How exactly are python tests parallelized? I do see that pants is executing multiple processes. Is this one pytest invocation or multiple? The reason I am asking is that we have some expensive session scoped fixtures and project startup is also slowish, so we would like to keep number of invocations low

Each test run is a separate pytest invocation. By default this is running files individually, but there’s some options to tweak. See “batching and parallelism” in https://www.pantsbuild.org/docs/python-test-goal (I’m on mobile so can’t link directly) for more discussion