JavaScript API#

Backward compatibility of the API is not guaranteed at this point.

Globals#

Functions:

async loadPyodide(config)

Load the main Pyodide wasm module and initialize it.
asyncglobalThis.loadPyodide(config)#

Load the main Pyodide wasm module and initialize it.

Only one copy of Pyodide can be loaded in a given JavaScript global scope because Pyodide uses global variables to load packages. If an attempt is made to load a second copy of Pyodide, loadPyodide will throw an error. (This can be fixed once Firefox adopts support for ES6 modules in webworkers.)

Arguments

  • config.indexURL (string) – The URL from which Pyodide will load packages

  • config.homedir (string) – The home directory which Pyodide will use inside virtual file system Default: /home/pyodide

  • config.fullStdLib (boolean) – Load the full Python standard library. Setting this to false excludes following modules: distutils. Default: true

  • config.stdin (undefined|function) – Override the standard input callback. Should ask the user for one line of input. Default: undefined

  • config.stdout (undefined|function) – Override the standard output callback. Default: undefined

  • config.stderr (undefined|function) – Override the standard error output callback. Default: undefined

Returns

The pyodide module.

pyodide#

Attributes:

FS

An alias to the Emscripten File System API.

globals

An alias to the global Python namespace.

loadedPackages

The list of packages that Pyodide has loaded.

pyodide_py

An alias to the Python pyodide package.

version

The Pyodide version.

Functions:

checkInterrupt()

Throws a KeyboardInterrupt error if a KeyboardInterrupt has been requested via the interrupt buffer.

isPyProxy(jsobj)

Is the argument a PyProxy?

async loadPackage(names, messageCallback, errorCallback)

Load a package or a list of packages over the network.

async loadPackagesFromImports(code, messageCallback, errorCallback)

Inspect a Python code chunk and use pyodide.loadPackage() to install any known packages that the code chunk imports.

pyimport(mod_name)

Imports a module and returns it.

registerComlink(Comlink)

Tell Pyodide about Comlink.

registerJsModule(name, module)

Registers the JavaScript object module as a JavaScript module named name.

runPython(code, globals)

Runs a string of Python code from JavaScript.

async runPythonAsync(code, globals)

Runs Python code using PyCF_ALLOW_TOP_LEVEL_AWAIT.

setInterruptBuffer(interrupt_buffer)

Sets the interrupt buffer to be interrupt_buffer.

toPy(obj, options)

Convert the JavaScript object to a Python object as best as possible.

unpackArchive(buffer, format, extract_dir)

Unpack an archive into a target directory.

unregisterJsModule(name)

Unregisters a JavaScript module with given name that has been previously registered with pyodide.registerJsModule() or pyodide.register_js_module().

Classes:

PyBuffer

A class to allow access to a Python data buffers from JavaScript.

PythonError

A JavaScript error caused by a Python exception.
pyodide.FS#

type: FS

An alias to the Emscripten File System API.

This provides a wide range of POSIX-like file/device operations, including mount which can be used to extend the in-memory filesystem with features like persistence.

While all the file systems implementations are enabled, only the default MEMFS is guaranteed to work in all runtime settings. The implementations are available as members of FS.filesystems: IDBFS, NODEFS, PROXYFS, WORKERFS.

pyodide.globals#

type: PyProxy

An alias to the global Python namespace.

For example, to access a variable called foo in the Python global scope, use pyodide.globals.get("foo")

pyodide.loadedPackages#

type: object

The list of packages that Pyodide has loaded. Use Object.keys(pyodide.loadedPackages) to get the list of names of loaded packages, and pyodide.loadedPackages[package_name] to access install location for a particular package_name.

pyodide.pyodide_py#

type: PyProxy

An alias to the Python pyodide package.

You can use this to call functions defined in the Pyodide Python package from JavaScript.

pyodide.version#

type: string

The Pyodide version.

