Frequently Asked Questions#

How can I load external files in Pyodide?#

See Accessing Files Quick Reference.

Why can’t I load files from the local file system?#

For security reasons JavaScript in the browser is not allowed to load local data files (for example, file:///path/to/local/ You will run into Network Errors, due to the Same Origin Policy. There is a File System API supported in Chrome but not in Firefox or Safari. See (Experimental) Using the native file system in the browser for experimental local file system support.

For development purposes, you can serve your files with a web server.

How can I execute code in a custom namespace?#

The second argument to pyodide.runPython() is an options object which may include a globals element which is a namespace for code to read from and write to. The provided namespace must be a Python dictionary.

let my_namespace = pyodide.globals.get("dict")();
pyodide.runPython(`x = 1 + 1`, { globals: my_namespace });
pyodide.runPython(`y = x ** x`, { globals: my_namespace });
my_namespace.get("y"); // ==> 4

You can also use this approach to inject variables from JavaScript into the Python namespace, for example:

let my_namespace = pyodide.toPy({ x: 2, y: [1, 2, 3] });
  assert x == y[1]
  z = x ** x
  { globals: my_namespace }
my_namespace.get("z"); // ==> 4

How to detect that code is run with Pyodide?#

At run time, you can check if Python is built with Emscripten (which is the case for Pyodide) with,

import sys

if sys.platform == 'emscripten':
    # running in Pyodide or other Emscripten based build

To detect that a code is running with Pyodide specifically, you can check for the loaded pyodide module,

import sys

if "pyodide" in sys.modules:
   # running in Pyodide

This however will not work at build time (i.e. in a due to the way the Pyodide build system works. It first compiles packages with the host compiler (e.g. gcc) and then re-runs the compilation commands with emsdk. So the is never run inside the Pyodide environment.

To detect Pyodide, at build time use,

import os

if "PYODIDE" in os.environ:
    # building for Pyodide

How do I create custom Python packages from JavaScript?#

Put a collection of functions into a JavaScript object and use pyodide.registerJsModule(): JavaScript:

let my_module = {
  f: function (x) {
    return x * x + 1;
  g: function (x) {
    console.log(`Calling g on argument ${x}`);
    return x;
  submodule: {
    h: function (x) {
      return x * x - 1;
    c: 2,
pyodide.registerJsModule("my_js_module", my_module);

You can import your package like a normal Python package:

import my_js_module
from my_js_module.submodule import h, c
assert my_js_module.f(7) == 50
assert h(9) == 80
assert c == 2

How can I send a Python object from my server to Pyodide?#

The best way to do this is with pickle. If the version of Python used in the server exactly matches the version of Python used in the client, then objects that can be successfully pickled can be sent to the client and unpickled in Pyodide. If the versions of Python are different then for instance sending AST is unlikely to work since there are breaking changes to Python AST in most Python minor versions.

Similarly when pickling Python objects defined in a Python package, the package version needs to match exactly between the server and pyodide.

Generally, pickles are portable between architectures (here amd64 and wasm32). The rare cases when they are not portable, for instance currently tree based models in scikit-learn, can be considered as a bug in the upstream library.

Security Issues with pickle

Unpickling data is similar to eval. On any public-facing server it is a really bad idea to unpickle any data sent from the client. For sending data from client to server, try some other serialization format like JSON.

How can I use a Python function as an event handler?#

Note that the most straight forward way of doing this will not work:

from js import document
def f(*args):
    document.querySelector("h1").innerHTML += "(>.<)"

document.body.addEventListener('click', f)

Now every time you click, an error will be raised (see Calling JavaScript functions from Python).

To do this correctly use create_proxy() as follows:

from js import document
from pyodide.ffi import create_proxy
def f(*args):
    document.querySelector("h1").innerHTML += "(>.<)"

proxy_f = create_proxy(f)
document.body.addEventListener('click', proxy_f)
# Store proxy_f in Python then later:
document.body.removeEventListener('click', proxy_f)

How can I use fetch with optional arguments from Python?#

The most obvious translation of the JavaScript code won’t work:

import json
resp = await js.fetch('/someurl', {
  "method": "POST",
  "body": json.dumps({ "some" : "json" }),
  "credentials": "same-origin",
  "headers": { "Content-Type": "application/json" }

The fetch() API ignores the options that we attempted to provide. You can do this correctly in one of two ways:

import json
from pyodide.ffi import to_js
from js import Object
resp = await js.fetch('',
  method= "POST",
  body= json.dumps({ "some" : "json" }),
  credentials= "same-origin",
  headers= Object.fromEntries(to_js({ "Content-Type": "application/json" })),


import json
from pyodide.ffi import to_js
from js import Object
resp = await js.fetch('', to_js({
  "method": "POST",
  "body": json.dumps({ "some" : "json" }),
  "credentials": "same-origin",
  "headers": { "Content-Type": "application/json" }
}, dict_converter=Object.fromEntries)

How can I control the behavior of stdin / stdout / stderr?#

If you wish to override stdin, stdout or stderr for the entire Pyodide runtime, you can pass options to loadPyodide(): If you say

  stdin: stdin_func,
  stdout: stdout_func,
  stderr: stderr_func,

then every time a line is written to stdout (resp. stderr), stdout_func (resp stderr_func) will be called on the line. Every time stdin is read, stdin_func will be called with zero arguments. It is expected to return a string which is interpreted as a line of text.

You can also use the functions pyodide.setStdin(), pyodide.setStdout(), and pyodide.setStderr().

