Stream custom generator functions

You can use generator functions (ie. functions that use the yield keyword, and behave like iterators) in a LCEL pipeline.

The signature of these generators should be Iterator[Input] -> Iterator[Output]. Or for async generators: AsyncIterator[Input] -> AsyncIterator[Output].

These are useful for: - implementing a custom output parser - modifying the output of a previous step, while preserving streaming capabilities

Let’s implement a custom output parser for comma-separated lists.

Sync version

from typing import Iterator, List

from langchain.prompts.chat import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser
from langchain_openai import ChatOpenAI

prompt = ChatPromptTemplate.from_template(
    "Write a comma-separated list of 5 animals similar to: {animal}"
)
model = ChatOpenAI(temperature=0.0)

str_chain = prompt | model | StrOutputParser()

for chunk in str_chain.stream({"animal": "bear"}):
    print(chunk, end="", flush=True)

lion, tiger, wolf, gorilla, panda

str_chain.invoke({"animal": "bear"})

'lion, tiger, wolf, gorilla, panda'

# This is a custom parser that splits an iterator of llm tokens
# into a list of strings separated by commas
def split_into_list(input: Iterator[str]) -> Iterator[List[str]]:
    # hold partial input until we get a comma
    buffer = ""
    for chunk in input:
        # add current chunk to buffer
        buffer += chunk
        # while there are commas in the buffer
        while "," in buffer:
            # split buffer on comma
            comma_index = buffer.index(",")
            # yield everything before the comma
            yield [buffer[:comma_index].strip()]
            # save the rest for the next iteration
            buffer = buffer[comma_index + 1 :]
    # yield the last chunk
    yield [buffer.strip()]

list_chain = str_chain | split_into_list

for chunk in list_chain.stream({"animal": "bear"}):
    print(chunk, flush=True)

['lion']
['tiger']
['wolf']
['gorilla']
['panda']

list_chain.invoke({"animal": "bear"})

['lion', 'tiger', 'wolf', 'gorilla', 'panda']

Async version

from typing import AsyncIterator


async def asplit_into_list(
    input: AsyncIterator[str],
) -> AsyncIterator[List[str]]:  # async def
    buffer = ""
    async for (
        chunk
    ) in input:  # `input` is a `async_generator` object, so use `async for`
        buffer += chunk
        while "," in buffer:
            comma_index = buffer.index(",")
            yield [buffer[:comma_index].strip()]
            buffer = buffer[comma_index + 1 :]
    yield [buffer.strip()]


list_chain = str_chain | asplit_into_list

async for chunk in list_chain.astream({"animal": "bear"}):
    print(chunk, flush=True)

['lion']
['tiger']
['wolf']
['gorilla']
['panda']

await list_chain.ainvoke({"animal": "bear"})

['lion', 'tiger', 'wolf', 'gorilla', 'panda']