# 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
  {ref}`load it with micropip <micropip-installing-from-arbitrary-urls>`.
- fetch the python code as a string and evaluate it in Python,
  ```js
  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](https://wicg.github.io/file-system-access/) supported in Chrome but not in Firefox. [There is a discussion about implementing it for Firefox here.](https://github.com/mozilla/standards-positions/issues/154)


## 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:
```python
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,
```py
import sys

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

More generally you can detect Python built with Emscripten (which includes
Pyodide) with,
```py
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,
```python
import os

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