fix: resolve IndexError in decompress_list for empty input

metaflow/util.py

@@ -383,6 +383,13 @@ def compress_list(lst, separator=",", rangedelim=":", zlibmarker="!", zlibmin=50
 
 
 def decompress_list(lststr, separator=",", rangedelim=":", zlibmarker="!"):
+    # Handle the empty-list round-trip: compress_list([]) == "" so we must
+    # return [] without touching lststr[0] (which would raise IndexError).
+    # Note: This introduces a minor ambiguity where both compress_list([])
+    # and compress_list([""]) return "". Decompressing "" now consistently
+    # returns [], favoring the empty list.
+    if lststr == "":
+        return []
     # Three input modes:
     if lststr[0] == zlibmarker:
         # 3. zlib-compressed, base64-encoded

test/unit/test_util.py

@@ -0,0 +1,51 @@
+from metaflow.util import compress_list, decompress_list
+
+
+def test_compress_decompress_empty_list():
+    # round-trip must not raise and must return []
+    assert decompress_list(compress_list([])) == []
+
+
+def test_decompress_empty_string():
+    # direct empty-string input
+    assert decompress_list("") == []
+
+
+def test_compress_decompress_single_element():
+    lst = ["abc"]
+    assert decompress_list(compress_list(lst)) == lst
+
+
+def test_compress_decompress_plain_csv():
+    lst = ["a", "b", "c"]
+    assert decompress_list(compress_list(lst)) == lst
+
+
+def test_compress_decompress_prefix_encoded():
+    # Test with a longer list that triggers prefix encoding if applicable
+    # or just test with a manual prefix string
+    # Prefix encoding (Mode 2)
+    lst = ["test_1", "test_2", "test_3"]
+    # Round-trip test for a list with a shared prefix ("test_") - tests Mode 2 indirectly
+    compressed = compress_list(lst)
+    assert decompress_list(compressed) == lst
+
+
+def test_compress_decompress_zlib():
+    # Test with a very long list to trigger zlib compression (Mode 3)
+    lst = [str(i) for i in range(1000)]
+    compressed = compress_list(lst)
+    ZLIB_MARKER = "!"
+    assert compressed.startswith(ZLIB_MARKER)
+    assert decompress_list(compressed) == lst
+
+
+def test_compress_empty_string_element_ambiguity():
+    # Document the current behavior/limitation:
+    # both [] and [""] compress to ""
+    assert compress_list([]) == ""
+    assert compress_list([""]) == ""
+    # decompressing "" returns []
+    assert decompress_list("") == []
+    # Known limitation: round-trip for [""] loses data (returns [] instead of [""])
+    assert decompress_list(compress_list([""])) == []