It can be either the exact release version (e.g. 0.1.0), or the latest release version followed by the number of commits since, and the git hash of the current commit (e.g. 0.1.0-1-bd84646).

pyodide.checkInterrupt()#

Throws a KeyboardInterrupt error if a KeyboardInterrupt has been requested via the interrupt buffer.

This can be used to enable keyboard interrupts during execution of JavaScript code, just as PyErr_CheckSignals is used to enable keyboard interrupts during execution of C code.

pyodide.isPyProxy(jsobj)#

Is the argument a PyProxy?

Arguments

  • jsobj (any) – Object to test.

Returns

boolean – Is jsobj a PyProxy?

asyncpyodide.loadPackage(names, messageCallback, errorCallback)#

Load a package or a list of packages over the network. This installs the package in the virtual filesystem. The package needs to be imported from Python before it can be used.

Arguments

  • names (string|Array.<string>|PyProxy) – Either a single package name or URL or a list of them. URLs can be absolute or relative. The URLs must have file name <package-name>.js and there must be a file called <package-name>.data in the same directory. The argument can be a PyProxy of a list, in which case the list will be converted to JavaScript and the PyProxy will be destroyed.

  • messageCallback (LogFn) – A callback, called with progress messages (optional)

  • errorCallback (LogFn) – A callback, called with error/warning messages (optional)

asyncpyodide.loadPackagesFromImports(code, messageCallback, errorCallback)#

Inspect a Python code chunk and use pyodide.loadPackage() to install any known packages that the code chunk imports. Uses the Python API pyodide.find_imports() to inspect the code.

For example, given the following code as input

import numpy as np x = np.array([1, 2, 3])

loadPackagesFromImports() will call pyodide.loadPackage(['numpy']).

Arguments

  • code (string) – The code to inspect.

  • messageCallback (LogFn) – The messageCallback argument of pyodide.loadPackage (optional).

  • errorCallback (LogFn) – The errorCallback argument of pyodide.loadPackage (optional).

pyodide.pyimport(mod_name)#

Imports a module and returns it.

Warning

This function has a completely different behavior than the old removed pyimport function!

pyimport is roughly equivalent to:

pyodide.runPython(`import ${pkgname}; ${pkgname}`);

except that the global namespace will not change.

Example:

let sysmodule = pyodide.pyimport("sys");
let recursionLimit = sys.getrecursionlimit();
Arguments

  • mod_name (string) – The name of the module to import

Returns

A PyProxy for the imported module

Tell Pyodide about Comlink. Necessary to enable importing Comlink proxies into Python.

pyodide.registerJsModule(name, module)#

Registers the JavaScript object module as a JavaScript module named name. This module can then be imported from Python using the standard Python import system. If another module by the same name has already been imported, this won’t have much effect unless you also delete the imported module from sys.modules. This calls the pyodide_py API pyodide.register_js_module().

Arguments

  • name (string) – Name of the JavaScript module to add

  • module (object) – JavaScript object backing the module

pyodide.runPython(code, globals)#

Runs a string of Python code from JavaScript.

The last part of the string may be an expression, in which case, its value is returned.

Arguments

  • code (string) – Python code to evaluate

  • globals (PyProxy) – An optional Python dictionary to use as the globals. Defaults to pyodide.globals. Uses the Python API pyodide.eval_code to evaluate the code.

Returns

Py2JsResult – The result of the Python code translated to JavaScript. See the documentation for pyodide.eval_code for more info.

asyncpyodide.runPythonAsync(code, globals)#

Runs Python code using PyCF_ALLOW_TOP_LEVEL_AWAIT.

Python imports

Since pyodide 0.18.0, you must call loadPackagesFromImports() to import any python packages referenced via import statements in your code. This function will no longer do it for you.

For example:

let result = await pyodide.runPythonAsync(`
    from js import fetch
    response = await fetch("./packages.json")
    packages = await response.json()
    # If final statement is an expression, its value is returned to JavaScript
    len(packages.packages.object_keys())
`);
console.log(result); // 79
Arguments
Returns

Py2JsResult – The result of the Python code translated to JavaScript.

pyodide.setInterruptBuffer(interrupt_buffer)#

Sets the interrupt buffer to be interrupt_buffer. This is only useful when Pyodide is used in a webworker. The buffer should be a SharedArrayBuffer shared with the main browser thread (or another worker). To request an interrupt, a 2 should be written into interrupt_buffer (2 is the posix constant for SIGINT).

