Change Log

Version 0.17.0

April 21, 2020

See the Version 0.17.0 Release Notes for more information.

Improvements to package loading and dynamic linking

  • Enhancement Uses the emscripten preload plugin system to preload .so files in packages

  • Enhancement Support for shared library packages. This is used for CLAPACK which makes scipy a lot smaller. #1236

  • Fix Pyodide and included packages can now be used with Safari v14+. Safari v13 has also been observed to work on some (but not all) devices.

Python / JS type conversions

  • Feature A JsProxy of a Javascript Promise or other awaitable object is now a Python awaitable. #880

  • API Change Instead of automatically converting Python lists and dicts into Javascript, they are now wrapped in PyProxy. Added a new PyProxy.toJs API to request the conversion behavior that used to be implicit. #1167

  • API Change Added JsProxy.to_py API to convert a Javascript object to Python. #1244

  • Feature Flexible jsimports: it now possible to add custom Python “packages” backed by Javascript code, like the js package. The js package is now implemented using this system. #1146

  • Feature A PyProxy of a Python coroutine or awaitable is now an awaitable Javascript object. Awaiting a coroutine will schedule it to run on the Python event loop using asyncio.ensure_future. #1170

  • Enhancement Made PyProxy of an iterable Python object an iterable Js object: defined the [Symbol.iterator] method, can be used like for(let x of proxy). Made a PyProxy of a Python iterator an iterator: is translated to next(it). Made a PyProxy of a Python generator into a Javascript generator: is translated to gen.send(val). #1180

  • API Change Updated PyProxy so that if the wrapped Python object supports __getitem__ access, then the wrapper has get, set, has, and delete methods which do obj[key], obj[key] = val, key in obj and del obj[key] respectively. #1175

  • API Change The pyodide.pyimport function is deprecated in favor of using pyodide.globals.get('key'). #1367

  • API Change Added PyProxy.getBuffer API to allow direct access to Python buffers as Javascript TypedArrays. #1215

  • API Change The innermost level of a buffer converted to Javascript used to be a TypedArray if the buffer was contiguous and otherwise an Array. Now the innermost level will be a TypedArray unless the buffer format code is a ‘?’ in which case it will be an Array of booleans, or if the format code is a “s” in which case the innermost level will be converted to a string. #1376

  • Enhancement Javascript BigInts are converted into Python int and Python ints larger than 2^53 are converted into BigInt. #1407

  • API Change Added pyodide.isPyProxy to test if an object is a PyProxy. #1456

  • Enhancement PyProxy and PyBuffer objects are now garbage collected if the browser supports FinalizationRegistry. #1306

  • Enhancement Automatic conversion of Javascript functions to CPython calling conventions. #1051, #1080

  • Enhancement Automatic detection of fatal errors. In this case Pyodide will produce both a Javascript and a Python stack trace with explicit instruction to open a bug report. pr{1151}, pr{1390}, pr{1478}.

  • Enhancement Systematic memory leak detection in the test suite and a large number of fixed to memory leaks. pr{1340}

  • Fix getattr and dir on JsProxy now report consistent results and include all names defined on the Python dictionary backing JsProxy. #1017

  • Fix JsProxy.__bool__ now produces more consistent results: both bool(window) and bool(zero-arg-callback) were False but now are True. Conversely, bool(empty_js_set) and bool(empty_js_map) were True but now are False. #1061

  • Fix When calling a Javascript function from Python without keyword arguments, Pyodide no longer passes a PyProxy-wrapped NULL pointer as the last argument. #1033

  • Fix JsBoundMethod is now a subclass of JsProxy, which fixes nested attribute access and various other strange bugs. #1124

  • Fix Javascript functions imported like from js import fetch no longer trigger “invalid invocation” errors (issue #461) and js.fetch("some_url") also works now (issue #768). #1126

  • Fix Javascript bound method calls now work correctly with keyword arguments. #1138

  • Fix Javascript constructor calls now work correctly with keyword arguments. #1433

pyodide-py package

  • Feature Added a Python event loop to support asyncio by scheduling coroutines to run as jobs on the browser event loop. This event loop is available by default and automatically enabled by any relevant asyncio API, so for instance asyncio.ensure_future works without any configuration. #1158

  • API Change Removed as_nested_list API in favor of JsProxy.to_py. #1345


  • API Change Removed iodide-specific code in pyodide.js. This breaks compatibility with iodide. #878, #981

  • API Change Removed the pyodide.autocomplete API, use Jedi directly instead. #1066

  • API Change Removed pyodide.repr API. #1067

  • Fix If messageCallback and errorCallback are supplied to pyodide.loadPackage, pyodide.runPythonAsync and pyodide.loadPackagesFromImport, then the messages are no longer automatically logged to the console.

