Dash uses the CommonMark
specification of Markdown.
Check out their 60 Second Markdown Tutorial
if this is your first introduction to Markdown!

Core Components

dash_core_components (dash.dcc as of Dash v2.0) includes a set of
higher-level components like dropdowns, graphs, markdown blocks, and more.

Like all Dash components, they are described entirely declaratively.
Every option that is configurable is available as a keyword argument
of the component.

We’ll see many of these components throughout the tutorial.
You can view all of the available components in the
Dash Core Components Gallery.

Here are a few of the available components.
Create a file named app.py with the following code:

# Run this app with `python app.py` and
# visit <a href="http://127.0.0.1:8050/">http://127.0.0.1:8050/</a> in your web browser.

import dash
import dash_core_components as dcc
import dash_html_components as html

app = dash.Dash(__name__)

app.layout = html.Div([
    html.Label('Dropdown'),
    dcc.Dropdown(
        options=[
            {'label': 'New York City', 'value': 'NYC'},
            {'label': u'Montréal', 'value': 'MTL'},
            {'label': 'San Francisco', 'value': 'SF'}
        ],
        value='MTL'
    ),

    html.Label('Multi-Select Dropdown'),
    dcc.Dropdown(
        options=[
            {'label': 'New York City', 'value': 'NYC'},
            {'label': u'Montréal', 'value': 'MTL'},
            {'label': 'San Francisco', 'value': 'SF'}
        ],
        value=['MTL', 'SF'],
        multi=True
    ),

    html.Label('Radio Items'),
    dcc.RadioItems(
        options=[
            {'label': 'New York City', 'value': 'NYC'},
            {'label': u'Montréal', 'value': 'MTL'},
            {'label': 'San Francisco', 'value': 'SF'}
        ],
        value='MTL'
    ),

    html.Label('Checkboxes'),
    dcc.Checklist(
        options=[
            {'label': 'New York City', 'value': 'NYC'},
            {'label': u'Montréal', 'value': 'MTL'},
            {'label': 'San Francisco', 'value': 'SF'}
        ],
        value=['MTL', 'SF']
    ),

    html.Label('Text Input'),
    dcc.Input(value='MTL', type='text'),

    html.Label('Slider'),
    dcc.Slider(
        min=0,
        max=9,
        marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(1, 6)},
        value=5,
    ),
], style={'columnCount': 2})

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

Calling help

Dash components are declarative: every configurable aspect of these
components is set during instantiation as a keyword argument.
Call help in your Python console on any of the components to
learn more about a component and its available arguments.

>>> help(dcc.Dropdown)
class Dropdown(dash.development.base_component.Component)
|  A Dropdown component.
|  Dropdown is an interactive dropdown element for selecting one or more
|  items.
|  The values and labels of the dropdown items are specified in the `options`
|  property and the selected item(s) are specified with the `value` property.
|
|  Use a dropdown when you have many options (more than 5) or when you are
|  constrained for space. Otherwise, you can use RadioItems or a Checklist,
|  which have the benefit of showing the users all of the items at once.
|
|  Keyword arguments:
|  - id (string; optional)
|  - className (string; optional)
|  - disabled (boolean; optional): If true, the option is disabled
|  - multi (boolean; optional): If true, the user can select multiple values
|  - options (list; optional)
|  - placeholder (string; optional): The grey, default text shown when no option is selected
|  - value (string | list; optional): The value of the input. If `multi` is false (the default)
|  then value is just a string that corresponds to the values
|  provided in the `options` property. If `multi` is true, then
|  multiple values can be selected at once, and `value` is an
|  array of items with values corresponding to those in the
|  `options` prop.

Summary

The layout of a Dash app describes what the app looks like.
The layout is a hierarchical tree of components.
The dash_html_components library provides classes for all of the HTML
tags and the keyword arguments describe the HTML attributes like style,
className, and id.
The dash_core_components library generates higher-level
components like controls and graphs.

For reference, see:

The next part of the Dash tutorial covers how to make these apps interactive.
Dash Tutorial Part 3: Basic Callbacks