For now, only one `pyodide` can be loaded in a given global scope because we need to use a global variable for package loading. This changes `loadPyodide` so that it no longer defines `globalThis.pyodide` in favor of just returning `pyodide` from `loadPyodide`. `Module` is placed on the global object as `globalThis.__pyodide_module` for the purposes of package loading. I added documentation to explicitly state that `loadPyodide` can only be run once in each global Javascript context, an error will be thrown otherwise.

This has two advantages: 
1. The returned `pyodide` module is understandable to Typescript and other code analysis, whereas the global variable is not understandable.
2. If in the future we are able to remove the global variable, no further API change will be needed (other than relaxing the documented "pyodide must be globally-unique" constraint). I figure it's better to make this API change earlier rather than later.

It will be possible to completely remove the global variable dependence when Firefox implements support for using ES6 modules in webworkers (see https://bugzilla.mozilla.org/show_bug.cgi?id=1247687).
API Don't expose pyodide as a global variable in loadPyodide. For now, only one `pyodide` can be loaded in a given global scope because we need to use a global variable for package loading. This changes `loadPyodide` so that it no longer defines `globalThis.pyodide` in favor of just returning `pyodide` from `loadPyodide`. `Module` is placed on the global object as `globalThis.__pyodide_module` for the purposes of package loading. I added documentation to explicitly state that `loadPyodide` can only be run once in each global Javascript context, an error will be thrown otherwise.

This has two advantages: 
1. The returned `pyodide` module is understandable to Typescript and other code analysis, whereas the global variable is not understandable.
2. If in the future we are able to remove the global variable, no further API change will be needed (other than relaxing the documented "pyodide must be globally-unique" constraint). I figure it's better to make this API change earlier rather than later.

It will be possible to completely remove the global variable dependence when Firefox implements support for using ES6 modules in webworkers (see https://bugzilla.mozilla.org/show_bug.cgi?id=1247687).
(loading_packages)=
# Loading packages

Only the Python standard library is available after importing Pyodide.
To use other packages, you’ll need to load them using either:
 - {any}`pyodide.loadPackage` for packages built with Pyodide, or
 - {any}`micropip.install` for pure Python packages with wheels available on PyPi or
   from other URLs.

```{note}
{mod}`micropip` can also be used to load packages built in Pyodide (in
which case it relies on {any}`pyodide.loadPackage`).
```

If you use {any}`pyodide.loadPackagesFromImports` Pyodide will automatically
download all packages that the code snippet imports. This is particularly useful
for making a repl since users might import unexpected packages. At present,
`loadPackagesFromImports` will not download packages from PyPi, it will only
download packages included in the Pyodide distribution.

## Loading packages with {any}`pyodide.loadPackage`

Packages included in the official Pyodide repository can be loaded using
{any}`pyodide.loadPackage`:
```js
pyodide.loadPackage('numpy');
```
It is also possible to load packages from custom URLs:
```js
pyodide.loadPackage('https://foo/bar/numpy.js');
```
The file name in the URL must be `/<package-name>.js` and there must be an
accompanying `.data` file in the same directory.

When you request a package from the official repository, all of that package's
dependencies are also loaded. Dependency resolution is not yet implemented when
loading packages from custom URLs.

In general, loading a package twice is not permitted. However, one can override
a dependency by loading a custom URL with the same package name before loading
the dependent package.

Multiple packages can also be loaded at the same time by passing a list to `loadPackage.
```js
pyodide.loadPackage(['cycler', 'pytz']);
```

{any}`pyodide.loadPackage` returns a `Promise` which resolves when all of the
packages are finished loading:
```javascript
async function main(){
  await loadPyodide({ 'indexURL' : '<some-url>' });
  await pyodide.loadPackage('matplotlib');
  // matplotlib is now available
}
main();
```

(micropip)=
## Micropip

### Installing packages from PyPI

Pyodide supports installing pure Python wheels from PyPI with {mod}`micropip`.
{func}`micropip.install` returns a Python `Future
<https://docs.python.org/3/library/asyncio-future.html>`_ so you can await the
future or otherwise use the Python future API to do work once the packages have
finished loading:

```pyodide
pyodide.runPythonAsync(`
  import micropip
  await micropip.install('snowballstemmer')
  import snowballstemmer
  stemmer = snowballstemmer.stemmer('english')
  print(stemmer.stemWords('go goes going gone'.split()))
`);
```

Micropip implements file integrity validation by checking the hash of the
downloaded wheel against pre-recorded hash digests from the PyPi JSON API.

(micropip-installing-from-arbitrary-urls)=

### Installing wheels from arbitrary URLs

Pure Python wheels can also be installed from any URL with {mod}`micropip`,
```py
import micropip
micropip.install(
    'https://example.com/files/snowballstemmer-2.0.0-py2.py3-none-any.whl'
)
```
Micropip decides whether a file is a URL based on whether it ends in ".whl" or not.
The wheel name in the URL must follow [PEP 427 naming
convention](https://www.python.org/dev/peps/pep-0427/#file-format), which will
be the case if the wheels is made using standard Python tools (`pip wheel`,
`setup.py bdist_wheel`).

All required dependencies must have been previously installed with {mod}`micropip`
or {any}`pyodide.loadPackage`.

