RapydScript Documentation

Recent Changes that Could Cause Slight Incompatibility with Older Code

  • new keyword should no longer be used with native JavaScript objects (Image, RegExp, Error, etc.)
  • comma should no longer be on the same line as last line of function definition in dictionaries and object literals (unless preceeded by a semi-colon)

What is RapydScript?

RapydScript is a pre-compiler for JavaScript, similar to CoffeeScript, but with cleaner, more readable syntax. The syntax is mostly Python, but allows JavaScript as well. This project was written as an alternative to Pyjamas for those wishing Python-like JavaScript without the extra overhead and complexity Pyjamas introduces.

RapydScript allows to write your front-end in Python without the overhead that other similar frameworks introduce (the performance is the same as with pure JavaScript). To allow this, RapydScript leverages a customized version of PyvaScript (a compiler designed to do nothing more than add Pythonic syntax sugar to JavaScript), adding some much needed improvements. To those familiar with PyvaScript, the best way to describe RapydScript is PyvaScript++ (it really is like comparing C++ to C in terms of what it brings to the table). To those familiar with CoffeeScript, RapydScript is like CoffeeScript with syntax (and some features) of Python. To those familiar with Pyjamas, RapydScript brings many of the same features and support for Python syntax without the same overhead. Don't worry if you've never used either of the above-mentioned compilers, if you've ever had to write your code in pure JavaScript you'll appreciate RapydScript. RapydScript combines the best features of Python as well as JavaScript, bringing you features most other Pythonic JavaScript replacements overlook. Here are a few features of RapydScript:

  • classes that work and feel similar to Python
  • optional function arguments that work similar to Python
  • inheritance system that's both, more powerful than Python and cleaner than JavaScript
  • support for object literals with anonymous functions, like in JavaScript
  • ability to invoke any JavaScript/DOM object/function/method as if it's part of the same framework, without the need for special syntax
  • variable and object scoping that make sense (no need for repetitive 'var' or 'new' keywords)
  • ability to use both, Python's methods/functions and JavaScript's alternatives
  • similar to above, ability to use both, Python's and JavaScript's tutorials (as well as widgets)

Let's not waste any more time with the introductions, however. The best way to learn a new language/framework is to dive in.


If you have questions, bug reports, or feature requests, feel free to post them on our mailing list: (


You can use RapydScript without installing it, from the same directory you unpack it to, or 'install' it, which will copy the files to your system path, making it accessible from anywhere (that way you don't have to be in RapydScript's directory to perform compilation).

Current implementation of RapydScript relies on PyvaScript (this might change in the future, I'm considering using CoffeeScript as base instead). You will first need to install PyMeta (Ometa Tokenizer used by PyvaScript), and then RapydScript (PyvaScript is already included as part of RapydScript). If you're using Linux or BSD, just run the following command to install PyMeta (you will need PyMeta whether you run RapydScript from directory or install it):

wget; unzip; cd wkornewald-pymeta*; sudo python install; cd ..; rm

Now run the following to install RapydScript:

wget; unzip; cd pyjeon-rapydscript*; sudo python install; cd ..; rm

Alternatively, you can also install RapydScript from PIP repository.

If you're using OSX, you can probably use the same commands (let me know if that's not the case). If you're using Windows, just download the files I linked to in the above URLs and install them in a similar manner.

To run rapydscript without installing it, you will need to set your PYTHONPATH to point to the rapydscript_repo_location directory.


Once you have installed RapydScript, compiling your application is as simple as running the following command:

rapydscript <location of main file>

This will generate a JavaScript file with the same name as your main file, in the same directory. Now all you have to do is reference it in your html page the same way as you would with a typical JavaScript file. If you're only using RapydScript for classes and functions, then you're all set. If you're using additional Python methods, such as range(), print(), list.append(), list.remove(), then you will want to link RapydScript's stdlib.js in your html page as well. There are two ways of doing this, one is to include it as a JavaScript file in your HTML, the other is to include it as an import in your source code and let RapydScript pull it in automatically.

Getting Started

Like JavaScript, RapydScript can be used to create anything from a quick function to a complex web-app. RapydScript can access anything regular JavaScript can, in the same manner. Let's say we want to write a function that greets us with a "Hello World" pop-up. The following code will do it:

def greet():
    alert("Hello World!")

