[simple][test] Added test case for schema.py (#3692)

- added unittest for schema.py covering utility functions and token
counting.
- fixed a nit. based on huggingface doc, the tokenizer model is gpt-2.
[link](https://huggingface.co/transformers/v4.8.2/_modules/transformers/models/gpt2/tokenization_gpt2_fast.html)
- make lint && make format, passed on local
- screenshot of new test running result

<img width="1283" alt="Screenshot 2023-04-27 at 9 51 55 PM"
src="https://user-images.githubusercontent.com/62768671/235057441-c0ac3406-9541-453f-ba14-3ebb08656114.png">

tests/integration_tests/test_schema.py

@@ -0,0 +1,15 @@
+"""Test formatting functionality."""
+
+from langchain.schema import _get_num_tokens_default_method
+
+
+class TestTokenCountingWithGPT2Tokenizer:
+    def test_empty_token(self) -> None:
+        assert _get_num_tokens_default_method("") == 0
+
+    def test_multiple_tokens(self) -> None:
+        assert _get_num_tokens_default_method("a b c") == 3
+
+    def test_special_tokens(self) -> None:
+        # test for consistency when the default tokenizer is changed
+        assert _get_num_tokens_default_method("a:b_c d") == 6

tests/unit_tests/test_schema.py

@@ -0,0 +1,77 @@
+"""Test formatting functionality."""
+
+import unittest
+
+from langchain.schema import (
+    AIMessage,
+    HumanMessage,
+    SystemMessage,
+    get_buffer_string,
+    messages_from_dict,
+    messages_to_dict,
+)
+
+
+class TestGetBufferString(unittest.TestCase):
+    human_msg: HumanMessage = HumanMessage(content="human")
+    ai_msg: AIMessage = AIMessage(content="ai")
+    sys_msg: SystemMessage = SystemMessage(content="sys")
+
+    def test_empty_input(self) -> None:
+        self.assertEqual(get_buffer_string([]), "")
+
+    def test_valid_single_message(self) -> None:
+        expected_output = f"Human: {self.human_msg.content}"
+        self.assertEqual(
+            get_buffer_string([self.human_msg]),
+            expected_output,
+        )
+
+    def test_custom_human_prefix(self) -> None:
+        prefix = "H"
+        expected_output = f"{prefix}: {self.human_msg.content}"
+        self.assertEqual(
+            get_buffer_string([self.human_msg], human_prefix="H"),
+            expected_output,
+        )
+
+    def test_custom_ai_prefix(self) -> None:
+        prefix = "A"
+        expected_output = f"{prefix}: {self.ai_msg.content}"
+        self.assertEqual(
+            get_buffer_string([self.ai_msg], ai_prefix="A"),
+            expected_output,
+        )
+
+    def test_multiple_msg(self) -> None:
+        msgs = [self.human_msg, self.ai_msg, self.sys_msg]
+        expected_output = "\n".join(
+            [
+                f"Human: {self.human_msg.content}",
+                f"AI: {self.ai_msg.content}",
+                f"System: {self.sys_msg.content}",
+            ]
+        )
+        self.assertEqual(
+            get_buffer_string(msgs),
+            expected_output,
+        )
+
+
+class TestMessageDictConversion(unittest.TestCase):
+    human_msg: HumanMessage = HumanMessage(
+        content="human", additional_kwargs={"key": "value"}
+    )
+    ai_msg: AIMessage = AIMessage(content="ai")
+    sys_msg: SystemMessage = SystemMessage(content="sys")
+
+    def test_multiple_msg(self) -> None:
+        msgs = [
+            self.human_msg,
+            self.ai_msg,
+            self.sys_msg,
+        ]
+        self.assertEqual(
+            messages_from_dict(messages_to_dict(msgs)),
+            msgs,
+        )