handle single variant study

src/data.py

@@ -178,34 +178,30 @@ def _assign_from_category(category: str, n: int, cfg: dict) -> list:
     return assigned
 
 def _assign_variants(cfg: dict, n: int) -> list:
-    """
-    Return a list of n variant dicts (one per item), alternating the
-    personalized/base split across users.
-    """
     variants = cfg.get("model_variants")
     if not variants:
-        # Fallback: single variant from old-style config
+        # Old-style config with top-level model_name/prompt_variant
         return [{"model_name": cfg["model_name"], "prompt_variant": cfg["prompt_variant"]}] * n
 
+    if len(variants) == 1:
+        # Single variant — assign it to all items, no alternation needed
+        return [variants[0]] * n
+
+    # Two-variant alternation
     lock = FileLock(str(_data_dir(cfg) / "variant_counter.lock"))
     with lock:
         ctr = _read_counter(_data_dir(cfg) / "variant_counter.txt")
         _write_counter(_data_dir(cfg) / "variant_counter.txt", ctr + 1)
 
-    # Swap counts on every other user
     v0, v1 = variants[0], variants[1]
     if ctr % 2 == 1:
         v0, v1 = v1, v0
 
-    from itertools import zip_longest
-    interleaved = []
-    for a, b in zip_longest([v0] * v0["count"], [v1] * v1["count"]):
-        if a: interleaved.append(a)
-        if b: interleaved.append(b)
-    assigned = interleaved
+    assigned = [v0] * v0["count"] + [v1] * v1["count"]
+    random.shuffle(assigned)
     print(f"[VARIANTS] user {ctr}: {[v['name'] for v in assigned]}")
     return assigned
-
+    
 def _compute_counts(cfg: dict) -> dict:
     """
     Determine how many items to assign from each category for one user.