Arguments

  • interrupt_buffer (TypedArray) –

pyodide.toPy(obj, options)#

Convert the JavaScript object to a Python object as best as possible.

This is similar to JsProxy.to_py but for use from JavaScript. If the object is immutable or a PyProxy, it will be returned unchanged. If the object cannot be converted into Python, it will be returned unchanged.

See JavaScript to Python for more information.

Arguments

  • obj (*) –

  • options (object) –

  • options.depth (number) – Optional argument to limit the depth of the conversion.

Returns

PyProxy – The object converted to Python.

pyodide.unpackArchive(buffer, format, extract_dir)#

Unpack an archive into a target directory.

Arguments

  • buffer (ArrayBuffer) – The archive as an ArrayBuffer (it’s also fine to pass a TypedArray).

  • format (string) – The format of the archive. Should be one of the formats recognized by shutil.unpack_archive. By default the options are ‘bztar’, ‘gztar’, ‘tar’, ‘zip’, and ‘wheel’. Several synonyms are accepted for each format, e.g., for ‘gztar’ any of ‘.gztar’, ‘.tar.gz’, ‘.tgz’, ‘tar.gz’ or ‘tgz’ are considered to be synonyms.

  • extract_dir (string) – The directory to unpack the archive into. Defaults to the working directory.

pyodide.unregisterJsModule(name)#

Unregisters a JavaScript module with given name that has been previously registered with pyodide.registerJsModule() or pyodide.register_js_module(). If a JavaScript module with that name does not already exist, will throw an error. Note that if the module has already been imported, this won’t have much effect unless you also delete the imported module from sys.modules. This calls the pyodide_py API pyodide.unregister_js_module().

Arguments

  • name (string) – Name of the JavaScript module to remove

class pyodide.PyBuffer()#

A class to allow access to a Python data buffers from JavaScript. These are produced by PyProxy.getBuffer and cannot be constructed directly. When you are done, release it with the release method. See Python buffer protocol documentation for more information.

To find the element x[a_1, ..., a_n], you could use the following code:

function multiIndexToIndex(pybuff, multiIndex){
   if(multindex.length !==pybuff.ndim){
      throw new Error("Wrong length index");
   }
   let idx = pybuff.offset;
   for(let i = 0; i < pybuff.ndim; i++){
      if(multiIndex[i] < 0){
         multiIndex[i] = pybuff.shape[i] - multiIndex[i];
      }
      if(multiIndex[i] < 0 || multiIndex[i] >= pybuff.shape[i]){
         throw new Error("Index out of range");
      }
      idx += multiIndex[i] * pybuff.stride[i];
   }
   return idx;
}
console.log("entry is", pybuff.data[multiIndexToIndex(pybuff, [2, 0, -1])]);

Contiguity

If the buffer is not contiguous, the data TypedArray will contain data that is not part of the buffer. Modifying this data may lead to undefined behavior.

Readonly buffers

If buffer.readonly is true, you should not modify the buffer. Modifying a readonly buffer may lead to undefined behavior.

Converting between TypedArray types

The following naive code to change the type of a typed array does not work:

// Incorrectly convert a TypedArray.
// Produces a Uint16Array that points to the entire WASM memory!
let myarray = new Uint16Array(buffer.data.buffer);

Instead, if you want to convert the output TypedArray, you need to say:

// Correctly convert a TypedArray.
let myarray = new Uint16Array(
    buffer.data.buffer,
    buffer.data.byteOffset,
    buffer.data.byteLength
);
PyBuffer.PyBuffer#
PyBuffer.c_contiguous#

type: boolean

Is it C contiguous?

PyBuffer.data#

type: TypedArray

The actual data. A typed array of an appropriate size backed by a segment of the WASM memory.

