pyodide/docs/usage/wasm-constraints.md

Pyodide Python compatibility

Python Standard library

Most of the Python standard library is functional, except for the modules listed in the sections below. A large part of the CPython test suite passes except for tests skipped in src/tests/python_tests.yaml or via patches.

Optional modules

The following stdlib modules are unvendored by default, in order to reduce initial download size of Python distribution. You can load all unvendored stdlib modules when initializing Pyodide with, loadPyodide({ fullStdLib : true }). However this has a significant impact on the download size. Instead, it is better to load individual modules as needed using {js:func}pyodide.loadPackage or {py:func}micropip.install.

• ssl
• lzma
• sqlite3
• test: it is an exception to the above, since it is not loaded even if fullStdLib is set to true.

Modules with limited functionality

• hashlib: Hash algorithms that are depending on OpenSSL are not available by default. See Python hashlib documentation for list of algorithms that are dependent on OpenSSL. If you need those algorithms, you need to call pyodide.loadPackage('hashlib') or micropip.install('hashlib') before importing hashlib.

• decimal: The decimal module has C (_decimal) and Python (_pydecimal) implementations with the same functionality. The Python implementation is not available by default. If you need a Python implementation of decimal, you need to call pyodide.loadPackage('pydecimal') or micropip.install('pydecimal'), then explicitly import _pydecimal.

• pydoc: Help messages for Python builtins are not available by default in order to reduce the initial download size. You need to call pyodide.loadPackage('pydoc_data') or micropip.install('pydoc_data') to enable them.

• webbrowser: The original webbrowser module is not available. Instead, Pyodide includes some method stubs based on browser APIs: webbrowser.open(), webbrowser.open_new(), webbrowser.open_new_tab().

• zoneinfo: The zoneinfo package will only work if you install the timezone data using the tzdata package (i.e. by calling pyodide.loadPackage("tzdata"))

Synchronous HTTP requests support

Packages for urllib3 and requests are included in pyodide. In browser, these function roughly the same as on other operating systems with some limitations. In node.js, they are currently untested, they will require at least a polyfill for synchronous XMLHttpRequest, and WebWorker.

The first limitation is that streaming download of files only works in very specific circumstances, which are that pyodide has to be running in a web-worker, and it has to be on a cross-origin isolated website. If either of these conditions are not met, it will do a non-streaming request, i.e. download the full request body before it returns from the initial request call.

Secondly, all network calls are done via the browser. This means you are subject to the same limitations as any JavaScript network call. This means you have very little or no control over certificates, timeouts, proxies and other network related settings. You also are constrained by browser policies relating to cross-origin requests, sometimes things will be blocked by CORS policies if the server doesn't serve them with the correct headers.

Removed modules

The following modules are removed from the standard library to reduce download size and since they currently wouldn't work in the WebAssembly VM,

• curses
• dbm
• ensurepip
• fcntl
• grp
• idlelib
• lib2to3
• msvcrt
• pwd
• resource
• syslog
• termios
• tkinter
• turtle.py
• turtledemo
• venv
• winreg
• winsound

Included but not working modules

The following modules can be imported, but are not functional due to the limitations of the WebAssembly VM:

• multiprocessing
• threading
• sockets

as well as any functionality that requires these.

The following are present but cannot be imported due to a dependency on the termios package which has been removed:

• pty
• tty