[Bugfix][Frontend] Raise exception when file-like chat template fails…

… to be opened (vllm-project#4292)

tests/async_engine/test_chat_template.py

@@ -76,20 +76,29 @@ def test_load_chat_template():
 {% if add_generation_prompt and messages[-1]['role'] != 'assistant' %}{{ '<|im_start|>assistant\\n' }}{% endif %}"""  # noqa: E501
 
 
-def test_no_load_chat_template():
+def test_no_load_chat_template_filelike():
     # Testing chatml template
     template = "../../examples/does_not_exist"
     tokenizer = MockTokenizer()
 
+    mock_serving_chat = MockServingChat(tokenizer)
+
+    with pytest.raises(ValueError, match="looks like a file path"):
+        OpenAIServingChat._load_chat_template(mock_serving_chat,
+                                              chat_template=template)
+
+
+def test_no_load_chat_template_literallike():
+    # Testing chatml template
+    template = "{{ messages }}"
+    tokenizer = MockTokenizer()
+
     mock_serving_chat = MockServingChat(tokenizer)
     OpenAIServingChat._load_chat_template(mock_serving_chat,
                                           chat_template=template)
     template_content = tokenizer.chat_template
 
-    # Test assertions
-    assert template_content is not None
-    # Hard coded value for template_chatml.jinja
-    assert template_content == """../../examples/does_not_exist"""
+    assert template_content == template
 
 
 @pytest.mark.asyncio

vllm/entrypoints/openai/serving_chat.py

@@ -319,23 +319,30 @@ async def chat_completion_full_generator(
         return response
 
     def _load_chat_template(self, chat_template):
+        tokenizer = self.tokenizer
+
         if chat_template is not None:
             try:
                 with open(chat_template, "r") as f:
-                    self.tokenizer.chat_template = f.read()
-            except OSError:
+                    tokenizer.chat_template = f.read()
+            except OSError as e:
+                JINJA_CHARS = "{}\n"
+                if not any(c in chat_template for c in JINJA_CHARS):
+                    msg = (f"The supplied chat template ({chat_template}) "
+                           f"looks like a file path, but it failed to be "
+                           f"opened. Reason: {e}")
+                    raise ValueError(msg) from e
+
                 # If opening a file fails, set chat template to be args to
                 # ensure we decode so our escape are interpreted correctly
-                self.tokenizer.chat_template = codecs.decode(
+                tokenizer.chat_template = codecs.decode(
                     chat_template, "unicode_escape")
 
             logger.info(
-                f"Using supplied chat template:\n{self.tokenizer.chat_template}"
-            )
-        elif self.tokenizer.chat_template is not None:
+                f"Using supplied chat template:\n{tokenizer.chat_template}")
+        elif tokenizer.chat_template is not None:
             logger.info(
-                f"Using default chat template:\n{self.tokenizer.chat_template}"
-            )
+                f"Using default chat template:\n{tokenizer.chat_template}")
         else:
             logger.warning(
                 "No chat template provided. Chat API will not work.")