Once compiled, the above code will turn into the following JavaScript:

function greet() {
    alert("Hello World!");

Now you can reference this function from other JavaScript or the page itself (using "onclick", for example). For our next example, let's say you want a function that computes factorial of a number:

def factorial(n):
    if n == 0:
        return 1
    return n * factorial(n-1)

Now all we need is to tie it into our page so that it's interactive. Let's add an input field to the page body and a cell for displaying the factorial of the number in the input once the input loses focus.

<input id="user-input" onblur="computeFactorial()"></input>
<div id="result"></div>

NOTE: To complement RapydScript, I have also written RapydML (, which is a pre-compiler for HTML (just like RapydScript is a pre-compiler for JavaScript).

Now let's implement computeFactorial() function in RapydScript:

def computeFactorial():
    n = document.getElementById("user-input").value
    document.getElementById("result").innerHTML = factorial(n)

Again, notice that we have access to everything JavaScript has access to, including direct DOM manipulation. Once compiled, this function will look like this:

function computeFactorial() {
    var n;
    n = document.getElementById("user-input").value;
    document.getElementById("result").innerHTML = factorial(n);

Notice that RapydScript automatically declares variables in local scope when you try to assign to them. This not only makes your code shorter, but saves you from making common JavaScript mistake of overwriting a global. For more information on controlling variable scope, see Scope Control section.

Leveraging other APIs

Aside from Python-like stdlib, RapydScript does not have any of its own APIs. Nor does it need to, there are already good options available that we can leverage instead. If we wanted, for example, to rewrite the above factorial logic using jQuery, we could easily do so:

def computeFactorial():
    n = $("#user-input").val()

Many of these external APIs, however, take object literals as input. Like with JavaScript, you can easily create those with RapydScript, the same way you would create one in JavaScript, or a dictionary in Python:

styles = {
    'background-color': '#ffe',
    'border-left':      '5px solid #ccc',
    'width':            50,

Now you can pass it to jQuery:


Another feature of RapydScript is ability to have functions as part of your object literal. JavaScript APIs often take callback/handler functions as part of their input parameters, and RapydScript lets you create such object literal without any quirks/hacks:

params = {
    width:  50,
    height: 30,
    onclick:    def(event):
        alert("you clicked me"),
    onmouseover:    def(event):
        $(self).css('background', 'red')
    onmouseout: def(event):
        # reset the background
        $(self).css('background', '')

Note the comma on a new line following a function declaration, it needs to be there to let the compiler know there are more attributes in this object literal, yet it can't go on the same line as the function since it would get parsed as part of the function block. Like Python, however, RapydScript supports new-line shorthand using a ;, which you could use to place the comma on the same line:

hash = {
    'foo':  def():
    'bar':  def():

NOTE: If you used RapydScript in the past, you might have noticed that the comma is no longer on the same line as the function. This was changed in a recent version of RapydScript to be more consistent (and to not require a comma at the end) (I noticed that PyvaScript now supports anonymous functions within object literals and its solution seemed cleaner than RapydScript, so I removed the extra bit of pre-processing RapydScript was doing for these).

Anonymous Functions

Like JavaScript, RapydScript allows the use of anonymous functions. In fact, you've already seen the use of anonymous functions in previous section when creating an object literal ('onmouseover' and 'onmouseout' assignments). This is similar to Python's lambda function, except that the syntax isn't awkward like lambda, and the function isn't limited to one line. The following two function declarations are equivalent:

def factorial(n):
    if n == 0:
        return 1
    return n * factorial(n-1)

factorial = def(n):
    if n == 0:
        return 1
    return n * factorial(n-1)

This might not seem like much at first, but if you're familiar with JavaScript, you know that this can be extermely useful to the programmer, especially when dealing with nested functions, which are a bit syntactically awkward in Python (it's not immediatelly obvious that those can be copied and assigned to other objects). To illustrate the usefulness, let's create a method that creates and returns an element that changes color while the user keeps the mouse pressed on it.

def makeDivThatTurnsGreen():
    div = $('<div></div>')
    turnGreen = def(event):
        div.css('background', 'green')
    resetColor = def(event):
        div.css('background', '')
    return div

At first glance, anonymous functions might not seem that useful. We could have easily created nested functions and assigned them instead. By using anonymous functions, however, we can quickly identify that these functions will be bound to a different object. They belong to the div, not the main function that created them, nor the logic that invoked it. The best use case for these is creating an element inside another function/object without getting confused which object the function belongs to.

