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A custom Tornado request handler that generates and serves the Bokeh autoload JavaScript chunk for embedding Bokeh applications in web pages.

Purpose

This handler extends Bokeh's built-in AutoloadJsHandler to provide custom session management and resource handling for Panel/Bokeh applications. It generates JavaScript code that can be embedded in HTML pages to dynamically load and initialize Bokeh visualizations. The handler manages session state, resolves resource URLs, and produces the autoload script with proper configuration for the target element.

Source Code

class AutoloadJsHandler(BkAutoloadJsHandler, SessionPrefixHandler):
    ''' Implements a custom Tornado handler for the autoload JS chunk

    '''

    async def get(self, *args, **kwargs):
        element_id = self.get_argument("bokeh-autoload-element", default=None)
        if not element_id:
            self.send_error(status_code=400, reason='No bokeh-autoload-element query parameter')
            return

        app_path = self.get_argument("bokeh-app-path", default="/")
        absolute_url = self.get_argument("bokeh-absolute-url", default=None)

        if absolute_url:
            server_url = '{uri.scheme}://{uri.netloc}'.format(uri=urlparse(absolute_url))
        else:
            server_url = None

        with self._session_prefix():
            session = await self.get_session()
            state.curdoc = session.document
            try:
                resources = Resources.from_bokeh(self.application.resources(server_url))
                js = autoload_js_script(
                    session.document, resources, session.token, element_id,
                    app_path, absolute_url
                )
            finally:
                state.curdoc = None

        self.set_header("Content-Type", 'application/javascript')
        self.write(js)

Parameters

Parameter Details

bases: Inherits from BkAutoloadJsHandler (Bokeh's base autoload handler) and SessionPrefixHandler (provides session prefix management). No explicit __init__ parameters are defined, so it uses the parent classes' constructors which expect Tornado application and request objects.

Return Value

The class itself returns an instance when instantiated. The get() method doesn't return a value but writes JavaScript content directly to the HTTP response via self.write(). On error, it sends a 400 status code with an error message.

Class Interface

Methods

Attributes

Dependencies

• tornado
• bokeh
• param
• urllib

Required Imports

from bokeh.server.views.autoload_js_handler import AutoloadJsHandler as BkAutoloadJsHandler
from urllib.parse import urlparse
from resources import Resources
from state import state

Conditional/Optional Imports

These imports are only needed under specific conditions:

from bokeh.embed.bundle import Script

from bokeh.core.templates import AUTOLOAD_JS

Usage Example

# This handler is typically registered with Tornado/Bokeh server, not instantiated directly
# Example of URL pattern registration:
from tornado.web import Application
from bokeh.server.urls import per_app_patterns

# Handler is registered in URL patterns
app_patterns = per_app_patterns(
    '/app',
    [(r'/autoload.js', AutoloadJsHandler)]
)

# Client-side usage - embed in HTML:
# <div id="myplot"></div>
# <script src="http://localhost:5006/autoload.js?bokeh-autoload-element=myplot&bokeh-app-path=/app&bokeh-absolute-url=http://localhost:5006/app"></script>

# The handler processes GET requests with query parameters:
# - bokeh-autoload-element: ID of the target DOM element
# - bokeh-app-path: Path to the Bokeh application (default: '/')
# - bokeh-absolute-url: Absolute URL for resource resolution (optional)

Best Practices

• This handler should be registered with Tornado's URL routing system, not instantiated directly by users
• Always provide the 'bokeh-autoload-element' query parameter when requesting the autoload script, or a 400 error will be returned
• Use 'bokeh-absolute-url' parameter when the application needs to resolve resources from a specific server URL
• The handler manages document state via state.curdoc - ensure proper cleanup in finally blocks to prevent state leakage
• Session management is handled via the SessionPrefixHandler mixin's context manager (_session_prefix())
• The handler is async and should be used in an async Tornado application context
• Content-Type is automatically set to 'application/javascript' for proper browser handling
• The generated JavaScript should be included in HTML pages via <script> tags with appropriate query parameters

Tags

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