Stop needs_merlin_expansion extending the caller's label list

merlin/utils.py

@@ -805,7 +805,8 @@ def needs_merlin_expansion(cmd: str, restart_cmd: str, labels: List[str], includ
     """
     sample_keywords = ["MERLIN_SAMPLE_ID", "MERLIN_SAMPLE_PATH", "merlin_sample_id", "merlin_sample_path"]
     if include_sample_keywords:
-        labels += sample_keywords
+        # Not `labels +=`, which extends the caller's list in place.
+        labels = labels + sample_keywords
 
     for label in labels:
         if f"$({label})" in cmd:

tests/unit/utils/test_needs_merlin_expansion.py

@@ -0,0 +1,30 @@
+##############################################################################
+# Copyright (c) Lawrence Livermore National Security, LLC and other Merlin
+# Project developers. See top-level LICENSE and COPYRIGHT files for dates and
+# other details. No copyright assignment is required to contribute to Merlin.
+##############################################################################
+
+from merlin.utils import needs_merlin_expansion
+
+
+def test_does_not_modify_the_labels_it_was_given() -> None:
+    """The caller's list is read, not extended with the sample keywords."""
+    labels = ["X", "Y"]
+
+    needs_merlin_expansion("echo $(X)", "", labels)
+
+    assert labels == ["X", "Y"]
+
+
+def test_sample_keywords_are_still_matched() -> None:
+    assert needs_merlin_expansion("echo $(MERLIN_SAMPLE_ID)", "", ["X"])
+    assert not needs_merlin_expansion("echo $(MERLIN_SAMPLE_ID)", "", ["X"], include_sample_keywords=False)
+
+
+def test_repeated_calls_keep_the_same_answer() -> None:
+    labels = ["X"]
+
+    for _ in range(3):
+        assert not needs_merlin_expansion("echo $(Z)", "", labels)
+
+    assert labels == ["X"]