Additionally, as you already noticed in the previous section, anonymous functions can be used to avoid creating excessive temporary variables and make your code cleaner:

math_ops = {
    add:    def(a, b): return a+b;,
    sub:    def(a, b): return a-b;,
    mul:    def(a, b): return a*b;,
    div:    def(a, b): return a/b;,
    roots:  def(a, b, c):
        r = Math.sqrt(b*b - 4*a*c)
        d = 2*a
        return (-b + r)/d, (-b - r)/d

I'm sure you will agree that the above code is cleaner than declaring 5 temporary variables first and assigning them to the object literal keys after. Note that the example puts the function header (def()) and content on the same line. I'll refer to it as function inlining. This is meant as a feature of RapydScript to make the code cleaner in cases like the example above. While you can use it in longer functions by chaining statements together using ;, a good rule of thumb (to keep your code clean) is if your function needs semi-colons ask yourself whether you should be inlining, and if it needs more than 2 semi-colons, the answer is probably no (note that you can also use semi-colons as newline separators within functions that aren't inlined, as in the example in the previous section).

Chaining Blocks

RapydScript wouldn't be useful if it required work-arounds for things that JavaScript handled easily. If you've worked with JavaScript or jQuery before, you've probably seen the following syntax:

    // some logic here

This code calls the function immediatelly after declaring it instead of assigning it to a variable. Python doesn't have any way of doing this. The closest work-around is this:

def tmp():
    # some logic here

While it's not horrible, it did litter our namespace with a temporary variable. If we have to do this repeatedly, this pattern does get annoying. This is where RapydScript decided to be a little unorthodox and implement the JavaScript-like solution:

    # some logic here

RapydScript will bind any lines beginning with . to the outside of the block with the matching indentation. This logic isn't limited to the .call() method, you can use it with .apply() or any other method/property the function has assigned to it. This can be used for jQuery as well:

.css('background-color', 'red')

The only limitation is that the indentation has to match, if you prefer to indent your chained calls, you can still do so by using the \ delimiter:

    .css('background-color', 'red')\

RapydScript also allows an alternative syntax for the same feature, for those prefering Python's traditional, hanging-indent look:

def(one, two) and call(this, 1, 2):

Which is equivalent to the following:

def(one, two):
.call(this, 1, 2)

Some of you might welcome this feature, some of you might not. RapydScript always aims to make its unique features unobtrusive to regular Python, which means that you don't have to use them if you disagree with them. Recently, we have enhanced this feature to handle do/while loops as well:

a = 0
    a += 1
.while a < 1

In my opinion, this is something even Python could benefit from. Like with functions, you could use the hanging-indent form as well:

a = 0
do and while a < 1:
    a += 1

Optional Arguments

Like Python, Javascript allows optional arguments to be passed into a function. Unlike Python, however, JavaScript doesn't assign default values to these, nor does it work well with optional arguments. If you forget to pass an argument, JavaScript will simply set the variable to undefined, and it's up to you to handle it. Should you forget about it, you'll probably pay the price later, when you use this variable in any sort of mathematical computation. By that time, you'll either end up with NaN (not a number), infinity, or some other weird value, causing an error a few hundred lines after the line responsible for causing it (good luck debugging it). Luckily, RapydScript allows sane optional arguments, similar to Python. The following function, for example, allows you to pass in a color for the background, or it will default to black (the format is r,g,b,a):

def setColor(color=[0,0,0,1]):
    $('body').css('background', 'rgba('+','.join(color)+')')

One thing to note here, is that unlike Python, RapydScript will create a separate object for the optional argument each time the function is called. This makes it slightly less efficient, but prevents it from messing up existing objects.

Variable number of arguments (*args)

Like Python, Javascript allows the user to write a function that takes variable number of arguments and performs its logic on all of them. Some examples of this are Math.max(), console.log() function and array's concat() method. Unlike, Python, however, JavaScript does not make this intuitive. You have to use a special iterable element inside the function called arguments, which has some properties of an array but doesn't support all of the functionality. Likewise, if you want to unfold an array into a list of arguments for a function during a function call, you have to use .apply() method instead of invoking the function normally. RapydScript converts Pythonic way of *args declaration to JavaScript. For example, the following function definition will take 2 named arguments, and dump the rest into an array (which DOES support all of array functionality):

def doSomething(a, b, *args):

Likewise, the following function call will unpack the array into separate arguments for the function:


This is useful for cases where you have multiple elements inside a list, but don't know the size of this list. Here are two equivalent ways of calling the print function, for example:

# first way
a = 'I was here'
b = 'and there'
print(a, b)

# second way
a = 'I was here'
b = 'and there'
args = [a, b]

In this particular case, there is no advantage to using *, but if args gets passed in from outside, and we don't know its size, * becomes very handy. Likewise, if you're writing a function that can take variable number of arguments, it's cleaner to use *args rather than forcing the developer using it to pass in an array.