  • Feature runPythonAsync now runs the code with eval_code_async. In particular, it is possible to use top-level await inside of runPythonAsync.

  • eval_code now accepts separate globals and locals parameters. #1083

  • Added the pyodide.setInterruptBuffer API. This can be used to set a SharedArrayBuffer to be the keyboard interupt buffer. If Pyodide is running on a webworker, the main thread can signal to the webworker that it should raise a KeyboardInterrupt by writing to the interrupt buffer. #1148 and #1173

  • Changed the loading method: added an async function loadPyodide to load Pyodide to use instead of languagePluginURL and languagePluginLoader. The change is currently backwards compatible, but the old approach is deprecated. #1363


  • Feature micropip now supports installing wheels from relative URLs. #872

  • API Change micropip.install now returns a Python Future instead of a Javascript Promise. #1324

  • micropip.install now interacts correctly with pyodide.loadPackage. #1457

  • micropip.install now handles version constraints correctly even if there is a version of the package available from the Pyodide indexURL.

Build system

  • Enhancement Updated to latest emscripten 2.0.13 with the updstream LLVM backend #1102

  • API Change Use upstream, and stop checking package abi versions. The PYODIDE_PACKAGE_ABI environment variable is no longer used, but is still set as some packages use it to detect whether it is being built for Pyodide. This usage is deprecated, and a new environment variable PYODIDE is introduced for this purpose.

    As part of the change, Module.checkABI is no longer present. #991

  • uglifyjs and lessc no longer need to be installed in the system during build #878.

  • Enhancement Reduce the size of the core Pyodide package #987.

  • Enhancement Optionally to disable docker port binding #1423.

  • Enhancement Run arbitrary command in docker #1424

  • Docker images for Pyodide are now accessible at pyodide/pyodide-env and pyodide/pyodide.


  • Fix In console.html: sync behavior, full stdout/stderr support, clean namespace, bigger font, correct result representation, clean traceback #1125 and #1141

  • Fix Switched from ̀Jedi to rlcompleter for completion in pyodide.console.InteractiveConsole and so in console.html. This fixes some completion issues (see #821 and #1160)

  • Enhancement Support top-level await in the console #1459


  • six, jedi and parso are no longer vendored in the main Pyodide package, and need to be loaded explicitly #1010, #987.

  • Updated packages #1021, #1338, #1460.

  • Added Plotly version 4.14.3 and retrying dependency #1419

Version 0.16.1

December 25, 2020

Note: due to a CI deployment issue the 0.16.0 release was skipped and replaced by 0.16.1 with identical contents.

  • Pyodide files are distributed by JsDelivr, The previous CDN still works and there are no plans for deprecating it. However please use JsDelivr as a more sustainable solution, including for earlier Pyodide versions.

Python and the standard library

  • Pyodide includes CPython 3.8.2 #712

  • ENH Patches for the threading module were removed in all packages. Importing the module, and a subset of functionality (e.g. locks) works, while starting a new thread will produce an exception, as expected. #796. See #237 for the current status of the threading support.

  • ENH The multiprocessing module is now included, and will not fail at import, thus avoiding the necessity to patch included packages. Starting a new process will produce an exception due to the limitation of the WebAssembly VM with the following message: Resource temporarily unavailable #796.

Python / JS type conversions

  • FIX Only call Py_INCREF() once when proxied by PyProxy #708

  • Javascript exceptions can now be raised and caught in Python. They are wrapped in pyodide.JsException. #891

pyodide-py package and micropip

  • The file was transformed to a pyodide-py package. The imports remain the same so this change is transparent to the users #909.

  • FIX Get last version from PyPi when installing a module via micropip #846.

  • Suppress REPL results returned by pyodide.eval_code by adding a semicolon #876.

  • Enable monkey patching of eval_code and find_imports to customize behavior of runPython and runPythonAsync #941.