The type argument of PyProxy.getBuffer determines which sort of TypedArray this is. By default PyProxy.getBuffer will look at the format string to determine the most appropriate option.

PyBuffer.f_contiguous#

type: boolean

Is it Fortran contiguous?

PyBuffer.format#

type: string

The format string for the buffer. See the Python documentation on format strings.

PyBuffer.itemsize#

type: number

How large is each entry (in bytes)?

PyBuffer.nbytes#

type: number

The total number of bytes the buffer takes up. This is equal to buff.data.byteLength.

PyBuffer.ndim#

type: number

The number of dimensions of the buffer. If ndim is 0, the buffer represents a single scalar or struct. Otherwise, it represents an array.

PyBuffer.offset#

type: number

The offset of the first entry of the array. For instance if our array is 3d, then you will find array[0,0,0] at pybuf.data[pybuf.offset]

PyBuffer.readonly#

type: boolean

If the data is readonly, you should not modify it. There is no way for us to enforce this, but it may cause very weird behavior.

PyBuffer.shape#

type: number[]

The shape of the buffer, that is how long it is in each dimension. The length will be equal to ndim. For instance, a 2x3x4 array would have shape [2, 3, 4].

PyBuffer.strides#

type: number[]

An array of of length ndim giving the number of elements to skip to get to a new element in each dimension. See the example definition of a multiIndexToIndex function above.

PyBuffer.release()#

Release the buffer. This allows the memory to be reclaimed.

class pyodide.PythonError()#

A JavaScript error caused by a Python exception.

In order to reduce the risk of large memory leaks, the PythonError contains no reference to the Python exception that caused it. You can find the actual Python exception that caused this error as sys.last_value.

See Errors for more information.

Avoid Stack Frames

If you make a PyProxy of sys.last_value, you should be especially careful to destroy() it when you are done. You may leak a large amount of memory including the local variables of all the stack frames in the traceback if you don’t. The easiest way is to only handle the exception in Python.

PythonError.PythonError#
PythonError.message#

type: string

The Python traceback.

PyProxy#

A PyProxy is an object that allows idiomatic use of a Python object from JavaScript. See Proxying from Python into JavaScript.

Attributes:

length

The length of the object.

type

The name of the type of the object.

Functions:

callKwargs(jsargs)

Call the function with key word arguments.

catch(onRejected)

Runs asyncio.ensure_future(awaitable) and executes onRejected(error) if the future fails.

copy()

Make a new PyProxy pointing to the same Python object.

delete(key)

This translates to the Python code del obj[key].

destroy(destroyed_msg)

Destroy the PyProxy.

finally(onFinally)

Runs asyncio.ensure_future(awaitable) and executes onFinally(error) when the future resolves.

get(key)

This translates to the Python code obj[key].

getBuffer(type)

Get a view of the buffer data which is usable from JavaScript.

has(key)

This translates to the Python code key in obj.

isAwaitable()

Check whether the PyProxy is awaitable.

isBuffer()

Check whether the PyProxy is a buffer.

isCallable()

Check whether the PyProxy is a Callable.

isIterable()

Check whether the PyProxy is iterable.

isIterator()

Check whether the PyProxy is iterable.

[Symbol.iterator]()

This translates to the Python code iter(obj).

next(value)

This translates to the Python code next(obj).

set(key, value)

This translates to the Python code obj[key] = value.

supportsGet()

Check whether the PyProxy.get method is available on this PyProxy.

supportsHas()

Check whether the PyProxy.has method is available on this PyProxy.

supportsLength()

Check whether the PyProxy.length getter is available on this PyProxy.

supportsSet()

Check whether the PyProxy.set method is available on this PyProxy.

then(onFulfilled, onRejected)

Runs asyncio.ensure_future(awaitable), executes onFulfilled(result) when the Future resolves successfully, executes onRejected(error) when the Future fails.

toJs(options)

Converts the PyProxy into a JavaScript object as best as possible.

toString()

PyProxy.length#

The length of the object.