Inferred Tuple Packing/Unpacking

Like Python, RapydScript allows inferred tuple packing/unpacking and assignment. While inferred/implicit logic is usually bad, it can sometimes make the code more cleaner, and based on the order of statements in the Zen of Python, 'beautiful' takes priority over 'explicit'. For example, if you wanted to swap two variables, the following looks cleaner than explicitly declaring a temporary variable:

a, b = b, a

Likewise, if a function returns multiple variables, it's cleaner to say:

a, b, c = fun()

rather than:

tmp = fun()
a = tmp[0]
b = tmp[1]
c = tmp[2]

Since JavaScript doesn't have tuples, RapydScript uses arrays for tuple packing/unpacking behind the scenes, but the functionality stays the same. Note that unpacking only occurs when you're assigning to multiple arguments:

a, b, c = fun()     # gets unpacked
tmp = fun()         # no unpacking, tmp will store an array of length 3

Unpacking can also be done in for loops (which you can read about in later section):

for index, value in enumerate(items):
    print(index+': '+value)

Tuple packing is the reverse operation, and is done to the variables being assigned, rather than the ones being assigned to. This can occur during assignment or function return:

def fun():
    return 1, 2, 3

To summarize packing and unpacking, it's basically just syntax sugar to remove obvious assignment logic that would just litter the code. For example, the swap operation shown in the beginning of this section is equivalent to the following code:

tmp = [b, a]
a = tmp[0]
b = tmp[1]

Python vs JavaScript

RapydScript allows you use both, Python and JavaScript names for the methods. For example, we can 'push()' a value to array, as well as 'append()' it:

arr = []
print(arr) # outputs [2,4]

In order to use Python's methods, you will have to include RapydScript's stdlib.js. There are two ways of doing this. One is by adding the following line in your html page:

<script type="text/javascript" src='stdlib.js'></script>

The other way is to include the following line at the top of your main RapydScript file:

import stdlib

The advantage of the second method is that the library will automatically be pulled in without having to manually copy it over into your JavaScript directory. The first include method, however, might be useful when you have multiple independently compiled RapydScript programs on your page and don't want the overhead of the same stdlib included in each one.

In order to simulate Python-like methods, I had to bastardize a couple of JavaScript's own methods. For example, array.pop() has been overwritten to work like Python's pop() (or JavaScript's splice()):

arr.pop()       # removes last element (expected behavior in JavaScript and Python)
arr.pop(2)      # removes third element (expected behavior in Python, but not JavaScript)
arr.splice(2,1) # removes third element (expected behavior in JavaScript, but not Python)

There is a subtle difference between the last 2 lines above, arr.pop(2) will return a single element, arr.splice(2,1) will return an array containing that single element. Most of the keywords are interchangeable as well:

RapydScript     JavaScript

self            this
None            null
False           false
True            true

The JavaScript operators, however, are not supported. You will have to use Python versions of those. If you're unfamiliar with them, here is the mapping RapydScript uses:

RapydScript     JavaScript

and             &&
or              ||
not             !
is              ===
is not          !==
+=1             ++
-=1             --

Admittedly, is is not exactly the same thing in Python as === in JavaScript, but JavaScript is quirky when it comes to comparing objects anyway.

