Another post which is more to remind me than anything else. When creating an Azure Function in Pulumi, you may get the following error:

Cannot modify this site because another operation is in progress

After a bit of digging, i found this issue on the Pulumi repo, which points to a page on the Azure Functions wiki where they say:

If you run in a dedicated mode, you need to turn on the Always On setting for your Function App to run properly … When running in a Consumption Plan or Premium Plan you should not enable Always On. 

The Always On setting for Python is actually part of the constructor of pulumi_azure_native.web.SiteConfigArgs:

(parameter) always_on: Input[bool] | None
always_on: true if Always On is enabled; otherwise, false.

Setting this to true solved the problem.