core: RunnablePick may not return a dict if keys is a string

[cbornet]

Checked other resources

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Example Code

def test_runnablepick() -> None:
    chain = RunnablePick("foo")
    chain.invoke({"foo": "bar"})
    assert chain.invoke({"foo": "bar"}) == "bar"

Error Message and Stack Trace (if applicable)

No response

Description

According to its typing and documentation RunnnablePick should return a dict:

class RunnablePick(RunnableSerializable[dict[str, Any], dict[str, Any]]):

but if its keys param is a string and not a list, it returns the value of the key, not a dict filtered on the key.

To fix, we can either:

1. Change the signature to

class RunnablePick(RunnableSerializable[dict[str, Any], Any):

2. Or change the behavior to always return a dict. But that would be a breaking change...

System Info

System Information
------------------
> OS:  Darwin
> OS Version:  Darwin Kernel Version 24.1.0: Thu Oct 10 21:06:57 PDT 2024; root:xnu-11215.41.3~3/RELEASE_ARM64_T6041
> Python Version:  3.13.2 (main, Feb 18 2025, 17:10:43) [Clang 16.0.0 (clang-1600.0.26.6)]

Package Information
-------------------
> langchain_core: 0.3.47
> langchain: 0.3.21
> langchain_community: 0.3.20
> langsmith: 0.3.18
> langchain_astradb: 0.5.3
> langchain_text_splitters: 0.3.7

Optional packages not installed
-------------------------------
> langserve

Other Dependencies
------------------
> aiohttp<4.0.0,>=3.8.3: Installed. No version info available.
> astrapy: 1.5.2
> async-timeout<5.0.0,>=4.0.0;: Installed. No version info available.
> dataclasses-json<0.7,>=0.5.7: Installed. No version info available.
> httpx: 0.28.1
> httpx-sse<1.0.0,>=0.4.0: Installed. No version info available.
> jsonpatch<2.0,>=1.33: Installed. No version info available.
> langchain-anthropic;: Installed. No version info available.
> langchain-aws;: Installed. No version info available.
> langchain-azure-ai;: Installed. No version info available.
> langchain-cohere;: Installed. No version info available.
> langchain-community;: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.45: Installed. No version info available.
> langchain-deepseek;: Installed. No version info available.
> langchain-fireworks;: Installed. No version info available.
> langchain-google-genai;: Installed. No version info available.
> langchain-google-vertexai;: Installed. No version info available.
> langchain-groq;: Installed. No version info available.
> langchain-huggingface;: Installed. No version info available.
> langchain-mistralai;: Installed. No version info available.
> langchain-ollama;: Installed. No version info available.
> langchain-openai;: Installed. No version info available.
> langchain-text-splitters<1.0.0,>=0.3.7: Installed. No version info available.
> langchain-together;: Installed. No version info available.
> langchain-xai;: Installed. No version info available.
> langchain<1.0.0,>=0.3.21: Installed. No version info available.
> langsmith-pyo3: Installed. No version info available.
> langsmith<0.4,>=0.1.125: Installed. No version info available.
> langsmith<0.4,>=0.1.17: Installed. No version info available.
> numpy: 1.26.4
> numpy<3,>=1.26.2: Installed. No version info available.
> openai-agents: Installed. No version info available.
> opentelemetry-api: Installed. No version info available.
> opentelemetry-exporter-otlp-proto-http: Installed. No version info available.
> opentelemetry-sdk: Installed. No version info available.
> orjson: 3.10.15
> packaging: 24.2
> packaging<25,>=23.2: Installed. No version info available.
> pydantic: 2.10.6
> pydantic-settings<3.0.0,>=2.4.0: Installed. No version info available.
> pydantic<3.0.0,>=2.5.2;: Installed. No version info available.
> pydantic<3.0.0,>=2.7.4: Installed. No version info available.
> pydantic<3.0.0,>=2.7.4;: Installed. No version info available.
> pytest: 8.1.1
> PyYAML>=5.3: Installed. No version info available.
> requests: 2.32.3
> requests-toolbelt: 1.0.0
> requests<3,>=2: Installed. No version info available.
> rich: 13.9.4
> SQLAlchemy<3,>=1.4: Installed. No version info available.
> tenacity!=8.4.0,<10,>=8.1.0: Installed. No version info available.
> tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
> typing-extensions>=4.7: Installed. No version info available.
> zstandard: 0.23.0

[neel2299]

Hello, I was wondering if you preferred swastik's PR 31309 Or whether permanently returning dictionaries is being considered?

[eyurtsev]

@cbornet i assume there's no way to introduce type dependency in this case? (i.e., Any if passed as a string, and dict if passed a list of str?)

[cbornet]

I don't know how to do it. I'm not sure it's possible...

[mdrxy]

Any movement on this? Are we totally blocked? If so we should close this and #31321

[cbornet]

We are not blocked.
We would need a decision between changing the signature and changing the behaviour.

[eugeneo17]

I'd like to take this issue and submit a fix. Let me know if anyone is already working on it!

[mdrxy]

@eugeneo17 go for it!

[cbornet]

Note that there is a PR opened if the choice is to change the signature : #31321

[eugeneo17]

Thanks. I'm starting work on #31309