Build system

  • Updated docker image to Debian buster, resulting in smaller images. #815

  • Pre-built docker images are now available as iodide-project/pyodide #787

  • Host Python is no longer compiled, reducing compilation time. This also implies that Python 3.8 is now required to build Pyodide. It can for instance be installed with conda. #830

  • FIX Infer package tarball directory from source URL #687

  • Updated to emscripten 1.38.44 and binaryen v86 (see related commits)

  • Updated default --ldflags argument to pyodide_build scripts to equal what Pyodide actually uses. #817

  • Replace C lz4 implementation with the (upstream) Javascript implementation. #851

  • Pyodide deployment URL can now be specified with the PYODIDE_BASE_URL environment variable during build. The pyodide_dev.js is no longer distributed. To get an equivalent behavior with pyodide.js, set

    window.languagePluginUrl = './';

    before loading it. #855

  • Build runtime C libraries (e.g. libxml) via package build system with correct dependency resolution #927

  • Pyodide can now be built in a conda virtual environment #835

Other improvements

  • Modifiy MEMFS timestamp handling to support better caching. This in particular allows to import newly created Python modules without invalidating import caches #893


  • New packages: freesasa, lxml, python-sat, traits, astropy, pillow, scikit-image, imageio, numcodecs, msgpack, asciitree, zarr

    Note that due to the large size and the experimental state of the scipy package, packages that depend on scipy (including scikit-image, scikit-learn) will take longer to load, use a lot of memory and may experience failures.

  • Updated packages: numpy 1.15.4, pandas 1.0.5, matplotlib 3.3.3 among others.

  • New package pyodide-interrupt, useful for handling interrupts in Pyodide (see project description for details).

Backward incompatible changes

  • Dropped support for loading .wasm files with incorrect MIME type, following #851

List of contributors

abolger, Aditya Shankar, Akshay Philar, Alexey Ignatiev, Aray Karjauv, casatir, chigozienri, Christian glacet, Dexter Chua, Frithjof, Hood Chatham, Jan Max Meyer, Jay Harris, jcaesar, Joseph D. Long, Matthew Turk, Michael Greminger, Michael Panchenko, mojighahar, Nicolas Ollinger, Ram Rachum, Roman Yurchak, Sergio, Seungmin Kim, Shyam Saladi, smkm, Wei Ouyang

Version 0.15.0

May 19, 2020

  • Upgrades Pyodide to CPython 3.7.4.

  • micropip no longer uses a CORS proxy to install pure Python packages from PyPi. Packages are now installed from PyPi directly.

  • micropip can now be used from web workers.

  • Adds support for installing pure Python wheels from arbitrary URLs with micropip.

  • The CDN URL for Pyodide changed to It now supports versioning and should provide faster downloads. The latest release can be accessed via

  • Adds messageCallback and errorCallback to pyodide.loadPackage.

  • Reduces the initial memory footprint (TOTAL_MEMORY) from 1 GiB to 5 MiB. More memory will be allocated as needed.

  • When building from source, only a subset of packages can be built by setting the PYODIDE_PACKAGES environment variable. See partial builds documentation for more details.

  • New packages: future, autograd

Version 0.14.3

Dec 11, 2019

  • Convert Javascript numbers containing integers, e.g. 3.0, to a real Python long (e.g. 3).

  • Adds __bool__ method to for JsProxy objects.

  • Adds a Javascript-side auto completion function for Iodide that uses jedi.

  • New packages: nltk, jeudi, statsmodels, regex, cytoolz, xlrd, uncertainties

Version 0.14.0

Aug 14, 2019

  • The built-in sqlite and bz2 modules of Python are now enabled.

  • Adds support for auto-completion based on jedi when used in iodide

Version 0.13.0

May 31, 2019

  • Tagged versions of Pyodide are now deployed to Netlify.

Version 0.12.0

May 3, 2019

User improvements:

  • Packages with pure Python wheels can now be loaded directly from PyPI. See Micropip for more information.

  • Thanks to PEP 562, you can now import js from Python and use it to access anything in the global Javascript namespace.

  • Passing a Python object to Javascript always creates the same object in Javascript. This makes APIs like removeEventListener usable.

  • Calling dir() in Python on a Javascript proxy now works.

  • Passing an ArrayBuffer from Javascript to Python now correctly creates a memoryview object.

  • Pyodide now works on Safari.

Version 0.11.0

Apr 12, 2019

User improvements:

  • Support for built-in modules:

    • sqlite, crypt

  • New packages: mne

Developer improvements:

  • The mkpkg command will now select an appropriate archive to use, rather than just using the first.

  • The included version of emscripten has been upgraded to 1.38.30 (plus a bugfix).

  • New packages: jinja2, MarkupSafe

Version 0.10.0

Mar 21, 2019

User improvements:

  • New packages: html5lib, pygments, beautifulsoup4, soupsieve, docutils, bleach, mne

Developer improvements:

  • console.html provides a simple text-only interactive console to test local changes to Pyodide. The existing notebooks based on legacy versions of Iodide have been removed.

  • The run_docker script can now be configured with environment variables.