Extending Bokeh¶

Python Models¶

For the most part, Python Bokeh models are completely declarative classes. Custom extensions are created by making a subclass Model (or one of its subclasses), and including special class attributes to declare the properties that are mirrored on the JavaScript side. All of the available property types are documented in the bokeh.core.properties section of the Reference Guide.

A small example that creates a Custom readout for a slider is presented below:

from bokeh.core.properties import String, Instance
from bokeh.models import LayoutDOM, Slider

class Custom(LayoutDOM):

    text = String(default="Custom text")

    range = Instance(Slider)

Since we would like to create a custom extension that can participate in DOM layout, we subclass from LayoutDOM. We also added two properties: a String to configure a text message for the readout, and an Instance that can hold a Slider. The JavaScript Slider object that corresponds to the Python Slider will be made available to use.

JavaScript Models and Views¶

While the Python side is mostly declarative, without much or any real code, the JavaScript side requires code to implement the model. When appropriate, code for a corresponding view must also be provided. Currently BokehJS models and views are subclasses of Models and View from the Backbone JavaScript library.

Below is an annotated JavaScript implementation for Custom and its CustomView. For built-in models, this code is included directly in the final bokeh.js library. We will see how to connect this code to custom extensions in the next section.

import {div, empty} from "core/dom"

# The "core/properties" module has all the property types
import * as p from "core/properties"

# We will subclass in JavaScript from the same class that was subclassed
# from in Python
import {LayoutDOM, LayoutDOMView} from "models/layouts/layout_dom"

# This model will actually need to render things, so we must provide
# view. The LayoutDOM model has a view already, so we will start with that
export class CustomView extends LayoutDOMView

  initialize: (options) ->
    super(options)

    @render()

    # Set Backbone listener so that when the Bokeh slider has a change
    # event, we can process the new data
    @listenTo(@model.slider, 'change', () => @render())

  render: () ->
    # Backbone Views create <div> elements by default, accessible as @el.
    # Many Bokeh views ignore this default <div>, and instead do things
    # like draw to the HTML canvas. In this case though, we change the
    # contents of the <div>, based on the current slider value.
    empty(@el)
    @el.appendChild(div({
      style: {
        color: '#686d8e'
        'background-color': '#2a3153'
      }
    }, "#{@model.text}: #{@model.slider.value}"))

export class Custom extends LayoutDOM

  # If there is an associated view, this is boilerplate.
  default_view: CustomView

  # The ``type`` class attribute should generally match exactly the name
  # of the corresponding Python class.
  type: "Custom"

  # The @define block adds corresponding "properties" to the JS model. These
  # should basically line up 1-1 with the Python model class. Most property
  # types have counterparts, e.g. bokeh.core.properties.String will be
  # p.String in the JS implementation. Where the JS type system is not yet
  # as rich, you can use p.Any as a "wildcard" property type.
  @define {
    text:   [ p.String ]
    slider: [ p.Any    ]
  }

Putting it Together¶

For built-in Bokeh models, the implementation in BokehJS is automatically matched with the corresponding Python model by the build process. In order connect JavaScript implementations to Python models, one additional step is needed. The Python class should have have a class attribute called __implementation__ whose value is the JavaScript (or CoffeeScript) code that the defined the client-side model (and optional view).

Assuming the CoffeeScript code above was saved in a file custom.coffee, then the complete Python class might look like:

from bokeh.core.properties import String, Instance
from bokeh.models import LayoutDOM, Slider

class Custom(LayoutDOM):

    __implementation__ = "custom.coffee"

    text = String(default="Custom text")

    slider = Instance(Slider)

Then, if this class is defined in a Python module custom.py then the custom extension can now be used exactly like any built-in Bokeh model:

from bokeh.io import show
from bokeh.layouts import column
from bokeh.models import Slider

from custom import Custom

slider = Slider(start=0, end=10, step=0.1, value=0, title="value")

custom = Custom(text="Special Slider Display", slider=slider)

layout = column(slider, custom)

show(layout)

Which results in the output below. The JavaScript code for the implementation is automatically included in the rendered document. Scrub the slider to see the special header update as the slider moves:

Supplying External Resources¶

As part of implementing a custom model in Bokeh, there may be the need to include third-party javascript libraries or css resources. Bokeh supports supplying external resources through the Python class attributes __javascript__ and __css__ of custom models.

Including the URL paths to external resources will causes Bokeh to add the resources to the html document head, causing the Javascript library to be available in the global namespace and the custom CSS styling to be applied.

One example is including the JS and CSS files for KaTex (a Javascript-based typesetting library that supports LaTex) in order to create a LaTexLabel custom model.

class LatexLabel(Label):
    """A subclass of the Bokeh built-in `Label` that supports rendering
    LaTex using the KaTex typesetting library.
    """
    __javascript__ = "https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.6.0/katex.min.js"
    __css__ = "https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.6.0/katex.min.css"
    __implementation__ = """
    # do something here
    """

See the LaTex example in the extensions gallery below to see the full implementation and resulting output.

Integration with Bokeh Server¶

No special work or modification is needed to integrate custom user extensions with the Bokeh server. As for standalone documents, the JavaScript implementation is automatically included in the rendered application. Additionally the standard synchronization of Bokeh model properties that happens for all built-in models happens transparently for custom user extensions as well.