Python bindings to a JavaScript environment
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Python bindings to js-host, enabling you to send requests and data from your python process to a performant JavaScript environment.

There are a variety of libraries which provide low-level access to JS engines - PyExecJS et al - but they tend to suffer from limited functionality and poor performance. Rather than relying on basic evaluation of JavaScript, this library enables you to configure a persistent JS processes which responds to data sent from your python process.

To reduce the cost of integrating yet another technology, a manager process is provided which can automatically spawn js-host instances in the background.



pip install js-host

js-host requires access to a Node.js or io.js binary.

On OSX, you can run brew install node. On Linux, you can install node with your package manager.

Quick start

Create a package.json file with

npm init

Install the js-host JavaScript library with

npm install --save js-host@0.12

Create a file named host.config.js and insert

module.exports = {
  functions: {
    hello_world: function(data, cb) {
      var name = || 'World';
      cb(null, 'Hello, ' + name + '!');

Open a python shell and run

from js_host.conf import settings


If everything went ok, you should see some output as the manager process spins up and then spawns a host which uses your host.config.js file.

In the same python shell, run

from js_host.function import Function

hello_world = Function('hello_world')'Foo')


Settings can be defined by importing js_host.conf.settings and calling the configure method with keyword arguments matching the setting that you want to define. For example

from js_host.conf import settings


If you are using js-host in a Django project, add 'js_host' to your INSTALLED_APPS

    # ...

And configure js-host by placing a dictionary named JS_HOST into your settings files

    'SOURCE_ROOT': '/path/to/your/project',


An absolute path to the directory which contains your node_modules directory.

Default: os.getcwd() # Your current working directory


A path to the default config file used for hosts and managers.

If the path is relative, it is appended to SOURCE_ROOT.

Default: 'host.config.js'


Indicates that a manager should be used to spawn host instances.


Default: False


A path to a node binary.

Default: 'node'


A path to the js-host binary used to control hosts and managers.

If the path is relative, it is appended to the SOURCE_ROOT setting

Default: os.path.join('node_modules', '.bin', 'js-host')


Indicates how many seconds Function objects will wait for a response before raising exceptions.

Default: 10.0


Indicates that once this library has been configured, it should attempt to connect to a host.

This setting enables any config or connection errors to be raised during startup rather than runtime. It also enables connections to managed hosts to be preserved between restarts of your python process.

If you want to run multiple hosts and/or control the connection process, set this to False, but be aware that managed hosts may not preserve connections when your python process restarts.

Default: True


Overrides the root url which requests are sent to. By default, the root url is inferred from the config that the host generates from your config file and the js-host defaults.

If you want to route requests manually to a host, set it to a string such as ''.

Default: None


Indicates how much information the host should print the terminal. By default the library will print to the terminal whenever processes are started or connected to.

If you want to suppress all output, set it to js_host.utils.verbosity.SILENT.

Default: js_host.utils.verbosity.PROCESS_START

Usage in development

In development, you can take advantage of the manager process to abstract away the overhead of starting and stopping processes. To use a manager to spawn hosts, set the USE_MANAGER setting to True.

If you are writing functions that are on a host, you are recommended to start hosts manually, as it will provide more immediate feedback, as well as easier control of a process. To start a host manually, refer to js-host's CLI usage.

Usage in production

In a production environment, you are strongly recommended to not use the manager. Ensure that the USE_MANAGER setting is set to False before you start your python process.

In a production environment, you should run your hosts under a supervisor system, such as supervisor or PM2. You can refer to js-host's CLI usage for the necessary incantation to spin up a process. It will generally boil down to something like

node_modules/.bin/js-host host.config.js


By default, js-host instances will only write their logs to stdout and stderr. To log to files, refer to js-host's documentation on logging.


js-host does not have its own caching layer, but an upstream caching layer can be implemented easily. All communication between the python processes and the js-host processes is performed via HTTP, so placing a reverse proxy such as varnish between the two can massively boost the performance of your requests.

By default, the python layer will infer a host's url from the host's config. If you want to route all requests through another address, define the ROOT_URL setting. For example

    # ...

The python layer will now send all requests through, rather than connecting directly to the host.

Caching requests

Requests are sent to hosts according to js-host's endpoint definition.

When the python layer sends requests to functions, it appends a hash paramater to the url which is a sha1 hash of the serialized data sent to the function. For example, a request to a function named hello_world with the data {'foo': 'bar'} will be sent as:

POST: /function/hello_world?hash=bc4919c6adf7168088eaea06e27a5b23f0f9f9da

This url convention enables you to easily cache a specific function's output by the data that was sent in.



js_host.function.Function objects enable you to pass data from your python process to a JavaScript environment. Functions must be instantiated with a string which matches a name specified in your config file.

