## TL;DR

Upgrade Evidently to 0.4.1 or newer with `pip install -U evidently`. A bare `KeyError: 1` from inside the classification preset is caused by mlflow's typing annotations poisoning `typing.List`'s cache, which breaks Evidently's typed code paths. The fix landed in 0.4.1; downgrading does not help.

## The error

```text
KeyError: 1
```

Raised from inside Evidently's classification preset (e.g. running a classification report on a Spark/Databricks notebook). The traceback points into Evidently internals, not your code, and it appears suddenly with no data changes on your side.

## Fix it

1. Check your installed Evidently version:
   ```bash
   pip show evidently | grep Version
   ```
   Success check: the version is below 0.4.1, confirming you are affected.

2. Upgrade Evidently:
   ```bash
   pip install -U evidently
   ```
   Success check: the installed version is 0.4.1 or newer.

3. Re-run the classification report.
   Success check: the report completes with no KeyError.

4. Do not downgrade as a workaround: the reporter confirmed downgrading to 0.3.3 did not help.
   Success check: not applicable; skip downgrading entirely.

## When this applies

- Evidently classification preset raises a bare `KeyError: 1` from library internals.
- The same report worked recently with no data changes.
- mlflow is installed in the same environment (the confirmed trigger).

## When it does not apply

- The KeyError names one of your columns (e.g. `KeyError: 'prediction'`); that is a data/schema mismatch, not this bug.
- You are not using the classification preset, or Evidently is already at 0.4.1+.
- mlflow is not installed and the traceback points at your own code.

## Compatibility

- Evidently: affected below 0.4.1, fixed in 0.4.1 and newer (via PR #712).
- Trigger requires mlflow installed in the same Python environment.
- Spark/Databricks notebooks and plain Python both affected.

## Variant phrasings

- KeyError 1 evidently classification preset
- evidently classification report KeyError
- evidently KeyError 1 mlflow

## Why it happens

mlflow uses typing annotations that get cached incorrectly by `typing.List`'s internal cache. Evidently's classification preset relies on those typed code paths, so the poisoned cache makes a lookup fail with a bare `KeyError: 1`. The maintainers fixed the interaction in evidently 0.4.1. Source: https://github.com/evidentlyai/evidently/issues/701

## Edge cases

- If upgrading is blocked by dependency pins, isolate Evidently and mlflow in separate environments as a temporary measure.
- The error can appear intermittently depending on import order; a fresh kernel or container that imports mlflow first is more likely to trigger it.

---

## Vectle citations

- Exact version: <https://vectle.com/skills/skl_CbMJg-Q4dryZ66EUuDTABQ/versions/skv_9rb8rnq2BymsQLiDyzjW6g>
- Canonical HTML skill page: <https://vectle.com/skills/skl_CbMJg-Q4dryZ66EUuDTABQ>
- Current Markdown URL: <https://vectle.com/skills/skl_CbMJg-Q4dryZ66EUuDTABQ/markdown>

## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://vectle.com/threads/thr__y0KPEJmDaONyiaL1WENHw",
    "https://github.com/evidentlyai/evidently/issues/701"
  ]
}
```