Temporary redirection works much the same as it does in native Python: you can overwrite sys.stdin, sys.stdout, and sys.stderr respectively. If you want to do it temporarily, it’s recommended to use contextlib.redirect_stdout() and contextlib.redirect_stderr() There is no contextlib.redirect_stdin() but it is easy to make your own as follows:

from contextlib import _RedirectStream
class redirect_stdin(_RedirectStream):
    _stream = "stdin"

For example, if you do:

from io import StringIO
with redirect_stdin(StringIO("\n".join(["eval", "asyncio.ensure_future", "functools.reduce", "quit"]))):

it will print:

Welcome to Python 3.10's help utility!
Help on built-in function eval in module builtins:
eval(source, globals=None, locals=None, /)
    Evaluate the given source in the context of globals and locals.
Help on function ensure_future in asyncio:
asyncio.ensure_future = ensure_future(coro_or_future, *, loop=None)
    Wrap a coroutine or an awaitable in a future.
Help on built-in function reduce in functools:
functools.reduce = reduce(...)
    reduce(function, sequence[, initial]) -> value
    Apply a function of two arguments cumulatively to the items of a sequence,
You are now leaving help and returning to the Python interpreter.

Why can’t Micropip find a “pure Python wheel” for a package?#

When installing a Python package from PyPI, micropip will produce an error if it cannot find a pure Python wheel. To determine if a package has a pure Python wheel manually, you can open its PyPi page (for instance and go to the “Download files” tab. If this tab doesn’t contain a file *py3-none-any.whl then the pure Python wheel is missing.

This can happen for two reasons,

  1. either the package is pure Python (you can check language composition for a package on Github), and its maintainers didn’t upload a wheel. In this case, you can report this issue to the package issue tracker. As a temporary solution, you can also build the wheel yourself, upload it to some temporary location and install it with micropip from the corresponding URL.

  2. or the package has binary extensions (e.g. C, Fortran or Rust), in which case it needs to be packaged in Pyodide. Please open an issue after checking that an issue for this package doesn’t exist already. Then follow Creating a Pyodide package.

How can I change the behavior of runPython() and runPythonAsync()?#

You can directly call Python functions from JavaScript. For most purposes it makes sense to make your own Python function as an entrypoint and call that instead of redefining runPython. The definitions of runPython() and runPythonAsync() are very simple:

function runPython(code) {
  pyodide.pyodide_py.code.eval_code(code, pyodide.globals);
async function runPythonAsync(code) {
  return await pyodide.pyodide_py.code.eval_code_async(code, pyodide.globals);

To make your own version of runPython() you could do:

const my_eval_code = pyodide.runPython(`
  from pyodide.code import eval_code
  def my_eval_code(code, globals=None, locals=None):
    extra_info = None
    result = eval_code(code, globals, locals)
    return globals["extra_info"], result

function myRunPython(code){
  return my_eval_code(code, pyodide.globals);

Then myRunPython("2+7") returns [None, 9] and myRunPython("extra_info='hello' ; 2 + 2") returns ['hello', 4]. If you want to change which packages pyodide.loadPackagesFromImports() loads, you can monkey patch pyodide.code.find_imports() which takes code as an argument and returns a list of packages imported.

Why can’t I import a file I just wrote to the file system?#

For example:

from pathlib import Path
def hello():
  print("hello world!")
from mymodule import hello # may raise "ModuleNotFoundError: No module named 'mymodule'"

If you see this error, call importlib.invalidate_caches() before importing the module:

import importlib
from pathlib import Path
def hello():
  print("hello world!")
importlib.invalidate_caches() # Make sure Python notices the new .py file
from mymodule import hello

Why changes made to IndexedDB don’t persist?#

Unlike other filesystems, IndexedDB (pyodide.FS.filesystem.IDBFS) is an asynchronous filesystem. This is because browsers offer only asynchronous interfaces for IndexedDB. So in order to persist changes, you have to call pyodide.FS.syncfs(). See Emscripten File System API for more details.

How can I access JavaScript objects/attributes in Python if their names are Python keywords?#

Some JavaScript objects may have names or attributes which are also Python Keywords, making them difficult to interact with when importing them into Python. For example, all three of the following uses of runPython will throw a SyntaxError:

//The built-in method Array.from() overlaps with Python's "from"
pyodide.runPython(`from js import Array; print(Array.from([1,2,3]))`);

//"global" is a valid attribute name in JS, but a reserved keyword in Python
people = {global: "lots and lots"};
pyodide.runPython(`from js import people; print(`);

//"lambda" is a valid object name in JS, but a reserved keyword in Python
lambda = (x) => {return x + 1};
pyodide.runPython(`from js import lambda; print(lambda(1))`);

If you try to access a Python reserved word followed by one or more underscores on a JsProxy, Pyodide will remove a single underscore:

    from js import Array

If you meant to access the keyword with an underscore at the end, you’ll have to add an extra one:

globalThis.lambda = 7;
globalThis.lambda_ = 8;
    from js import lambda_, lambda__
    print(lambda_, lambda__) # 7, 8

Another example:

people = {global: "lots and lots"};
    from js import people
    # the dir contains global_ but not global:
    assert "global_" in dir(people)
    assert "global" not in dir(people)
    people.global_ = 'even more'

You can also use getattr, setattr, and delattr to access the attribute:

    from js import Array
    fromFunc = getattr(Array, 'from')

people = {global: "lots and lots"};
    from js import people
    setattr(people, 'global', 'even more')
    print(getattr(people, 'global'))

For JavaScript globals whose names are keywords, one can similarly use getattr() on the js module itself:

globalThis.lambda = 7;
globalThis.lambda_ = 8;
    import js
    js_lambda = getattr(js, 'lambda')
    js_lambda_ = getattr(js, 'lambda_')
    js_lambda__ = getattr(js, 'lambda__')
    print(js_lambda, js_lambda_, js_lambda__) # 7, 7, 8