In rare cases RapydScript might not allow you to do what you need to, and you need access to pure JavaScript. When that's the case, you can wrap your JavaScript in a string, passing it to JS() method. Code inside JS() method is not a sandbox, you can still interact with it from normal RapydScript:

JS('a = {foo: "bar", baz: 1};')
print(    # prints "bar"

One last thing to note is the difference between print() and console.log, print() is part of RapydScript's stdlib, and is designed to work similar to Python's print statement, console.log() is JavaScript's version of debug output, which is more powerful but also more tedious when you just want a quick output. Here are some examples:

arr = [1,2,3,4]
print(arr)          # [1,2,3,4]
console.log(arr)    # [1,2,3,4]

arr2 = [[1,2],[3,4]]
print(arr2)         # [[1,2],[3,4]]
console.log(arr2)   # [Array[2], Array[2]]

hash = {'dogs': 1, 'cats': 2}
print(hash)         # {"dogs":1,"cats":2}
console.log(hash)   # Object


RapydScript's loops work like Python, not JavaScript. You can't, for example use 'for(i=0;i<max;i++)' syntax. You can, however, loop through arrays using 'for ... in' syntax without worrying about the extra irrelevant attributes regular JavaScript returns.

animals = ['cat', 'dog', 'mouse', 'horse']
for animal in animals:
    print('I have a '+animal)

If you need to use the index in the loop as well, you can do so by using enumerate():

for index, animal in enumerate(animals):
    print("index:"+index, "animal:"+animal)

Like in Python, if you just want the index, you can use range:

for index in range(len(animals)):           # or range(animals.length)
    print("animal "+index+" is a "+animals[index])

List Comprehensions

RapydScript also supports list comprehensions, using Python syntax. Instead of the following, for example:

myArray = []
for index in range(1,20):
    if index*index % 3 == 0:

You could write this:

myArray = [i*i for i in range(1,20) if i*i%3 == 0]

Which is not only shorter, but easier to read too. There are a few gotchas you might want to be aware of. RapydScript implements list comprehensions via a combination of map() and filter() functions, which are now part of the stdlib. For example, the above line compiles to:

var myArray;
myArray = range(1,20).filter(function(i) { return i*i%3 == 0; }).map(function(i) { return i*i; });

I have not tested RapydScript with more complex list comprehensions, and given its current implementation (regex), it's likely to break when you start nesting them (however, this in general is a bad idea anyway, when your list comprehensions start getting incomprehensibly long, you're better off with a loop anyway). Second, RapydScript takes the i*i and i*i%3 == 0 portions and inserts them into the output verbatim. This means that Python-only syntax, such as i**2 might not get compiled correctly here, but ** is not supported by RapydScript anyway, and I couldn't think of another case, so this is a low-priority bug for now (also, the work-around is quite simple, define a function doing the same operation before-hand). Which brings me to my last point. To accomodate filter() and map() syntax, RapydScript wraps this logic in an additional function call. So i*i becomes:

function (i) {
    return i*i;

This makes sense. However, this also means that if you write the following comprehension:

[stuff(x) for x in [1,2,3]]

then stuff(x) also gets wrapped in an extra function. I'd rather deal with the overhead of an extra function call, however, than adding more special cases that could introduce additional bugs. The current list comprehension in RapydScript is not built with performance in mind. It does omit the filter() part if you don't include the if statement, and it always filters first, to reduce the number of elements it needs to run map() on. However, in the worst case scenario (when filter doesn't remove any elements), it will generate two extra arrays internally of the same size as original.

Inclusive/Exclusive Sequences

Like Python, RapydScript has a range() function. While powerful, the result it generates isn't immediatelly obvious when looking at the code. It's a minor pet peeve, but the couple extra seconds trying to visually parse it and remember that it's not inclusive can detract from the code flow. To remedy this, RapydScript borrows to/til operators from LiveScript (also known as human-readable versions of Ruby's ../...). The following 4 lines of code are equivalent, for example:

a = [3 to 8]
a = [3 til 9]
a = range(3, 9)
a = [3, 4, 5, 6, 7, 8]

You can also use sequences within loops:

for i in [1 to 5]:

Or in list comprehensions:

[i*i for i in [1 to 6] if i%2 == 0]

The to/til statement gets converted to range() at compile time, and therefore can support variables or even expressions for start and end of the range:

num = 5
rng = [num to num * 2]

The to/til operators bind less tightly than arithmetic operators, so parentheses are optional.


