PScript introduction

The pscript module provides functionality for transpiling Python code to JavaScript.

Quick intro

This is a brief intro for using PScript. For more details see the sections below.

PScript is a tool to write JavaScript using (a subset) of the Python language. All relevant builtins, and the methods of list, dict and str are supported. Not supported are set, slicing with steps, yield and imports. Other than that, most Python code should work as expected … mostly, see caveats below. If you try hard enough the JavaScript may shine through. As a rule of thumb, the code should behave as expected when correct, but error reporting may not be very Pythonic.

The most important functions you need to know about are py2js and evalpy. In principal you do not need knowledge of JavaScript to write PScript code, though it does help in corner cases.


There is an increase in Python projects that target web technology to handle visualization and user interaction. PScript grew out of a desire to allow writing JavaScript callbacks in Python, to allow user-defined interaction to be flexible, fast, and stand-alone.

This resulted in the following two main goals:

  • To make writing JavaScript easier and less frustrating, by letting people write it with the Python syntax and builtins, and fixing some of JavaScripts quirks.
  • To allow JavaScript snippets to be defined naturally inside a Python program.

Code produced by PScript works standalone. Any (PScript-compatible) Python snippet can be converted to JS; you don’t need another JS library to run it.

PScript can also be used to develop standalone JavaScript (AMD) modules.

PScript is just JavaScript

The purpose of projects like Skulpt or PyJS is to enable full Python support in the browser. This approach will always be plagued by a fundamental limitation: libraries that are not pure Python (like numpy) will not work.

PScript takes a more modest approach; it is a tool that allows one to write JavaScript with a Python syntax. PScript is just JavaScript.

This means that depending on what you want to achieve, you may still need to know a thing or two about how JavaScript works. Further, not all Python code can be converted (e.g. import is not supported), and lists and dicts are really just JavaScript arrays and objects, respectively.


PScript makes writing JS more “Pythonic”. Apart from allowing Python syntax for loops, classes, etc, all relevant Python builtins are supported, as well as the methods of list, dict and str. E.g. you can use print(), range(), L.append(), D.update(), etc.

The empty list and dict evaluate to false (whereas in JS it’s true), and isinstance() just works (whereas JS’ typeof is broken).

Deep comparisons are supported (e.g. for == and in), so you can compare two lists or dicts, or even a structure of nested lists/dicts. Lists can be combined with the plus operator, and lists and strings can be repeated with the multiply (star) operator. Class methods are bound functions.


PScript fixes some of JS’s quirks, but it’s still just JavaScript. Here’s a list of things to keep an eye out for. This list is likely incomplete. We recommend familiarizing yourself with JavaScript if you plan to make heavy use of PScript.

  • JavasScript has a concept of null (i.e. None), as well as undefined. Sometimes you may want to use if x is None or x is undefined: ....
  • Accessing an attribute that does not exist will not raise an AttributeError but yield undefined. Though this may change.
  • Keys in a dictionary are implicitly converted to strings.
  • Magic functions on classes (e.g. for operator overloading) do not work.
  • Calling an object that starts with a capital letter is assumed to be a class instantiation (using new): PScript classes must start with a capital letter, and any other callables must not.
  • A function can accept keyword arguments if it has a **kwargs parameter or named arguments after *args. Passing keywords to a function that does not handle keyword arguments might result in confusing errors.
  • Divide by zero results in inf instead of raising ZeroDivisionError.
  • In Python you can do a_list += a_string where each character in the string will be added to the list. In PScript this will convert a_list to a string.

PScript is valid Python

Other than e.g. RapydScript, PScript is valid Python. This allows creating modules that are a mix of real Python and PScript. You can easily write code that runs correctly both as Python and PScript, and raw JavaScript can be included where needed (e.g. for performance).

PScript’s compiler is written in Python. Perhaps PScript can at some point compile itself, so that it becomes possible to define PScript inside HTML documents.

There are things you can do, which you cannot do in Python:


Because PScript produces relatively bare JavaScript, it is pretty fast. Faster than CPython, and significantly faster than e.g. Brython. Check out examples/app/

Nevertheless, the overhead to realize the more Pythonic behavior can have a negative impact on performance in tight loops (in comparison to writing the JS by hand). The recommended approach is to write performance critical code in pure JavaScript (using RawJS) if necessary.

Using PSCRIPT_OVERLOAD to increase performance

To improve the performance of critical code, it’s possible to disable some of the overloading that make PScript more Pythonic. This increases the speed of code, but it also makes it more like JavaScript.

To use this feature, write PSCRIPT_OVERLOAD = False. Any code that follows will not be subject to overloading. This parser setting can only be used inside a function and applies only to (the scope of) that function (i.e. not to functions defined inside that function nor any outer scope). If needed, overloading can also be enabled again by writing PSCRIPT_OVERLOAD = True.

Things that are no longer overloaded:

  • The add operator (+), so list concatenation cannot be done with +.
  • The multiply operator (*), so repeating a list or string cannot be done with *.
  • The equals operator (==), so deep comparisons of tuples/lists and dicts does not work.
  • The implicit truthy operator (as e.g. used in an if-statement), so empty tuples/lists and dicts evaluate to True. Note that functions like bool(), all() and any() still use the overloaded truthy.


This is an overview of the language features that PScript supports/lacks.

Not currently supported:

  • import (maybe we should translate an import to require()?)
  • the set class (JS has no set, but we could create one?)
  • slicing with steps (JS does not support this)
  • Generators, i.e. yield (not widely supported in JS)

Supported basics:

  • numbers, strings, lists, dicts (the latter become JS arrays and objects)
  • operations: binary, unary, boolean, power, integer division, in operator
  • comparisons (== -> ==, is -> ===)
  • tuple packing and unpacking
  • basic string formatting
  • slicing with start end end (though not with step)
  • if-statements and single-line if-expressions
  • while-loops and for-loops supporting continue, break, and else-clauses
  • for-loops using range()
  • for-loop over arrays
  • for-loop over dict/object using .keys(), .values() and .items()
  • function calls can have *args
  • function defs can have default arguments and *args
  • function calls/defs can use keyword arguments and **kwargs, but use with care (see caveats).
  • lambda expressions
  • list comprehensions
  • classes, with (single) inheritance, and the use of super()
  • raising and catching exceptions, assertions
  • creation of “modules”
  • globals / nonlocal
  • The with statement (no equivalent in JS)
  • double underscore name mangling

Supported Python conveniences:

  • use of self is translated to this
  • print() becomes console.log() (also supports sep and end)
  • isinstance() Just Works (for primitive types as well as user-defined classes)
  • an empty list or dict evaluates to False as in Python.
  • all Python builtin functions that make sense in JS are supported: isinstance, issubclass, callable, hasattr, getattr, setattr, delattr, print, len, max, min, chr, ord, dict, list, tuple, range, pow, sum, round, int, float, str, bool, abs, divmod, all, any, enumerate, zip, reversed, sorted, filter, map.
  • all methods of list, dict and str are supported (except a few string methods: encode, format_map, isprintable, maketrans).
  • the default return value of a function is None/null instead of undefined.
  • list concatenation using the plus operator, and list/str repeating using the star operator.
  • deep comparisons.
  • class methods are bound functions (i.e. this is fixed to the instance).
  • functions that are defined in another function (a.k.a closures) that do not have self/this as a first argument, are bound the the same instance as the function in which it is defined.

Other functionality

The PScript package provides a few other “utilities” to deal with JS code, such as renaming function/class definitions, and creating JS modules (AMD, UMD, etc.).