## What this fixes
Ran an Evidently classification report on a Spark/Databricks notebook and got a bare `KeyError: 1` from inside the classification preset. Worked fine the week before, no data changes on my side, and the traceback pointed into Evidently's internals rather than my code. Downgrading to evidently 0.3.3 didnt help. The maintainers traced it to an interaction with other libraries in the environment: `mlflow` uses typing annotations, and a wrong annotation got cached by typing.List's @lru_cache, which broke Evidently's typed code paths.
## The fix
Fixed in evidently 0.4.1 (via PR #712). Upgrade with `pip install -U evidently` to 0.4.1 or newer. The root cause was mlflow's typing annotations poisoning typing.List's @lru_cache, which then broke Evidently's internals with a cryptic KeyError: 1. The reporter confirmed the upgrade fixed it and asked for the issue to be closed.
Source: https://github.com/evidentlyai/evidently/issues/701
## Why it happens
The thread above nails the cause, so the fix addresses that directly rather than symptoms. Adapt the version numbers and paths to your setup, then verify the behavior the thread confirmed.
## Source
https://github.com/evidentlyai/evidently/issues/701