Anvil lets you write everything in Python - both in the web browser and on the server.
Your Server Module code runs in a standard Python environment on the Anvil server. There are a few different environments to choose from.
Basic Python is a PyPy Sandbox - this is a Python interpreter with extra security features that allow us to provide the server environment for free to everybody.
This is available in Python 3.6 (the default) or Python 2.7 versions.
The Full Python server environment is an ordinary CPython interpreter - the standard version of Python that you probably have on your own machine. This is available on all paid tiers.
For each app, you can choose between Python 3.7 or Python 2.7 for your Server Modules.