If the file is on a remote server, the server must set Cross-Origin Resource Sharing
(CORS) headers to allow access. Otherwise, you can prepend a CORS proxy to the
URL. Note however that using third-party CORS proxies has security implications,
particularly since we are not able to check the file integrity, unlike with
installs from PyPi.


## Example

```html
<html>
<head>
  <meta charset="utf-8">
</head>
<body>
  <script type="text/javascript" src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
  <script type="text/javascript">
    async function main(){
      await loadPyodide({ indexURL : 'https://cdn.jsdelivr.net/pyodide/dev/full/' });
      await pyodide.runPythonAsync(`
        import micropip # runPythonAsync will load micropip automatically
        await micropip.install('snowballstemmer')
        import snowballstemmer
        stemmer = snowballstemmer.stemmer('english')
        print(stemmer.stemWords('go goes going gone'.split()))
      `);
    }
    main();
  </script>
</body>
</html>
```

(loading_packages)=
# Loading packages

Only the Python standard library is available after importing Pyodide.
To use other packages, you’ll need to load them using either:
 - {any}`pyodide.loadPackage` for packages built with Pyodide, or
 - {any}`micropip.install` for pure Python packages with wheels available on PyPi or
   from other URLs.

```{note}
{mod}`micropip` can also be used to load packages built in Pyodide (in
which case it relies on {any}`pyodide.loadPackage`).
```

If you use {any}`pyodide.loadPackagesFromImports` Pyodide will automatically
download all packages that the code snippet imports. This is particularly useful
for making a repl since users might import unexpected packages. At present,
`loadPackagesFromImports` will not download packages from PyPi, it will only
download packages included in the Pyodide distribution.

## Loading packages with {any}`pyodide.loadPackage`

Packages included in the official Pyodide repository can be loaded using
{any}`pyodide.loadPackage`:
```js
pyodide.loadPackage('numpy');
```
It is also possible to load packages from custom URLs:
```js
pyodide.loadPackage('https://foo/bar/numpy.js');
```
The file name in the URL must be `/<package-name>.js` and there must be an
accompanying `.data` file in the same directory.

When you request a package from the official repository, all of that package's
dependencies are also loaded. Dependency resolution is not yet implemented when
loading packages from custom URLs.

In general, loading a package twice is not permitted. However, one can override
a dependency by loading a custom URL with the same package name before loading
the dependent package.

Multiple packages can also be loaded at the same time by passing a list to `loadPackage.
```js
pyodide.loadPackage(['cycler', 'pytz']);
```

{any}`pyodide.loadPackage` returns a `Promise` which resolves when all of the
packages are finished loading:
```javascript
let pyodide;
async function main(){
  pyodide = await loadPyodide({ 'indexURL' : '<some-url>' });
  await pyodide.loadPackage('matplotlib');
  // matplotlib is now available
}
main();
```

(micropip)=
## Micropip

### Installing packages from PyPI

Pyodide supports installing pure Python wheels from PyPI with {mod}`micropip`.
{func}`micropip.install` returns a Python `Future
<https://docs.python.org/3/library/asyncio-future.html>`_ so you can await the
future or otherwise use the Python future API to do work once the packages have
finished loading:

```pyodide
pyodide.runPythonAsync(`
  import micropip
  await micropip.install('snowballstemmer')
  import snowballstemmer
  stemmer = snowballstemmer.stemmer('english')
  print(stemmer.stemWords('go goes going gone'.split()))
`);
```

Micropip implements file integrity validation by checking the hash of the
downloaded wheel against pre-recorded hash digests from the PyPi JSON API.

(micropip-installing-from-arbitrary-urls)=

### Installing wheels from arbitrary URLs

Pure Python wheels can also be installed from any URL with {mod}`micropip`,
```py
import micropip
micropip.install(
    'https://example.com/files/snowballstemmer-2.0.0-py2.py3-none-any.whl'
)
```
Micropip decides whether a file is a URL based on whether it ends in ".whl" or not.
The wheel name in the URL must follow [PEP 427 naming
convention](https://www.python.org/dev/peps/pep-0427/#file-format), which will
be the case if the wheels is made using standard Python tools (`pip wheel`,
`setup.py bdist_wheel`).

All required dependencies must have been previously installed with {mod}`micropip`
or {any}`pyodide.loadPackage`.

If the file is on a remote server, the server must set Cross-Origin Resource Sharing
(CORS) headers to allow access. Otherwise, you can prepend a CORS proxy to the
URL. Note however that using third-party CORS proxies has security implications,
particularly since we are not able to check the file integrity, unlike with
installs from PyPi.


## Example

```html
<html>
<head>
  <meta charset="utf-8">
</head>
<body>
  <script type="text/javascript" src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
  <script type="text/javascript">
    async function main(){
      let pyodide = await loadPyodide({ indexURL : 'https://cdn.jsdelivr.net/pyodide/dev/full/' });
      await pyodide.loadPackage("micropip");
      await pyodide.runPythonAsync(`
        import micropip
        await micropip.install('snowballstemmer')
        import snowballstemmer
        stemmer = snowballstemmer.stemmer('english')
        print(stemmer.stemWords('go goes going gone'.split()))
      `);
    }
    main();
  </script>
</body>
</html>
```
