Type conversions

Python to Javascript conversions occur:

  • when returning the final expression from a pyodide.runPython call (evaluating a Python cell in Iodide)

  • using pyodide.pyimport

  • passing arguments to a Javascript function from Python

Javascript to Python conversions occur:

  • when using the from js import ... syntax

  • returning the result of a Javascript function to Python

Basic types

The following basic types are implicitly converted between Javascript and Python. The values are copied and any connection to the original object is lost.



int, float









undefined, null

list, tuple




Typed arrays

Javascript typed arrays (Int8Array and friends) are converted to Python memoryviews. This happens with a single binary memory copy (since Python can’t access arrays on the Javascript heap), and the data type is preserved. This makes it easy to correctly convert it to a Numpy array using numpy.asarray:

array = Float32Array([1, 2, 3])
from js import array
import numpy as np
numpy_array = np.asarray(array)

Python bytes and buffer objects are converted to Javascript as Uint8ClampedArrays, without any memory copy at all, and is thus very efficient, but be aware that any changes to the buffer will be reflected in both places.

Numpy arrays are currently converted to Javascript as nested (regular) Arrays. A more efficient method will probably emerge as we decide on an ndarray implementation for Javascript.

Class instances

Any of the types not listed above are shared between languages using proxies that allow methods and some operators to be called on the object from the other language.

Javascript from Python

When passing a Javascript object to Python, an extension type is used to delegate Python operations to the Javascript side. The following operations are currently supported. (More should be possible in the future – work in ongoing to make this more complete):







x.foo = bar

x.foo = bar

del x.foo

delete x.foo






new X(...)





x[foo] = bar

x[foo] = bar

del x[foo]

delete x[foo]

x == y

x == y


typeof x

One important difference between Python objects and Javascript objects is that if you access a missing member in Python, an exception is raised. In Javascript, it returns undefined. Since we can’t make any assumptions about whether the Javascript member is missing or simply set to undefined, Python mirrors the Javascript behavior. For example:

// Javascript
class Point {
  constructor(x, y) {
    this.x = x;
    this.y = y;
point = new Point(42, 43))
# python
from js import point
assert point.y == 43
del point.y
assert point.y is None

Python from Javascript

When passing a Python object to Javascript, the Javascript Proxy API is used to delegate Javascript operations to the Python side. In general, the Proxy API is more limited than what can be done with a Python extension, so there are certain operations that are impossible or more cumbersome when using Python from Javascript than vice versa. The most notable limitation is that while Python has distinct ways of accessing attributes and items (x.foo and x[foo]), Javascript conflates these two concepts. The following operations are currently supported:



foo in x

hasattr(x, 'foo')


getattr(x, 'foo')

x.foo = bar

setattr(x, 'foo', bar)

delete x.foo

delattr(x, 'foo')







An additional limitation is that when passing a Python object to Javascript, there is no way for Javascript to automatically garbage collect that object. Therefore, custom Python objects must be manually destroyed when passed to Javascript, or they will leak. To do this, call .destroy() on the object, after which Javascript will no longer have access to the object.

var foo = pyodide.pyimport('foo');
foo.call_method(); // This will raise an exception, since the object has been
                   // destroyed

Using Python objects from Javascript

A Python object (in global scope) can be brought over to Javascript using the pyodide.pyimport function. It takes a string giving the name of the variable, and returns the object, converted to Javascript.

var sys = pyodide.pyimport('sys');

Using Javascript objects from Python

Javascript objects can be accessed from Python using the special js module. This module looks up attributes of the global (window) namespace on the Javascript side.

import js
js.document.title = 'New window title'

Performance considerations

Looking up and converting attributes of the js module happens dynamically. In most cases, where the value is small or results in a proxy, this is not an issue. However, if the value takes a long time to convert from Javascript to Python, you may want to store it in a Python variable or use the from js import ... syntax.

For example, given this large Javascript variable:

var x = new Array(1000).fill(0)

Use it from Python as follows:

import js
x = js.x  # conversion happens once here
for i in range(len(x)):
    item = x[i]  # we don't pay the conversion price each time here

Or alternatively:

from js import x  # conversion happens once here
for i in range(len(x)):
    item = x[i]  # we don't pay the conversion price each time here