Integrating Python and Javascript with PyV8

A hobby project of mine would be made much easier if I could run the same code on the server as I run in the web browser. Projects like Node.js have made Javascript on the server a more realistic prospect, but I don’t want to give up on Python and Django, my preferred web development tools.

The obvious solution to this problem is to embed Javascript in Python and to call the key bits of Javascript code from Python. There are two major Javascript interpreters, Mozilla’s SpiderMonkey and Google’s V8.

Unfortunately the python-spidermonkey project is dead and there’s no way of telling if it works with later version of SpiderMonkey. The PyV8 project by contrast is still undergoing active development.

Although PyV8 has a wiki page entitled How To Build it’s not simple to get the project built. They recommend using prebuilt packages, but there are none for recent version of Ubuntu. In this post I’ll describe how to build it on Ubuntu 11.11 and give a simple example of it in action.

The first step is make sure you have the appropriate packages. There may be others that are required and not part of the default install, but there are what I had to install.

sudo aptitude install scons libboost-python-dev

Next you need to checkout both the V8 and PyV8 projects using the commands below.

svn checkout http://v8.googlecode.com/svn/trunk/ v8
svn checkout http://pyv8.googlecode.com/svn/trunk/ pyv8

The key step before building PyV8 is to set the V8_HOME environment variable to the directory where you checked out the V8 code. This allows PyV8 to patch V8 and build it as a static library rather than the default dynamic library. Once you’ve set that you can use the standard Python setup.py commands to build and install the library.

cd v8
export PyV8=`pwd`
cd ../pyv8
python setup.py build
sudo python setup.py install

In future I’ll write more detailed posts about how to use PyV8, but let’s start with a simple example. Mustache is a simple template language that is ideal when you want to create templates in Javascript. There’s actually a Python implementation of Mustache, but let’s pretend that it doesn’t exist.

To start import the PyV8 library and create a JSContext object. These are equivalent to sub-interpreters so you have several instance of your Javascript code running at once.

>>> import PyV8
>>> ctxt = PyV8.JSContext()

Before you can run any Javascript code you need enter() the context. You should also exit() it when you are complete. JSContext objects can be used with with statements to automate this, but for a console session it’s simplest to call the method explicitly. Next we call eval() to run our Javascript code, first by reading in the Mustache library and then to set up our template as a variable.

>>> ctxt.enter()
>>> ctxt.eval(open("mustache.js").read())
>>> ctxt.eval("var template = 'Javascript in Python is ';")

The final stage is to render the template by dynamically created some Javascript code. The results of the expressions are returned as Python objects, so here rendered contains a Python string.

>>> import random
>>> opinion = random.choice(["cool", "great", "nice", "insane"])
>>> rendered = ctxt.eval("Mustache.to_html(template, { opinion: '%s' })" % (opinion, ))
>>> print rendered
Javascript in Python is nice

There’s much more to PyV8 than I’ve described in this post, including calling Python code from Javascript but unfortunately the V8 and PyV8 documentation is a bit lacking. I will post some more of my discoveries in future posts.

Photo Lasauvage History Vehicles - Ford GT40 V8 Engine by Robert GLOD.