> For the complete documentation index, see [llms.txt](https://ciusji.gitbook.io/irelia/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ciusji.gitbook.io/irelia/using-formulas/python-versions.md).

# Python versions

Irelia formulas are written in Python, the most popular language for data science. The entirety of [Python’s standard library](https://docs.python.org/3.9/library/) is available to you. For those with a spreadsheet background, we’ve also added a suite of Excel-like functions, with all-uppercase names. Here’s the full list of functions. Python formulas are evaluated in a sandbox, without internet access, and without a persistent filesystem.

## Supported versions

We currently support two versions of Python:

* Python 3 (specifically 3.9 at the time of writing)
* Python 2 (specifically 2.7)

Newly created documents on our hosted service use Python 3 by default, while older documents (created before November 2021 approximately) use Python 2 by default. ~~To tell which version of Python a specific document uses, look at its~~ [~~Document Settings~~](https://ciusji.gitbook.io/irelia/managing-documents/create-a-document#document-settings)~~.~~ The `Engine` setting may be `python2`, `python3`, or blank. A blank setting implies `python2`.

If you have editing rights on a document, you can change the `Engine` setting, and the document will then reload with all formulas now interpreted using the version of Python you have specified. We recommend caution in doing so. A formula that works as intended in one version of Python may give errors in another, or (worse) give the wrong results.