squad_v2.py: include unanswerable questions in evaluation Fixes #1184

Author: Matteovanypersele

src/lighteval/tasks/tasks/squad_v2.py

@@ -16,6 +16,14 @@
 when possible, but also determine when no answer is supported by the paragraph
 and abstain from answering.
 
+note:
+This is an LLM-friendly adaptation of the original SQuAD 2.0 evaluation.
+The original evaluation uses extractive span selection with a confidence-based
+"no answer" threshold, which does not apply to generative models.
+Here, the model is instead instructed to generate "unanswerable" when the
+question cannot be answered from the context. EM and F1 metrics are computed
+over both answerable and unanswerable questions.
+
 languages:
 english
 
@@ -28,12 +36,46 @@
 
 from lighteval.metrics.metrics import Metrics
 from lighteval.tasks.lighteval_task import LightevalTaskConfig
+from lighteval.tasks.requests import Doc
 from lighteval.tasks.templates.qa import get_qa_prompt_function
 from lighteval.utils.language import Language
 
+UNANSWERABLE = "unanswerable"
+
+
+def squad_v2_prompt(line, task_name: str = None):
+    answers = list(set(ans for ans in line["answers"]["text"] if len(ans) > 0))
+    is_unanswerable = len(answers) == 0
+
+    if is_unanswerable:
+        choices = [f" {UNANSWERABLE}"]
+    else:
+        choices = [f" {ans}" for ans in answers]
+
+    return Doc(
+        task_name=task_name,
+        query=f"Context: {line['context']}\nQuestion: {line['question']}\n"
+        f"Answer with a span from the context, or \"{UNANSWERABLE}\" if the question cannot be answered.\nAnswer:",
+        choices=choices,
+        gold_index=list(range(len(choices))),
+    )
+
 
 squad_v2 = LightevalTaskConfig(
     name="squad_v2",
+    prompt_function=squad_v2_prompt,
+    hf_repo="rajpurkar/squad_v2",
+    hf_subset="squad_v2",
+    evaluation_splits=("validation",),
+    few_shots_split="train",
+    stop_sequence=["\n", "Question:", "question:"],
+    generation_size=200,
+    metrics=[Metrics.exact_match, Metrics.f1_score],
+    version=2,
+)
+
+squad_v2_answerable = LightevalTaskConfig(
+    name="squad_v2:answerable",
     prompt_function=get_qa_prompt_function(
         Language.ENGLISH,
         lambda line: {
@@ -49,10 +91,11 @@
     few_shots_split="train",
     stop_sequence=["\n", "Question:", "question:"],
     generation_size=200,
-    metrics=[Metrics.exact_match],
+    metrics=[Metrics.exact_match, Metrics.f1_score],
     version=1,
 )
 
 TASKS_TABLE = [
     squad_v2,
+    squad_v2_answerable,
 ]