This is where RapydScript really starts to shine. JavaScript is known for having really crappy class implementation (it's basically a hack on top of a normal function, most experienced users suggest using external libraries for creating those instead of creating them in pure JavaScript). Luckily RapydScript fixes that. Let's imagine we want a special text field that takes in a user color string and changes color based on it. Let's create such field via a class.

class ColorfulTextField:
    def __init__(self):
        field = $('<input></input>')
        changeColor = def(event):
            field.css('backround', field.val())
        self.widget = field

This class abuses DOM's tolerant behavior, where it will default to the original setting when the passed-in color is invalid (saving us the extra error-checking logic). To append this field to our page we can run the following code:

textfield = ColorfulTextField()

If you're used to JavaScript, the code above probably set off a few red flags in your head. In pure JavaScript, you can't create an object without using a 'new' operator. Don't worry, the above code will compile to the following:

var textfield;
textfield = new ColorfulTextField()

RapydScript will automatically handle appending the 'new' keyword for you, assuming you used 'class' to create the class for your object. This also holds when creating an object inside a list or returning it as well. You could easily do the following, for example:

fields = [ColorfulTextField(), ColorfulTextField(), ColorfulTextField()]

This is very useful for avoiding a common JavaScript error of creating 'undefined' objects by forgetting this keyword. One other point to note here is that regular DOM/JavaScript objects are also covered by this. So if you want to create a DOM image element, you should not use the 'new' keyword either:

myImage = Image()

But RapydScript's capability doesn't end here. Like Python, RapydScript allows inheritance. Let's say, for example, we want a new field, which works similar to the one above. But in addition to changing color of the field, it allows us to change the color of a different item, with ID of 'target' after we press the 'apply' button, located right next to it. Not a problem, let's implement this guy:

class TextFieldAffectingOthers(ColorfulTextField):
    def __init__(self):
        field = self.widget
        submit = $('<button type="button">apply</button>')
        applyColor = def(event):
            $('#target').css('background', field.val())
        self.widget = $('<div></div>')\

A couple things to note here. We can invoke methods from the parent class the same way we would in Python, by using Parent.method(self, ...) syntax. This allows us to control when and how (assuming it requires additional arguments) the parent method gets executed. Also note the use of \ operator to break up a line. This is something Python allows for keeping each line short and legible. Likewise, RapydScript, being indentation-based, allows the same.

An important distinction between Python and RapydScript is that RapydScript does not allow multiple inheritance. This might seem like a big deal at first, but in actuality it barely matters. When using multiple inheritance, we usually only care about a few methods from the alternative classes. Leveraging JavaScript prototypical inheritance, RapydScript allows us to reuse methods from another class without even inheriting from it:

class Something(Parent):
    def method(self, var):
        Parent.method(self, var)
        SomethingElse.method(self, var)

Notice that Something class has no __init__ method. Like in Python, this method is optional for classes. If you omit it, an empty constructor will automatically get created for you by RapydScript (or when inheriting, the parent's constructor will be used). Also notice that we never inherited from SomethingElse class, yet we can invoke its methods. This brings us to the next point, the only real advantage of inheriting from another class (which you can't gain by calling the other classes method as shown above) is that the omitted methods are automatically copied from the parent. Admittedly, we might also care about instance() method, to have it work with the non-main parent, but we're already overwriting JavaScript's instanceof() method in stdlib, so feel free to tweak it further, if you have the need to. To summarize classes, assume they work the same way as in Python, plus a few bonus cases. The following, for example, are equivalent:

class Aclass:
    def __init__(self):

    def method(self):

class Aclass:
    def __init__(self):
        self.method = def():

Note that when binding the method to another object, self inside the bound function will no longer correspond to the original object. To remedy this, you can create another variable storing self from original class:

class Main:
    def __init__(self):
        main = self
        method = def():

    def doSomething(self):

Exception Handling

Like Python and JavaScript, RapydScript has exception handling logic. The following, for example, will warn the user if variable foo is not defined:

    print("Foo wasn't declared yet")

It's a good practice, however, to only catch exceptions we expect. Imagine, for example, if foo was defined, but as a circular structure (with one of its attributes referencing itself):

foo = {} = foo

We would still trigger an exception, but for a completely different reason. A better way to rewrite our try/except block would be:

except ReferenceError:
    print("Foo wasn't declared yet")

We could also handle circular structure exception, if we needed to:

except ReferenceError:
    print("Foo wasn't declared yet")
except TypeError:
    print("One of foo's attributes references foo")

Or we could just dump the error back to the user:

except as err:
    print( + ':' + err.message)

In this example, err is a JavaScript error object, it has name and message attributes, more information can be found at You can inherit from this object as if it was a class to create custom errors, just like you would in Python:

class MyError(Error):
    def __init__(self, message): = Error
        self.message = message

raise MyError('This is a custom error!')