Present only if the proxied Python object has a __len__ method.

PyProxy.type#

type: string

The name of the type of the object.

Usually the value is "module.name" but for builtins or interpreter-defined types it is just "name". As pseudocode this is:

ty = type(x)
if ty.__module__ == 'builtins' or ty.__module__ == "__main__":
    return ty.__name__
else:
    ty.__module__ + "." + ty.__name__
PyProxy.callKwargs(jsargs)#

Call the function with key word arguments. The last argument must be an object with the keyword arguments.

PyProxy.catch(onRejected)#

Runs asyncio.ensure_future(awaitable) and executes onRejected(error) if the future fails.

See the documentation for Promise.catch.

Present only if the proxied Python object is awaitable.

Arguments

  • onRejected (function) – A handler called with the error as an argument if the awaitable fails.

Returns

Promise – The resulting Promise.

PyProxy.copy()#

Make a new PyProxy pointing to the same Python object. Useful if the PyProxy is destroyed somewhere else.

Returns

PyProxy –

PyProxy.delete(key)#

This translates to the Python code del obj[key].

Present only if the proxied Python object has a __delitem__ method.

Arguments

  • key (any) – The key to delete.

PyProxy.destroy(destroyed_msg)#

Destroy the PyProxy. This will release the memory. Any further attempt to use the object will raise an error.

In a browser supporting FinalizationRegistry Pyodide will automatically destroy the PyProxy when it is garbage collected, however there is no guarantee that the finalizer will be run in a timely manner so it is better to destroy the proxy explicitly.

Arguments

  • destroyed_msg (string) – The error message to print if use is attempted after destroying. Defaults to “Object has already been destroyed”.

PyProxy.finally(onFinally)#

Runs asyncio.ensure_future(awaitable) and executes onFinally(error) when the future resolves.

See the documentation for Promise.finally.

Present only if the proxied Python object is awaitable.

Arguments

  • onFinally (function) – A handler that is called with zero arguments when the awaitable resolves.

Returns

Promise – A Promise that resolves or rejects with the same result as the original Promise, but only after executing the onFinally handler.

PyProxy.get(key)#

This translates to the Python code obj[key].

Present only if the proxied Python object has a __getitem__ method.

Arguments

  • key (any) – The key to look up.

Returns

Py2JsResult – The corresponding value.

PyProxy.getBuffer(type)#

Get a view of the buffer data which is usable from JavaScript. No copy is ever performed.

Present only if the proxied Python object supports the Python Buffer Protocol.

We do not support suboffsets, if the buffer requires suboffsets we will throw an error. JavaScript nd array libraries can’t handle suboffsets anyways. In this case, you should use the toJs api or copy the buffer to one that doesn’t use suboffets (using e.g., numpy.ascontiguousarray).

If the buffer stores big endian data or half floats, this function will fail without an explicit type argument. For big endian data you can use toJs. DataViews have support for big endian data, so you might want to pass 'dataview' as the type argument in that case.

Arguments

  • type (string) – The type of the PyBuffer.data field in the output. Should be one of: "i8", "u8", "u8clamped", "i16", "u16", "i32", "u32", "i32", "u32", "i64", "u64", "f32", "f64, or "dataview". This argument is optional, if absent getBuffer will try to determine the appropriate output type based on the buffer format string.

Returns

PyBuffer – PyBuffer

PyProxy.has(key)#

This translates to the Python code key in obj.

Present only if the proxied Python object has a __contains__ method.

Arguments

  • key (*) – The key to check for.

Returns

boolean – Is key present?

PyProxy.isAwaitable()#

Check whether the PyProxy is awaitable. A Typescript type guard, if this function returns true Typescript considers the PyProxy to be a Promise.

Returns

boolean –

PyProxy.isBuffer()#

Check whether the PyProxy is a buffer. A Typescript type guard for PyProxy.getBuffer.

