fix parsing pepxml with none of the standard scores

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [1.5.5] - 2026-06-05
 
+### Fixed
+
+- `io.pepxml`: Fix a `TypeError` when none of the standard search engine scores are found and the fallback (`score_keys[0]`) is accessed (fixes #150)
+
 ### Changed
 
 - Pin version bounds for all core dependencies. Most notably, `pyteomics` is now constrained to `< 5` to avoid breaking changes introduced in the upcoming major release.

psm_utils/io/pepxml.py

@@ -75,7 +75,7 @@ def _infer_score_name(self) -> str | None:
             for spectrum_query in reader:
                 if "search_hit" not in spectrum_query:
                     continue
-                score_keys = spectrum_query["search_hit"][0]["search_score"].keys()
+                score_keys = list(spectrum_query["search_hit"][0]["search_score"].keys())
                 break
             else:
                 score_keys = []