test: BOS occurrence in formatted conversation

Signed-off-by: Toshiki Kataoka <kataoka@pelements.jp>

tests/examples_tests/test_jinja.py

@@ -0,0 +1,47 @@
+from pathlib import Path
+
+import pytest
+import transformers
+
+jinja_paths = [
+    pytest.param(path, id=path.stem)
+    for path in sorted((Path(__name__).parent.parent /
+                        "examples").glob("*.jinja"))
+]
+
+
+@pytest.mark.parametrize("path", jinja_paths)
+@pytest.mark.parametrize("num_messages", [1, 3])
+def test_bos(path: Path, num_messages: int) -> None:
+    with path.open("r", encoding="utf-8") as f:
+        chat_template = f.read()
+    # We might guess an appropriate tokenizer model from the file name but we
+    # don't maintain such list.
+    # Use arbitrary BOS for testing. It doesn't have to match the str in the
+    # correct tokenizer.
+    bos_token = "=BOS="
+    tokenizer = transformers.PreTrainedTokenizerBase(
+        chat_template=chat_template, bos_token=bos_token, eos_token="=EOS=")
+    conversation = [
+        {
+            "role": "user",
+            "content": "1"
+        },
+        {
+            "role": "assistant",
+            "content": "2"
+        },
+        {
+            "role": "user",
+            "content": "3"
+        },
+    ][:num_messages]
+    try:
+        prompt: str = tokenizer.apply_chat_template(conversation=conversation,
+                                                    tokenize=False)
+    except Exception as e:
+        if str(e) == "Embedding models should only embed one message at a time":
+            pytest.skip(reason=str(e))
+        raise
+    assert prompt.startswith(bos_token)
+    assert prompt.count(bos_token) == 1