Fix serialization issue

libs/langchain/langchain/load/dump.py

@@ -15,10 +15,16 @@ def default(obj: Any) -> Any:
 
 def dumps(obj: Any, *, pretty: bool = False) -> str:
     """Return a json string representation of an object."""
-    if pretty:
-        return json.dumps(obj, default=default, indent=2)
-    else:
-        return json.dumps(obj, default=default)
+    try:
+        if pretty:
+            return json.dumps(obj, default=default, indent=2)
+        else:
+            return json.dumps(obj, default=default)
+    except TypeError:
+        if pretty:
+            return json.dumps(to_json_not_implemented(obj), indent=2)
+        else:
+            return json.dumps(to_json_not_implemented(obj))
 
 
 def dumpd(obj: Any) -> Dict[str, Any]:

libs/langchain/langchain/schema/prompt_template.py

@@ -68,7 +68,7 @@ def _format_value(formatters: FormattersType, value: Any) -> Any:
 class BasePromptTemplate(Serializable, Runnable[Dict, PromptValue], ABC):
     """Base class for all prompt templates, returning a prompt."""
 
-    formatters: FormattersType = PROMPT_DEFAULT_FORMATTERS
+    formatters: Optional[FormattersType] = None
     """A mapping of types to functions that format them into a string.
     The functions should take a single argument, the value to format, and
     return a string. If the function takes two arguments, the second argument
@@ -147,7 +147,12 @@ def _prepare_variables(self, **kwargs: Any) -> Dict[str, Any]:
             for k, v in self.partial_variables.items()
         }
         all_variables = {**partial_kwargs, **kwargs}
-        return {k: _format_value(self.formatters, v) for k, v in all_variables.items()}
+        formatters = (
+            self.formatters
+            if self.formatters is not None
+            else PROMPT_DEFAULT_FORMATTERS
+        )
+        return {k: _format_value(formatters, v) for k, v in all_variables.items()}
 
     @abstractmethod
     def format(self, **kwargs: Any) -> str:
@@ -174,7 +179,6 @@ def _prompt_type(self) -> str:
     def dict(self, **kwargs: Any) -> Dict:
         """Return dictionary representation of prompt."""
         prompt_dict = super().dict(**kwargs)
-        del prompt_dict["formatters"]
         prompt_dict["_type"] = self._prompt_type
         return prompt_dict
 

libs/langchain/tests/unit_tests/prompts/__snapshots__/test_prompt.ambr

@@ -0,0 +1,60 @@
+# serializer version: 1
+# name: test_default_formatters
+  '''
+  {
+    "lc": 1,
+    "type": "constructor",
+    "id": [
+      "langchain",
+      "prompts",
+      "prompt",
+      "PromptTemplate"
+    ],
+    "kwargs": {
+      "input_variables": [
+        "foo"
+      ],
+      "template": "This is a {foo} test.",
+      "template_format": "f-string",
+      "partial_variables": {}
+    }
+  }
+  '''
+# ---
+# name: test_default_formatters.1
+  '''
+  {
+    "lc": 1,
+    "type": "constructor",
+    "id": [
+      "langchain",
+      "prompts",
+      "prompt",
+      "PromptTemplate"
+    ],
+    "kwargs": {
+      "input_variables": [
+        "foo"
+      ],
+      "template": "This is a {foo} test.",
+      "template_format": "f-string",
+      "partial_variables": {},
+      "formatters": {}
+    }
+  }
+  '''
+# ---
+# name: test_default_formatters.2
+  '''
+  {
+    "lc": 1,
+    "type": "not_implemented",
+    "id": [
+      "langchain",
+      "prompts",
+      "prompt",
+      "PromptTemplate"
+    ]
+  }
+  '''
+# ---

libs/langchain/tests/unit_tests/prompts/test_prompt.py

@@ -1,6 +1,9 @@
 """Test functionality related to prompts."""
+from langchain.load.dump import dumps
 import pytest
 
+from syrupy import SnapshotAssertion
+
 from langchain.prompts.prompt import PromptTemplate
 from langchain.schema.document import Document
 
@@ -123,7 +126,7 @@ def test_partial_init_string() -> None:
     assert result == "This is a 1 test."
 
 
-def test_default_formatters() -> None:
+def test_default_formatters(snapshot: SnapshotAssertion) -> None:
     """Test prompt can be initialized with partial variables."""
     template = "This is a {foo} test."
     prompt = PromptTemplate.from_template(template)
@@ -132,12 +135,23 @@ def test_default_formatters() -> None:
 
     foo = [Document(page_content="Hello there", metadata={"some": "key"})]
     assert prompt.format(foo=foo) == "This is a ['Hello there'] test."
+    assert dumps(prompt, pretty=True) == snapshot
 
     prompt_no_formatters = PromptTemplate.from_template(template, formatters={})
     assert (
         prompt_no_formatters.format(foo=foo)
         == "This is a [Document(page_content='Hello there', metadata={'some': 'key'})] test."  # noqa: E501
     )
+    assert dumps(prompt_no_formatters, pretty=True) == snapshot
+
+    prompt_custom_formatters = PromptTemplate.from_template(
+        template, formatters={Document: lambda x: x.metadata}
+    )
+    assert (
+        prompt_custom_formatters.format(foo=foo[0])
+        == "This is a {'some': 'key'} test."  # noqa: E501
+    )
+    assert dumps(prompt_custom_formatters, pretty=True) == snapshot
 
 
 def test_partial_init_func() -> None: