Change test + simplify check.

Author: StanFromIreland

Lib/test/test_zoneinfo/data/tzif_invalid_lookahead

@@ -742,13 +742,35 @@ def test_empty_zone(self):
             self.klass.from_file(zf)
 
     def test_invalid_transition_index(self):
-        with open(DATA_DIR / "tzif_invalid_trans_idx", "rb") as f:
-            with self.assertRaises(ValueError):
-                self.klass.from_file(f)
+        STD = ZoneOffset("STD", ZERO)
+        DST = ZoneOffset("DST", ONE_H, ONE_H)
+
+        zf = self.construct_zone([
+            ZoneTransition(datetime(2026, 3, 1, 2), STD, DST),
+            ZoneTransition(datetime(2026, 11, 1, 2), DST, STD),
+        ], after="", version=1)
+
+        data = bytearray(zf.read())
+        timecnt = struct.unpack_from(">l", data, 32)[0]
+        idx_offset = 44 + timecnt * 4
+        data[idx_offset + 1] = 2  # typecnt is 2, so index 2 is OOB
+        f = io.BytesIO(bytes(data))
+
+        with self.assertRaises(ValueError):
+            self.klass.from_file(f)
 
     def test_transition_lookahead_out_of_bounds(self):
-        with open(DATA_DIR / "tzif_invalid_lookahead", "rb") as f:
-            zi = self.klass.from_file(f)
+        STD = ZoneOffset("STD", ZERO)
+        DST = ZoneOffset("DST", ONE_H, ONE_H)
+        EXT = ZoneOffset("EXT", ONE_H)
+
+        zf = self.construct_zone([
+            ZoneTransition(datetime(2026, 3, 1), STD, DST),
+            ZoneTransition(datetime(2026, 6, 1), DST, EXT),
+            ZoneTransition(datetime(2026, 9, 1), EXT, DST),
+        ], after="")
+
+        zi = self.klass.from_file(zf)
         self.assertIsNotNone(zi)
 
     def test_zone_very_large_timestamp(self):

Lib/test/test_zoneinfo/data/tzif_invalid_trans_idx

@@ -71,9 +71,9 @@ def load_data(fobj):
         trans_list_utc = ()
         trans_idx = ()
 
-    for idx in trans_idx:
-        if idx >= typecnt:
-            raise ValueError(f"Invalid transition index found while reading TZif: {idx}")
+    if trans_idx and max(trans_idx) >= typecnt:
+        raise ValueError("Invalid transition index found while reading TZif: "
+                         f"{max(trans_idx)}")
 
     # Read the ttinfo struct, (utoff, isdst, abbrind)
     if typecnt: