langchain/prompts/loading.py/_load_prompt_from_file
def _load_prompt_from_file(file: Union[str, Path]) -> BasePromptTemplate:
    """Load prompt from file."""
    # Convert file to Path object.
    if isinstance(file, str):
        file_path = Path(file)
    else:
        file_path = file
    # Load from either json or yaml.
    if file_path.suffix == ".json":
        with open(file_path) as f:
            config = json.load(f)
    elif file_path.suffix == ".yaml":
        with open(file_path, "r") as f:
            config = yaml.safe_load(f)
    elif file_path.suffix == ".py":
        spec = importlib.util.spec_from_loader(
            "prompt", loader=None, origin=str(file_path)
        )
        if spec is None:
            raise ValueError("could not load spec")
        helper = importlib.util.module_from_spec(spec)
        with open(file_path, "rb") as f:
            exec(f.read(), helper.__dict__)
        if not isinstance(helper.PROMPT, BasePromptTemplate):
            raise ValueError("Did not get object of type BasePromptTemplate.")
        return helper.PROMPT
    else:
        raise ValueError(f"Got unsupported file type {file_path.suffix}")
    # Load the prompt from the config now.
    return load_prompt_from_config(config)