wait_chain: reject empty strategy list with ValueError instead of IndexError

releasenotes/notes/fix-wait-chain-empty-strategies-d3a8f7c2b1e4f569.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    ``wait_chain()`` now raises ``ValueError`` at construction time when
+    called with no strategies, instead of raising a confusing
+    ``IndexError`` from ``__call__`` the first time it is used. Empty
+    wait chains have no useful behaviour, so failing early gives a
+    clearer error message.

tenacity/wait.py

@@ -104,6 +104,8 @@ def wait_chained():
     """
 
     def __init__(self, *strategies: wait_base) -> None:
+        if not strategies:
+            raise ValueError("wait_chain() requires at least one strategy")
         self.strategies = strategies
 
     def __call__(self, retry_state: "RetryCallState") -> float:

tests/test_tenacity.py

@@ -541,10 +541,13 @@ def always_return_1() -> int:
         self.assertEqual(sleep_intervals, [1.0, 2.0, 3.0, 3.0])
         sleep_intervals[:] = []
 
-        # Clear and restart retrying.
-        self.assertRaises(tenacity.RetryError, always_return_1)
-        self.assertEqual(sleep_intervals, [1.0, 2.0, 3.0, 3.0])
-        sleep_intervals[:] = []
+    def test_wait_chain_requires_at_least_one_strategy(self) -> None:
+        with self.assertRaises(ValueError):
+            tenacity.wait_chain()
+        # Confirm the wrapped Retrying path surfaces the same ValueError
+        # instead of an opaque IndexError from self.strategies[-1].
+        with self.assertRaises(ValueError):
+            Retrying(wait=tenacity.wait_chain())
 
     def test_wait_random_exponential(self) -> None:
         fn = tenacity.wait_random_exponential(0.5, 60.0)