community: avoid double templating in langchain_prompty
#25777
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Not relevant to the core logic, but I hesitated betwen sharing a dict for the mapping between I went with the latter, but if anybody has pointers on what would be the cleanest here I'd be happy to learn. |
Prompty templates the given `.prompty` file and parse it for messages. We convert those messages to langchain's Message objects. This avoids the second templating in the call to ChatPromptTemplate.
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| from pydantic import BaseModel | ||
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| from .core import Invoker, Prompty, SimpleModel | ||
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| class RoleMap: |
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Does this cover the full scope of roles? What about "tool"?
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Yes, I simply adapted the roles that were defined when integrating Prompty into LangChain. The latest versions of Prompty removed ai and human, so we could even delete those (but that would break some users' prompts)
Tools are defined in the YAML header of the .prompty files, here's an example
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| lc_messages.append( | ||
| MessagesPlaceholder( | ||
| variable_name=input_name_agent_scratchpad, optional=True | ||
| ) # type: ignore[arg-type] | ||
| ) | ||
| lc_p = ChatPromptTemplate.from_messages( | ||
| lc_messages, template_format=template_format |
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Can you clarify why specifying template_format with a default of "f-string" is breaking? It looks like that is the default in ChatPromptTemplate.from_messages:
langchain/libs/core/langchain_core/prompts/chat.py
Lines 1147 to 1151 in d6c4803
Or is it just unrelated?
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You're right that's it's unrelated.
Here the issue is that before initializing the ChatPromptTemplate, Prompty has already parsed and templated the prompt using the user input.
When invoking ChatPromptTemplate, it tries to template the messages and interprets text inside { } (or {{ }}) as missing variables. The fix proposed in this PR avoids this templating by converting to Messages objects, which are already templated.
Since we skip this second templating entirely, it does not make sense to expose one of its parameters to the user, as it won't have any effect. You can select the template engine used by Prompty directly in the .promptys file YAML header.
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Thanks for merging and for the time you spent on this! |
Thank you for the fix! Released in langchain-prompty 0.0.3, feel free to take a look. |
Description
In
langchain_prompty, messages are templated by Prompty. However, a call toChatPromptTemplatewas initiating a second templating. We now convert parsed messages toMessageobjects before callingChatPromptTemplate, signifying clearly that they are already templated.We also revert #25739 , which applied to this second templating, which we now avoid, and did not fix the original issue.
Issue
Closes #25703