From b37952400a6b8be45664c01f7eafbdaae74d17ba Mon Sep 17 00:00:00 2001
From: Anurag Kumar Singh <dr.anuragkr@gmail.com>
Date: Mon, 31 Aug 2026 14:04:41 +0530
Subject: [PATCH] Fix decompress_list IndexError on empty string input (#3017)

---
 metaflow/util.py                           |  2 +
 test/unit/test_compress_decompress_list.py | 45 ++++++++++++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 test/unit/test_compress_decompress_list.py

diff --git a/metaflow/util.py b/metaflow/util.py
index 8f310a63236..95b8f724f08 100644
--- a/metaflow/util.py
+++ b/metaflow/util.py
@@ -382,6 +382,8 @@ def compress_list(lst, separator=",", rangedelim=":", zlibmarker="!", zlibmin=50
 
 
 def decompress_list(lststr, separator=",", rangedelim=":", zlibmarker="!"):
+    if lststr == "":
+        return []
     # Three input modes:
     if lststr[0] == zlibmarker:
         # 3. zlib-compressed, base64-encoded
diff --git a/test/unit/test_compress_decompress_list.py b/test/unit/test_compress_decompress_list.py
new file mode 100644
index 00000000000..b14b609fea4
--- /dev/null
+++ b/test/unit/test_compress_decompress_list.py
@@ -0,0 +1,45 @@
+"""Tests for compress_list / decompress_list round-trip correctness.
+
+Regression test for https://github.com/Netflix/metaflow/issues/3017:
+decompress_list("") must return [] instead of raising IndexError.
+"""
+
+import pytest
+
+from metaflow.util import compress_list, decompress_list
+
+
+def test_compress_empty_list():
+    assert compress_list([]) == ""
+
+
+def test_decompress_empty_string():
+    """The bug: lststr[0] crashes on empty string."""
+    assert decompress_list("") == []
+
+
+def test_roundtrip_empty():
+    assert decompress_list(compress_list([])) == []
+
+
+@pytest.mark.parametrize(
+    "items",
+    [
+        pytest.param(["x"], id="single"),
+        pytest.param(["a", "b", "c"], id="simple"),
+        pytest.param(
+            ["run/step/task/1", "run/step/task/2", "run/step/task/3"],
+            id="common-prefix",
+        ),
+    ],
+)
+def test_roundtrip(items):
+    assert decompress_list(compress_list(items)) == items
+
+
+def test_roundtrip_zlib():
+    """Force the zlib-compressed path by setting zlibmin=0."""
+    items = ["a", "b"]
+    compressed = compress_list(items, zlibmin=0)
+    assert compressed.startswith("!")
+    assert decompress_list(compressed) == items