pyodide/docs/type_conversions.md

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.

Python	Javascript
int, float	Number
str	String
True	true
False	false
None	undefined, null
list, tuple	Array
dict	Object

Additionally, Python bytes and buffer objects are converted to/from Javascript Uint8ClampedArray typed arrays. In this case, however, the underlying data is not copied, and is shared between the Python and Javascript sides. This makes passing raw memory between the languages (which in practice can be quite large) very efficient.

Aside: This is the technology on which matplotlib images are passed to Javascript to render in a canvas, and will be the basis of sharing Numpy arrays with n-dimensional array data structures in 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):

Python	Javascript
repr(x)	x.toString()
x.foo	x.foo
x.foo = bar	x.foo = bar
del x.foo	delete x.foo
x(...)	x(...)
x.foo(...)	x.foo(...)
X.new(...)	new X(...)
len(x)	x.length
x[foo]	x[foo]
x[foo] = bar	x[foo] = bar
del x[foo]	delete x[foo]
x == y	x == y
x.typeof	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:

Javascript	Python
foo in x	hasattr(x, 'foo')
x.foo	getattr(x, 'foo')
x.foo = bar	setattr(x, 'foo', bar)
delete x.foo	delattr(x, 'foo')
x.ownKeys()	dir(x)
x(...)	x(...)
x.foo(...)	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();
foo.destroy();
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 (See type conversions).

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

Using Javascript objects from Python

Javascript objects can be accessed from Python using the from js import ... syntax. The object must be in the global (window) namespace.

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