BasePromptTemplate Class — langchain Architecture

Source Code

libs/core/langchain_core/prompts/base.py lines 39–393

class BasePromptTemplate(
    RunnableSerializable[dict, PromptValue], ABC, Generic[FormatOutputType]
):
    """Base class for all prompt templates, returning a prompt."""

    input_variables: list[str]
    """A list of the names of the variables whose values are required as inputs to the
    prompt.
    """

    optional_variables: list[str] = Field(default=[])
    """A list of the names of the variables for placeholder or `MessagePlaceholder` that
    are optional.

    These variables are auto inferred from the prompt and user need not provide them.
    """

    input_types: builtins.dict[str, Any] = Field(default_factory=dict, exclude=True)
    """A dictionary of the types of the variables the prompt template expects.

    If not provided, all variables are assumed to be strings.
    """

    output_parser: BaseOutputParser | None = None
    """How to parse the output of calling an LLM on this formatted prompt."""

    partial_variables: Mapping[str, Any] = Field(default_factory=dict)
    """A dictionary of the partial variables the prompt template carries.

    Partial variables populate the template so that you don't need to pass them in every
    time you call the prompt.
    """

    metadata: builtins.dict[str, Any] | None = None
    """Metadata to be used for tracing."""

    tags: list[str] | None = None
    """Tags to be used for tracing."""

    @model_validator(mode="after")
    def validate_variable_names(self) -> Self:
        """Validate variable names do not include restricted names."""
        if "stop" in self.input_variables:
            msg = (
                "Cannot have an input variable named 'stop', as it is used internally,"
                " please rename."
            )
            raise ValueError(
                create_message(message=msg, error_code=ErrorCode.INVALID_PROMPT_INPUT)
            )
        if "stop" in self.partial_variables:
            msg = (
                "Cannot have an partial variable named 'stop', as it is used "
                "internally, please rename."
            )
            raise ValueError(
                create_message(message=msg, error_code=ErrorCode.INVALID_PROMPT_INPUT)
            )

        overall = set(self.input_variables).intersection(self.partial_variables)
        if overall:
            msg = f"Found overlapping input and partial variables: {overall}"
            raise ValueError(
                create_message(message=msg, error_code=ErrorCode.INVALID_PROMPT_INPUT)
            )
        return self

    @classmethod
    def get_lc_namespace(cls) -> list[str]:
        """Get the namespace of the LangChain object.

        Returns:
            `["langchain", "schema", "prompt_template"]`
        """
        return ["langchain", "schema", "prompt_template"]

    @classmethod
    def is_lc_serializable(cls) -> bool:
        """Return `True` as this class is serializable."""
        return True