Frequently Asked Questions (FAQ)

How can I load external python files in Pyodide?

The two possible solutions are,

  • include these files in a python package, build a pure python wheel with python setup.py bdist_wheel and load it with micropip.

  • fetch the python code as a string and evaluate it in Python,

    pyodide.runPython(await fetch('https://some_url/...'))

In both cases, files need to be served with a web server and cannot be loaded from local file system.

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. You need to serve them with a web-browser. Recently there is a Native File System API supported in Chrome but not in Firefox. There is a discussion about implementing it for Firefox here.

How can I change the behavior of runPython and runPythonAsync?

Internally they use the pyodide-py apis eval_code and find_imports. You can monkey patch these. Run the following Python code:

import pyodide
old_eval_code = pyodide.eval_code
def eval_code(code, ns):
  extra_info = None
  result = old_eval_code(code, ns)
  return [ns["extra_info"], result]
pyodide.eval_code = eval_code

Then pyodide.runPython("2+7") returns 9 and pyodide.runPython("extra_info='hello' ; 2 + 2") will return ['hello', 4].

How to detect that code is run with Pyodide?

At run time, you can detect that a code is running with Pyodide using,

import sys

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

More generally you can detect Python built with Emscripten (which includes Pyodide) with,

import platform

if platform.system() == 'Emscripten':
    # running in Pyodide or other Emscripten based build

This however will not work at build time (i.e. in a setup.py) 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 setup.py is never run inside the Pyodide environement.

To detect pyodide, at build time use,

import os

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