LangChain教程 - 表达式语言 (LCEL) -构建智能链

LangChain提供了一种灵活且强大的表达式语言 (LangChain Expression Language, LCEL)，用于创建复杂的逻辑链。通过将不同的可运行对象组合起来，LCEL可以实现顺序链、嵌套链、并行链、路由以及动态构建等高级功能，从而满足各种场景下的需求。本文将详细介绍这些功能及其实现方式。

顺序链

LCEL的核心功能是将可运行对象按顺序组合起来，其中前一个对象的输出会自动传递给下一个对象作为输入。我们可以使用管道操作符 (|) 或显式的 .pipe() 方法来构建顺序链。

以下是一个简单的例子：

from langchain_ollama import OllamaLLM
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

model = OllamaLLM(model="qwen2.5:0.5b")
prompt = ChatPromptTemplate.from_template("tell me a joke about {topic}")

chain = prompt | model | StrOutputParser()

result = chain.invoke({"topic": "bears"})
print(result)

输出：

Here's a bear joke for you:

Why did the bear dissolve in water?
Because it was a polar bear!

在上述例子中，提示模板将输入格式化为聊天模型的输入格式，聊天模型生成笑话，最后通过输出解析器将结果转换为字符串。

嵌套链

嵌套链允许我们将多个链组合起来以创建更复杂的逻辑。例如，可以将一个生成笑话的链与另一个链组合，该链负责分析笑话的有趣程度。

analysis_prompt = ChatPromptTemplate.from_template("is this a funny joke? {joke}")
composed_chain = {"joke": chain} | analysis_prompt | model | StrOutputParser()

result = composed_chain.invoke({"topic": "bears"})
print(result)

输出：

Haha, that's a clever play on words! Using "polar" to imply the bear dissolved or became polar/polarized when put in water. Not the most hilarious joke ever, but it has a cute, groan-worthy pun that makes it mildly amusing.

并行链

RunnableParallel 使得可以并行运行多个链，并将每个链的结果组合成一个字典。这种方式适用于需要同时处理多个任务的场景。

from langchain_core.runnables import RunnableParallel

joke_chain = ChatPromptTemplate.from_template("tell me a joke about {topic}") | model
poem_chain = ChatPromptTemplate.from_template("write a 2-line poem about {topic}") | model

parallel_chain = RunnableParallel(joke=joke_chain, poem=poem_chain)

result = parallel_chain.invoke({"topic": "bear"})
print(result)

输出：

{
 'joke': "Why don't bears like fast food? Because they can't catch it!",
 'poem': "In the quiet of the forest, the bear roams free\nMajestic and wild, a sight to see."
}

路由

路由允许根据输入动态选择要执行的子链。LCEL提供了两种实现路由的方式：

使用自定义函数

通过 RunnableLambda 实现动态路由：

from langchain_core.prompts import PromptTemplate
from langchain_core.runnables import RunnableLambda

chain = (
    PromptTemplate.from_template(
        """Given the user question below, classify it as either being about `LangChain`, `Anthropic`, or `Other`.

Do not respond with more than one word.

<question>
{question}
</question>

Classification:"""
    )
    | OllamaLLM(model="qwen2.5:0.5b")
    | StrOutputParser()
)

langchain_chain = PromptTemplate.from_template(
    """You are an expert in langchain. \
Always answer questions starting with "As Harrison Chase told me". \
Respond to the following question:

Question: {question}
Answer:"""
) | OllamaLLM(model="qwen2.5:0.5b")
anthropic_chain = PromptTemplate.from_template(
    """You are an expert in anthropic. \
Always answer questions starting with "As Dario Amodei told me". \
Respond to the following question:

Question: {question}
Answer:"""
) | OllamaLLM(model="qwen2.5:0.5b")
general_chain = PromptTemplate.from_template(
    """Respond to the following question:

Question: {question}
Answer:"""
) | OllamaLLM(model="qwen2.5:0.5b")

def route(info):
    if "anthropic" in info["topic"].lower():
        return anthropic_chain
    elif "langchain" in info["topic"].lower():
        return langchain_chain
    else:
        return general_chain

full_chain = {"topic": chain, "question": lambda x: x["question"]} | RunnableLambda(route)

result = full_chain.invoke({"question": "how do I use LangChain?"})
print(result)

def route(info):
    if "anthropic" in info["topic"].lower():
        return anthropic_chain
    elif "langchain" in info["topic"].lower():
        return langchain_chain
    else:
        return general_chain

from langchain_core.runnables import RunnableLambda

full_chain = {"topic": chain, "question": lambda x: x["question"]} | RunnableLambda(route)

result = full_chain.invoke({"question": "how do I use LangChain?"})
print(result)

使用 `RunnableBranch`

RunnableBranch 通过条件匹配选择分支：

from langchain_core.runnables import RunnableBranch

branch = RunnableBranch(
    (lambda x: "anthropic" in x["topic"].lower(), anthropic_chain),
    (lambda x: "langchain" in x["topic"].lower(), langchain_chain),
    general_chain,
)

full_chain = {"topic": chain, "question": lambda x: x["question"]} | branch
result = full_chain.invoke({"question": "how do I use Anthropic?"})
print(result)

动态构建

动态构建链可以根据输入在运行时生成链的部分。通过 RunnableLambda 的返回值机制，可以返回一个新的 Runnable。

from langchain_core.runnables import chain, RunnablePassthrough

llm = OllamaLLM(model="qwen2.5:0.5b")

contextualize_instructions = """Convert the latest user question into a standalone question given the chat history. Don't answer the question, return the question and nothing else (no descriptive text)."""
contextualize_prompt = ChatPromptTemplate.from_messages(
    [
        ("system", contextualize_instructions),
        ("placeholder", "{chat_history}"),
        ("human", "{question}"),
    ]
)
contextualize_question = contextualize_prompt | llm | StrOutputParser()

