fix for LC

Author: nimarb

langfuse/model.py

@@ -2,7 +2,6 @@
 
 import re
 from abc import ABC, abstractmethod
-from collections.abc import Iterable
 from typing import Any, Dict, List, Literal, Optional, Tuple, TypedDict, Union
 
 from langfuse.api.resources.commons.types.dataset import (
@@ -372,12 +371,14 @@ def compile_with_placeholders(
         self,
         placeholders: Dict[str, List[ChatMessageDict]],
         variables: Optional[Dict[str, str]] = None,
+        persist_compilation: bool = False,
     ) -> List[ChatMessageDict]:
         """Compile chat prompt by first replacing placeholders, then expanding variables.
 
         Args:
             variables: Dictionary of variable names to values for template substitution
             placeholders: Dictionary of placeholder names to lists of ChatMessage objects
+            persist_compilation: If True, saves the compiled output to the internal state. Useful if using the output for langchain prompts.
 
         Returns:
             List[ChatMessageDict]: Compiled chat messages
@@ -411,7 +412,7 @@ def compile_with_placeholders(
                 )
 
         # Then, replace the variables in the ChatMessage content.
-        return [
+        compiled_messages = [
             ChatMessageDict(
                 content=TemplateParser.compile_template(
                     chat_message["content"],
@@ -422,6 +423,19 @@ def compile_with_placeholders(
             for chat_message in messages_with_placeholders_replaced
         ]
 
+        # Mutate the internal prompt object if requested
+        if persist_compilation:
+            self.prompt = [
+                ChatMessageWithPlaceholdersDict_Message(
+                    type="message",
+                    role=msg["role"],
+                    content=msg["content"],
+                )
+                for msg in compiled_messages
+            ]
+
+        return compiled_messages
+
     def get_langchain_prompt(self, **kwargs):
         """Convert Langfuse prompt into string compatible with Langchain ChatPromptTemplate.
 

tests/test_prompt_compilation.py

@@ -732,3 +732,73 @@ def test_chat_prompt_with_json_variables(self):
         assert len(formatted_messages) == 2
         assert formatted_messages[0].content == expected_system
         assert formatted_messages[1].content == expected_user
+
+    def test_chat_prompt_with_placeholders_langchain(self):
+        """Test that chat prompts with placeholders work correctly with Langchain."""
+        from langfuse.api.resources.prompts import Prompt_Chat
+
+        chat_messages = [
+            ChatMessage(
+                role="system",
+                content="You are a {{role}} assistant with {{capability}} capabilities.",
+            ),
+            {"type": "placeholder", "name": "examples"},
+            ChatMessage(
+                role="user",
+                content="Help me with {{task}}.",
+            ),
+        ]
+
+        prompt_client = ChatPromptClient(
+            Prompt_Chat(
+                type="chat",
+                name="chat_placeholder_langchain_test",
+                version=1,
+                config={},
+                tags=[],
+                labels=[],
+                prompt=chat_messages,
+            ),
+        )
+
+        placeholders = {
+            "examples": [
+                {"role": "user", "content": "Example: What is 2+2?"},
+                {"role": "assistant", "content": "2+2 equals 4."},
+            ],
+        }
+
+        # Test compile_with_placeholders with only placeholders (no variables)
+        compiled_messages = prompt_client.compile_with_placeholders(
+            placeholders=placeholders,
+        )
+
+        assert len(compiled_messages) == 4
+        assert (
+            compiled_messages[0]["content"]
+            == "You are a {{role}} assistant with {{capability}} capabilities."
+        )
+        assert compiled_messages[1]["content"] == "Example: What is 2+2?"
+        assert compiled_messages[2]["content"] == "2+2 equals 4."
+        assert compiled_messages[3]["content"] == "Help me with {{task}}."
+
+        compiled_messages = prompt_client.compile_with_placeholders(
+            placeholders=placeholders, persist_compilation=True,
+        )
+
+        langchain_messages = prompt_client.get_langchain_prompt(
+            role="helpful",
+            capability="math",
+            task="addition",
+        )
+        langchain_prompt = ChatPromptTemplate.from_messages(langchain_messages)
+        formatted_messages = langchain_prompt.format_messages()
+
+        assert len(formatted_messages) == 4
+        assert (
+            formatted_messages[0].content
+            == "You are a helpful assistant with math capabilities."
+        )
+        assert formatted_messages[1].content == "Example: What is 2+2?"
+        assert formatted_messages[2].content == "2+2 equals 4."
+        assert formatted_messages[3].content == "Help me with addition."