Returns

boolean –

PyProxy.isCallable()#

Check whether the PyProxy is a Callable. A Typescript type guard, if this returns true then Typescript considers the Proxy to be callable of signature (args... : any[]) => PyProxy | number | bigint | string | boolean | undefined.

Returns

boolean –

PyProxy.isIterable()#

Check whether the PyProxy is iterable. A Typescript type guard for PyProxy.[Symbol.iterator].

Returns

boolean –

PyProxy.isIterator()#

Check whether the PyProxy is iterable. A Typescript type guard for PyProxy.next.

Returns

boolean –

[Symbol.iterator]()#

This translates to the Python code iter(obj). Return an iterator associated to the proxy. See the documentation for Symbol.iterator.

Present only if the proxied Python object is iterable (i.e., has an __iter__ method).

This will be used implicitly by for(let x of proxy){}.

Returns

Iterator.<Py2JsResult, Py2JsResult, any> – An iterator for the proxied Python object.

PyProxy.next(value)#

This translates to the Python code next(obj). Returns the next value of the generator. See the documentation for Generator.prototype.next. The argument will be sent to the Python generator.

This will be used implicitly by for(let x of proxy){}.

Present only if the proxied Python object is a generator or iterator (i.e., has a send or __next__ method).

Arguments

  • value (any) – The value to send to the generator. The value will be assigned as a result of a yield expression.

Returns

IteratorResult.<Py2JsResult, Py2JsResult> – An Object with two properties: done and value. When the generator yields some_value, next returns {done : false, value : some_value}. When the generator raises a StopIteration(result_value) exception, next returns {done : true, value : result_value}.

PyProxy.set(key, value)#

This translates to the Python code obj[key] = value.

Present only if the proxied Python object has a __setitem__ method.

Arguments

  • key (any) – The key to set.

  • value (any) – The value to set it to.

PyProxy.supportsGet()#

Check whether the PyProxy.get method is available on this PyProxy. A Typescript type guard.

Returns

boolean –

PyProxy.supportsHas()#

Check whether the PyProxy.has method is available on this PyProxy. A Typescript type guard.

Returns

boolean –

PyProxy.supportsLength()#

Check whether the PyProxy.length getter is available on this PyProxy. A Typescript type guard.

Returns

boolean –

PyProxy.supportsSet()#

Check whether the PyProxy.set method is available on this PyProxy. A Typescript type guard.

Returns

boolean –

PyProxy.then(onFulfilled, onRejected)#

Runs asyncio.ensure_future(awaitable), executes onFulfilled(result) when the Future resolves successfully, executes onRejected(error) when the Future fails. Will be used implictly by await obj.

See the documentation for Promise.then

Present only if the proxied Python object is awaitable.

Arguments

  • onFulfilled (function) – A handler called with the result as an argument if the awaitable succeeds.

  • onRejected (function) – A handler called with the error as an argument if the awaitable fails.

Returns

Promise – The resulting Promise.

PyProxy.toJs(options)#

Converts the PyProxy into a JavaScript object as best as possible. By default does a deep conversion, if a shallow conversion is desired, you can use proxy.toJs({depth : 1}). See Explicit Conversion of PyProxy for more info.

Arguments

  • options (object) –

  • options.depth (number) – How many layers deep to perform the conversion. Defaults to infinite.

  • options.pyproxies (array) – If provided, toJs will store all PyProxies created in this list. This allows you to easily destroy all the PyProxies by iterating the list without having to recurse over the generated structure. The most common use case is to create a new empty list, pass the list as pyproxies, and then later iterate over pyproxies to destroy all of created proxies.

  • options.create_pyproxies (boolean) – If false, toJs will throw a ConversionError rather than producing a PyProxy.

  • options.dict_converter (boolean) – A function to be called on an iterable of pairs [key, value]. Convert this iterable of pairs to the desired output. For instance, Object.fromEntries would convert the dict to an object, Array.from converts it to an array of entries, and (it) => new Map(it) converts it to a Map (which is the default behavior).

Returns

any – The JavaScript object resulting from the conversion.

PyProxy.toString()#
Returns

string –