You can lump multiple errors in the same except block as well:

except ReferenceError, TypeError as e:
    print( + ':' + e.message)

Basically, try/except/finally in RapydScript works very similar to the way it does in Python 3, lacking only the else directive (it didn't seem useful enough to implement). Like in Python and JavaScript, you can nest multiple exceptions inside each other, and use raise to throw the error back at the user:

except ReferenceError as e:
    if e.message == 'foo is not defined':
        raise e
    # reset foo
    foo = 'bar'

Scope Control

Scope refers to the context of a variable. For example, a variable declared inside a function is not seen by the code outside the function. This variable's scope is local to the function. JavaScript controls scope via var keyword. Any variable that you start using without declaring with a var first will try to reference inner-most variable with the same name, if one doesn't exist, it will create a global variable with that name. For example, the following JavaScript code will not only return a incremented by 1, but also overwrite the global variable a with a+1:

a = 1;
a_plus_1 = function() {
    return ++a;

Basically, JavaScript defaults to outer or global scope if you omit var. This behavior can introduce some very frustrating bugs in large applications. To avoid this problem, RapydScript's scope preference works in reverse (same as Python's). RapydScript will prefer local-most scope, always creating a local variable if you perform any sort of assignment on it in a function (this is called variable shadowing). Shadowing can create another annoyance, however, of function's variable changes getting discarded. For example, at first, it looks like the following code will set a to 2:

a = 1
b = 1
increment = def():
    a += b

When executed, however, increment() function will discard any changes to a. This is because, like Python, RapydScript will not allow you to edit variables declared in outer scope. As soon as you use any sort of assignment with a in the inner scope, RapydScript will declare it as an internal variable, shadowing a in the outer scope. One way around this is to use the global keyword, declaring a as a global variable. This, however, must be done in every function that edits a. It also litters global scope, which it frowned upon because it can accidently overwrite an unrelated variable with the same name (declared by someone else or another library). RapydScript solves this by introducing nonlocal keyword (just like Python 3):

a = 1
b = 1
increment = def():
    nonlocal a
    a += b

Note that b is not affected by shadowing. It's the assignment operator that triggers shadowing, you can read outer-scope variables without having to use nonlocal. You can combine multiple non-local arguments by separating them with a comma: nonlocal a, b, c. You can also chain nonlocal declarations to escape multiple scopes:

def fun1():
    a = 5
    b = fun2():
        nonlocal a
        a *= 2
        c = fun3():
            nonlocal a
            a += 1

Shadowing is preferred in most cases, since it can't accidently damage outside logic, and if you want to edit an external variable, you're usually better off assigning function's return value to it. There are cases, however, when using nonlocal makes the code cleaner. There is also global, but it is rarely a good solution, and use of nonlocal is preferred to it.


Like Python, RapydScript allows you to import additional modules into your code. Unlike Python, however, (and like RapydML) the current implementation of the importing logic is naive. This means that RaoydScript doesn't separate different modules into separate namespaces, nor does it support module aliasing yet (eventually I do want that functionality).

For those unfamiliar with importing, let's imagine we're writing a very large program. This program is several thousand lines of code. We could dump it all into the same file, but that wouldn't be too clean (especially when we have multiple developers working on it). Alternatively, we could separate different chunks of the program into different files. Let's imagine, for example, that we're writing a videogame. We've already written a module that implements 'BasicCharacter' class, used by NPCs, monsters, and the main character. We've saved this class to Basic.pyj (that's the extension RapydScript prefers). Now let's create the main character in a different module:

import Basic

class MainCharacter(BasicCharacter):
    This is the main character class, similar to basic but it implements attack and hp
    def __init__(self):
        self.hp = 100
        self.damage = 10

    def attack(self, something):

    def getHurt(self, damage):
        self.hp -= damage

With RapydScript's importing, you don't need to worry about including each JavaScript file individually in your html. All your .pyj files will get concatenated into a single .js file that you can then include in your page. The name of the .js file will match the name of the file you used input for the RapydScript compiler.

RapydScript already adds several modules you can import by default, like stdlib and yql. Check out the examples or the documentation in the modules themselves for usage.

Advanced Usage Topics

This section contains various topics which might be of interest to the programmer writing large projects using RapydScript, but might not be relevant to a programmer who is just getting started with RapydScript. The topics in this section focus on coding conventions to keep your code clean, optimizations, and additional libraries that come with RapydScript, as well as suggestions for writing your own libraries.