If your host.config.js file resembled the following

module.exports = {
  functions: {
    greeter: function(data, cb) {
      cb(null, 'Hello');
    double: function(data, cb) {
      if (!data.number) {
        return cb(new Error('No number was provided');
      cb(null, data.number * 2);

You can call those functions from Python with the following

from js_host.function import Function

greeter = Function('greeter')  # returns 'Hello'

double = Function('double')  # returns '40'  # raises a error 'No number was provided'

Keyword arguments provided to call are passed to the host as JSON. The host deserializes the data sent, and then passes the data to your function as its first argument. The second argument to your function is a callback which allows you to pass data back to the python process.

Your functions can complete their task either synchronously or asynchronously. Once the callback has been called, the host assumes that the function has completed and sends a response back to the Python process.

Functions will lazily bind to the singleton unless you override the function's host attribute.

from js_host.function import Function

greeter = Function('greeter')  # returns `None`

greeter.get_host()  # returns `` = my_host

greeter.get_host()  # returns `my_host`

For more information on the API and behaviour of functions, refer to the js-host's documentation on functions.


JSHost objects read in your config files and act as bridges to JavaScript environments generated from your config files.

If you want to introspect a host, there are some utils provided

# Import the host generated from the values in `js_host.conf.settings`
from import host

# An absolute path to the config file used by the host

# Returns a URL which points to the location of the host on your network

# Returns a dict containing information that the host reported during startup

# Connects to a running host, requests information about it, and returns the data as a dict

# Returns True/False if a host process is running at the expected url

# Connect to an environment and validate that it matches your config

If you are using the manager to control your hosts, the following utils are also available

from import host

# the JSHostManager instance that the host uses

# An absolute path to a file that the host writes its logs to

# Connect to the manager and ask it to restart the host

# Connect to the manager and ask it to stop the host


js_host.manager.JSHostManager objects provide an interface to a detached process which runs on your local network and can spawn js-host instances on demand.

The primary benefit of using a manager is that it abstracts away the bother of spawning a host manually. During development, your primary goal is to build things - issues regarding stability and logging should be left to production environments.

To allow a manager to spawn instances automatically, set the USE_MANAGER setting to True.

If you are writing functions to be used on hosts, be aware that manually starting hosts will provide easier access to the hosts output streams and debugging functionality. Refer to js-host's CLI usage to manually start a host.

Note: managers should only ever be used in development environments. Do not use the manager in production. Please refer to the usage in production section before configuring your environment.

# The singleton manager and host which are provided by default
from import manager, host

host.manager == manager  # True

# Stops the manager and all managed hosts

Under the hood

Managers are spun up via a child process of your python process. The child process blocks python until the manager has been spawned in a third process which is completely detached from python. Using a detached process allows the manager and its hosts to persist even when your python process has restarted or exited.

Once a manager is running, it starts listening for incoming requests which indicate that your python process wants it to spawn a new js-host using a particular config file. Once the new instance has been spawned, the python process asks the manager to register a new connection to the host.

When your python process exits, it sends a disconnect signal to the manager, notifying it that any hosts spawned by your python process are no longer required. When a spawned host no longer has ony open connections, the manager waits for 5 seconds to allow any new connections to be opened. If the time period expires and no new connections have been opened, the manager will stop the host's process.

When all of a manager's hosts have been disconnected and stopped, the manager will stop its own process. This enables the manager to clean up after itself, and prevents rogue processes from running in the background.

The connect/disconnect method enables your python process to hook in to persistent JS environments which survive even when your python has restarted. Maintaining persistent environments enables you to integrate JS technologies which have a high startup cost that should only be incurred once. A typical use-case where this optimisation provides massive performance improvements is integrating a compiler - such as webpack or browserify - which have a sizable startup overhead as they parse all of your files.


Be aware that managers introduce some behaviour that you should be aware of:

  • Python processes will connect to managed hosts based on the path to your config file. If you open a python shell which triggers a connection to a host, the manager will not stop the host until your shell has exited.

    If you are running a server and a python shell which have both connected to the same host, the manager will not stop the host until both processes have exited.

  • Managed hosts can continue running after their config file has changed.

    To force a restart, call the restart method of a managed JSHost instance. For example

    from import host
  • Managers and managed hosts run in processes detached from your shell, which can make it difficult to inspect their output streams.

    To provide some measure of introspection, managed hosts write their output to logfiles which are stored in your temporary directory. The logfile attribute of a JSHost instance provides an absolute path to the file. For example

    from import host
  • If a managed host encounters an unhandled exception, the host will crash and the python process will raise errors until the host is respawned. By design, hosts will not respawn until you have restarted the python process.

    Preventing automatic respawning of crashed hosts simply hides the underlying issue, and makes it difficult to reason about a system's state.

    If a crash is detected, exceptions will be raised indicating that you should consult the host's logfile to inspect the stack traces produced during the unhandled exception.

Reading logs

If you want to inspect the log output of a managed host, a path to the host's logfile is accessible via

from import host

The following one liner will start (or connect to) a managed host and then tail -f its logfile.

$ python -c "from js_host.conf import settings; settings.configure(USE_MANAGER=True); from import host; import subprocess;['tail', '-f', host.logfile])"

Running the tests

pip install -r requirements.txt
cd tests
npm install
cd ..