@chain
def contextualize_if_needed(input_: dict):
    if input_.get("chat_history"):
        return contextualize_question
    else:
        return RunnablePassthrough() | itemgetter("question")

@chain
def fake_retriever(input_: dict):
    return "egypt's population in 2024 is about 111 million"

qa_instructions = (
    """Answer the user question given the following context:\n\n{context}."""
)
qa_prompt = ChatPromptTemplate.from_messages(
    [("system", qa_instructions), ("human", "{question}")]
)

full_chain = (
    RunnablePassthrough.assign(question=contextualize_if_needed).assign(
        context=fake_retriever
    )
    | qa_prompt
    | llm
    | StrOutputParser()
)

result = full_chain.invoke({
    "question": "what about egypt",
    "chat_history": [
        ("human", "what's the population of indonesia"),
        ("ai", "about 276 million"),
    ],
})
print(result)

输出：

According to the context provided, Egypt's population in 2024 is estimated to be about 111 million.

完整代码实例

from operator import itemgetter

from langchain_ollama import OllamaLLM
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

print("\n-----------------------------------\n")

# Simple demo
model = OllamaLLM(model="qwen2.5:0.5b")
prompt = ChatPromptTemplate.from_template("tell me a joke about {topic}")

chain = prompt | model | StrOutputParser()

result = chain.invoke({"topic": "bears"})
print(result)

print("\n-----------------------------------\n")

# Compose demo
analysis_prompt = ChatPromptTemplate.from_template("is this a funny joke? {joke}")
composed_chain = {"joke": chain} | analysis_prompt | model | StrOutputParser()

result = composed_chain.invoke({"topic": "bears"})
print(result)

print("\n-----------------------------------\n")

# Parallel demo
from langchain_core.runnables import RunnableParallel

joke_chain = ChatPromptTemplate.from_template("tell me a joke about {topic}") | model
poem_chain = ChatPromptTemplate.from_template("write a 2-line poem about {topic}") | model

parallel_chain = RunnableParallel(joke=joke_chain, poem=poem_chain)

result = parallel_chain.invoke({"topic": "bear"})
print(result)

print("\n-----------------------------------\n")

# Route demo
from langchain_core.prompts import PromptTemplate
from langchain_core.runnables import RunnableLambda

chain = (
    PromptTemplate.from_template(
        """Given the user question below, classify it as either being about `LangChain`, `Anthropic`, or `Other`.

Do not respond with more than one word.

<question>
{question}
</question>

Classification:"""
    )
    | OllamaLLM(model="qwen2.5:0.5b")
    | StrOutputParser()
)

langchain_chain = PromptTemplate.from_template(
    """You are an expert in langchain. \
Always answer questions starting with "As Harrison Chase told me". \
Respond to the following question:

Question: {question}
Answer:"""
) | OllamaLLM(model="qwen2.5:0.5b")
anthropic_chain = PromptTemplate.from_template(
    """You are an expert in anthropic. \
Always answer questions starting with "As Dario Amodei told me". \
Respond to the following question:

Question: {question}
Answer:"""
) | OllamaLLM(model="qwen2.5:0.5b")
general_chain = PromptTemplate.from_template(
    """Respond to the following question:

Question: {question}
Answer:"""
) | OllamaLLM(model="qwen2.5:0.5b")

def route(info):
    if "anthropic" in info["topic"].lower():
        return anthropic_chain
    elif "langchain" in info["topic"].lower():
        return langchain_chain
    else:
        return general_chain

full_chain = {"topic": chain, "question": lambda x: x["question"]} | RunnableLambda(route)

result = full_chain.invoke({"question": "how do I use LangChain?"})
print(result)

print("\n-----------------------------------\n")

# Branch demo
from langchain_core.runnables import RunnableBranch

branch = RunnableBranch(
    (lambda x: "anthropic" in x["topic"].lower(), anthropic_chain),
    (lambda x: "langchain" in x["topic"].lower(), langchain_chain),
    general_chain,
)

full_chain = {"topic": chain, "question": lambda x: x["question"]} | branch
result = full_chain.invoke({"question": "how do I use Anthropic?"})
print(result)

print("\n-----------------------------------\n")

# Dynamic demo
from langchain_core.runnables import chain, RunnablePassthrough

llm = OllamaLLM(model="qwen2.5:0.5b")

contextualize_instructions = """Convert the latest user question into a standalone question given the chat history. Don't answer the question, return the question and nothing else (no descriptive text)."""
contextualize_prompt = ChatPromptTemplate.from_messages(
    [
        ("system", contextualize_instructions),
        ("placeholder", "{chat_history}"),
        ("human", "{question}"),
    ]
)
contextualize_question = contextualize_prompt | llm | StrOutputParser()

@chain
def contextualize_if_needed(input_: dict):
    if input_.get("chat_history"):
        return contextualize_question
    else:
        return RunnablePassthrough() | itemgetter("question")

@chain
def fake_retriever(input_: dict):
    return "egypt's population in 2024 is about 111 million"

qa_instructions = (
    """Answer the user question given the following context:\n\n{context}."""
)
qa_prompt = ChatPromptTemplate.from_messages(
    [("system", qa_instructions), ("human", "{question}")]
)

full_chain = (
    RunnablePassthrough.assign(question=contextualize_if_needed).assign(
        context=fake_retriever
    )
    | qa_prompt
    | llm
    | StrOutputParser()
)

result = full_chain.invoke({
    "question": "what about egypt",
    "chat_history": [
        ("human", "what's the population of indonesia"),
        ("ai", "about 276 million"),
    ],
})
print(result)

print("\n-----------------------------------\n")