` Python to Javascript Compilation

Python to Javascript Compilation - README

py2js (Python to Javascript) is an experimental tool to assist developers in converting python source code to javascript. Note only a subset of python is (and will ever be) supported - python code will need to be made somewhat javascript-friendly to be successfully converted by py2js to javascript which performs an equivalent task.

py2js is an experimental prototype (not production quality code)

Download py2js (alpha release)

For credits and licenses please see the Credits and Licenses section at the end of this page.

py2js requires Python 2.5

Source code is hosted here: JavaScript for Python Programmers


Getting started

download py2js.zip and unzip it. cd to the py2js subdirectory.

To convert the sample python program helloworld.py to javascript:

python py2js.py helloworld.py > helloworld.js

You can now run helloworld.js using (for example) spidermonkey

Note: Depends on _ast module, and so requires Python2.5. Currently tested with Python 2.5, spidermonkey Targets Javascript 1.5

What is supported and what isn't

The following lists give a broad overview of python language support in py2js


Support planned in future

Not supported, no support planned

Customizing py2js

To customize/extend the mapping of built in functions and operators, you can modify the python->javascript mappings in jsmap.py. See the comments in this file for advice.

Module Equivalents

py2js can be configured with javascript and python equivalent implementations of a module. Place the code under modules/python and modules/javascript and configure py2js_modules in jslib.py

module equivalents are useful when you need to manually convert a module from python to javascript or when a javascript equivalent of a python module already exists.

when importing a python module into your python program, for py2js to correctly include the javascript equivalent code, you'll have to import the module using "from <module> import *"

for an example of module equivalents, see the MersenneTwister example:

... and the test example tests/modules/rng.py

for more details on the MersenneTwister python and javascript implementations see section "Credit and Licenses" below.

overriding py2js with py2js "macros"

In some cases py2js will not be able to convert python to javascript. In these situations you can use py2js "macros" to manually control the conversion.

To embed javascript equivalents into your python program (to allow for cases where py2js cannot convert) using the following "macros"

Use a triple-quoted string starting with the string "py2js-verbatim:" to supply some javascript that will be exported to the output of py2js verbatim:

function foo(x) {

Use string literals "py2js-skip-begin" and "py2js-skip-end" to bracket python code which you do not want py2js to attempt to convert:

def foo(x):
print x

Running unit-tests

Run unit tests using:

./unittest.sh (this sets up PYTHONPATH and runs py2js_test.py)

See the comments in py2js_test.py for more details on how the unit tests work

After running py2js_test.py, open the page tests/test.html in your browser to see if you get the same results. py2js_test generates the file tests/unittestdata.js which is executed by this web page.

Credits and Licenses

Using py2js

* py2js maps many python builtin methods and functions to javascript equivalents.

Avoid defining method and function names which match those of python builtin method and function names, because these will also be mapped.

* empty dictionary and list values in python evaluate to false, but the same
is not true in javascript.  Use explicit tests, eg.

if len(list_or_dict) > 0:
# do something with non-empty list/dict

... instead of

if list_or_dict:
# do something wit non-empty list/dict

* list and dictionary access which would cause an exception in python, return
an undefined value in javascript.  Consider guarding access to list and dictionary, eg. use:

if key in dict:
val = dict[key]
val = "not found"

... instead of

val = dict[key]
val = "not found"

* do not use negative subscripts when accessing arrays

eg. use:

x = ['a','b','c']
print x[len(x)-1] # prints 'c'

... instead of ...

x = ['a','b','c']
print x[-1]	  # prints 'c'

* in python 2.5 and earlier, integer division results (by default) in an
integer.  Use "from __future__ import division" to return a floating point
result from integer division in python (which matches the javascript

* do not use + in python to concatenate lists, + does not work on Javascript Arrays in the same way. 

eg use extend:

a = [1,2,3]
b = [4,5,6]
c = a[:]

... instead of
a = [1,2,3]
b = [4,5,6]
c = a + b

Appendix - Development History

26/10/2008	Initial version
01/11/2008	Add for ... in ... support, and other stuff
08/11/2008	Add basic class support, try/except/finally, more operators	
15/11/2008	Add a few builtin functions, list comprehension
22/11/2008	Improve support for classes - inheritance and static members
30/11/2008	Import support part 1 (not yet stable)
05/12/2008	Import support part 2 (more stable, but...)
12/12/2008	Run unit tests in browsers, support for del
20/12/2008	Support for generator classes
27/12/2008	Some bug fixes.  Support for module equivalents (Mersenne Twister example)
02/01/2009	Initial support for super(>class<,>instance<).
11/01/2009	Some clean up.  Add support for string formatting (mostly complete).
18/01/2009	Completed string formatting.


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