langchain/prompts/prompt.py/PromptTemplate/from_file
class PromptTemplate:    def from_file(
        cls, template_file: Union[str, Path], input_variables: List[str], **kwargs: Any
    ) -> PromptTemplate:
        """Load a prompt from a file.

        Args:
            template_file: The path to the file containing the prompt template.
            input_variables: A list of variable names the final prompt template
                will expect.
        Returns:
            The prompt loaded from the file.
        """
        with open(str(template_file), "r") as f:
            template = f.read()
        return cls(input_variables=input_variables, template=template, **kwargs)