LangChain中用JSON文本构建FewShotPromptTemplate触发KeyError问题

问题重现 #

尝试创建用于总结JSON内容的FewShotPromptTemplate时，无论传入什么输入都会触发KeyError: '\n "name"'，但单独调用example_prompt.invoke(examples[0]).to_string()能正常运行。相关代码如下：

from langchain_core.prompts import PromptTemplate, FewShotPromptTemplate

json_raw = """{
    "name": "Bernd",
    "type": "Bread"
}"""
examples = [
    {
        "question": json_raw,
        "answer": "A file describing a bread.",
    }
]
example_prompt = PromptTemplate.from_template("Input file:\n{question}\n\nOutput file:\n{answer}\n")
prompt = FewShotPromptTemplate(
    examples=examples,
    example_prompt=example_prompt,
    suffix="Input file:\n{input}",
    input_variables=["input"],
)
command = prompt.invoke({"input": "asdf"})
print(command)

完整报错信息：

Traceback (most recent call last):
  File "/Users/jan/meine/ai/bpmnbot/fewshotbug.py", line 20, in <module>
    command = prompt.invoke({"input": "input"})
  File "/Users/jan/meine/ai/bpmnbot/venv/lib/python3.13/site-packages/langchain_core/prompts/base.py", line 208, in invoke
    return self._call_with_config(
           ~~~~~~~~~~~~~~~~~~~~~~^
        self._format_prompt_with_error_handling,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<3 lines>...
        serialized=self._serialized,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/jan/meine/ai/bpmnbot/venv/lib/python3.13/site-packages/langchain_core/runnables/base.py", line 1925, in _call_with_config
    context.run(
    ~~~~~~~~~~~^
        call_func_with_variable_args,  # type: ignore[arg-type]
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<4 lines>...
        **kwargs,
        ^^^^^^^^^
    ),
    ^
  File "/Users/jan/meine/ai/bpmnbot/venv/lib/python3.13/site-packages/langchain_core/runnables/config.py", line 396, in call_func_with_variable_args
    return func(input, **kwargs)  # type: ignore[call-arg]
  File "/Users/jan/meine/ai/bpmnbot/venv/lib/python3.13/site-packages/langchain_core/prompts/base.py", line 183, in _format_prompt_with_error_handling
    return self.format_prompt(**_inner_input)
           ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/Users/jan/meine/ai/bpmnbot/venv/lib/python3.13/site-packages/langchain_core/prompts/string.py", line 286, in format_prompt
    return StringPromptValue(text=self.format(**kwargs))
                                  ~~~~~~~~~~~^^^^^^^^^^
  File "/Users/jan/meine/ai/bpmnbot/venv/lib/python3.13/site-packages/langchain_core/prompts/few_shot.py", line 197, in format
    return DEFAULT_FORMATTER_MAPPING[self.template_format](template, **kwargs)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/string.py", line 190, in format
    return self.vformat(format_string, args, kwargs)
           ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jan/meine/ai/bpmnbot/venv/lib/python3.13/site-packages/langchain_core/utils/formatting.py", line 33, in vformat
    return super().vformat(format_string, args, kwargs)
           ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/string.py", line 194, in vformat
    result, _ = self._vformat(format_string, args, kwargs, used_args, 2)
                ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/string.py", line 234, in _vformat
    obj, arg_used = self.get_field(field_name, args, kwargs)
                    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/string.py", line 299, in get_field
    obj = self.get_value(first, args, kwargs)
  File "/opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/string.py", line 256, in get_value
    return kwargs[key]
           ~~~~~~^^^^^
KeyError: '\n    "name"'

原因分析 #

FewShotPromptTemplate默认使用Python的str.format作为模板格式化方式，示例中json_raw包含JSON语法的{}，这些会被str.format误判为需要填充的变量占位符。当FewShotPromptTemplate拼接完所有示例生成最终模板后，JSON里的{"name": "Bernd"}会被解析为需要获取键'\n "name"'的值，但该键不存在于输入变量中，因此触发KeyError。

单独调用example_prompt.invoke(examples[0])时，json_raw是作为{question}变量的填充值传入，此时str.format只会处理模板中定义的{question}和{answer}占位符，不会解析JSON内部的{}，所以能正常运行。

解决方案 #

有两种可行的解决方式：

方式1：切换模板格式为Jinja2 #

Jinja2模板只会识别{{ }}作为变量占位符，不会解析JSON中的{}，只需在创建FewShotPromptTemplate时指定template_format="jinja2"即可：

prompt = FewShotPromptTemplate(
    examples=examples,
    example_prompt=example_prompt,
    suffix="Input file:\n{input}",
    input_variables=["input"],
    template_format="jinja2"  # 添加这一行
)

方式2：转义JSON中的{} #

将JSON里的{替换为{{，}替换为}}，让str.format将其视为普通字符：

json_raw = """{{
    "name": "Bernd",
    "type": "Bread"
}}"""

推荐使用方式1，无需手动转义，更简洁可靠。

内容的提问来源于stack exchange，提问作者Jan Berndt