Code Conventions

It's not hard to see that RapydScript is a cleaner language than JavaScript. However, like with all dynamically-typed languages (including Python), it's still easy to shoot yourself in the foot if you don't follow some sort of code conventions. Needless to say, they're called conventions for a reason, feel free to ignore them if you already have a set of conventions you follow or if you disagree with some.

Tabs vs Spaces

This seems to be a very old debate. Python code conventions suggest 4-space indent, most of the bundled RapydScript files use 1-tab for indentation. I use tabs for a couple reasons, that's the code convention at my work and I use a similar setup for my vim at home, and because I noticed some editors only delete 1 space when using backspace with 4-space setup. If you would rather use 4 spaces, that's fine, just stay consistent.

Object Literals vs Hashes/Dicts

JavaScript treats object literals and hashes as the same thing. I'm not a fan of this policy. Some of the problems you can see resulting from this is Google Closure compiler's ADVANACED_OPTIMIZATIONS breaking a lot of seemingly-good JavaScript code. The main problem for most of the code that breaks seems to be renaming of methods/variables in one place and not another. My suggestion is to ALWAYS treat object literals as object literals and ALWAYS treat hashes as hashes, basically be consistent about quoting your keys. As an added bonus, your code will have a much better chance of compiling correctly via Closure compiler. For example:

obj = {
    foo:    1,
    bar:    def(): print('bar' + str(foo))
hash = {
    'foo':  1,
    'bar':  def(): print('bar' + str(foo))
}       # good
obj['bar']()    # bad      # bad
hash['bar']()   # good

There is a reason RapydScript added ability to use unquoted object literal keys back into our version of PyvaScript, and this is it.


Don't abuse semi-colons. They're meant as a way to group related logic together, not to fit your entire web-app on one line. The following is fine:

X = 0; Y = 1

Anything that requires more than a couple semi-colons, however, or involves long mathematical computations, is better off on its own line. Use your discretion, if the logic requires more than one visual pass-through from the programmer to understand the flow, you probably shouldn't use semi-colons. A Fibanacci function, as shown below, would probably be the upper limit of the kind of logic you could sanely represent with semi-colons:

fib = def(x): if x<=1: return 1; return fib(x-1)+fib(x-2)

Even for this example, however, I'd personally prefer to use multiple lines.

jQuery-wrapped Elements

If you use jQuery with your app, you will probably be storing these into variables a lot. If you've written a decently sized app, you've probably mistaken a bare element with wrapped element at least once. This is especially true of objects like canvas, where you need to access object's attributes and methods directly. My solution for these is simple, prepend jQuery-wrapped elements with $:

$canvas = ('<canvas></canvas>')
canvas = $canvas.get(0)
ctx = canvas.getContext('2d')

This is especially useful with function definitions, since you will immediatelly know what kind of object the function takes in just by skimming its signature.


In a perfect world, software works flawlessly and doesn't have any special cases requiring workarounds on the user's part. In a less-perfect world, all quirks are due to the software itself and can be fixed. In our world, we not only have to deal with the quirks of the software we're using, but other software it interacts with. RapydScript is no exception, in addition to its own quirks there are a few quirks brought upon us by the browser as well as a few bugs in jQuery that affect us. Here is a list of things you need to be aware of:

  • RapydScript automatically appends 'new' keyword when using classes generated by it, it does not append it to objects created by another library, but it does append it to native JavaScript objects like Image and RegExp.
  • jQuery erroneously assumes that no other library will be modifying JavaScript's 'Object', and fails to do object.hasOwnProperty() check in multiple places where it should. To avoid breaking it, I had to implement stdlib such that dictionary methods are methods of a different object. Regular Python allows you to call hash.keys() as well as dict.keys(hash). RapydScript only supports the second notation - which is admittedly a bit more awkward.
  • List comprehensions could break when nesting multiple of them together on one line

Additional Features

In addition to the above features, RapydScript also checks for obvious errors during compilation. These errors include common mistakes made by developers as well as things that aren't really errors but PyvaScript will still break on (due to its own bugs). Some examples include mixing tabs and spaces in leading whitespace, using wrong library for JavaScript's Math methods (you want to use Math.sqrt() instead of math.sqrt() or sqrt()), reusing same names for global variables/classes/functions, omitting space after 'if' even if the next character is a bracket, and some other minor PyvaScript bugs. This should be enough to get you started with RapydScript. If I omitted anything that you think is useful, feel free to contact me.